Transcript Forces

Forces
• Newton’s Laws
of Motion
• Weight
• Free fall
• Force and motion
problems in 1-D
• Normal force
• Tension
• Free body diagrams
• Atwood device
• Static and kinetic friction
• Coefficients of friction
• Air resistance
• Terminal velocity
Examples of Forces
• A force is just a push or pull. Examples:
– an object’s weight
– tension in a rope
– a left hook to the schnozola
– friction
– attraction between an electron and proton
• Bodies don’t have to be in contact to
exert forces on each other, e.g., gravity.
Fundamental Forces of Nature
• Gravity
– Attraction between any two bodies w/ mass
– Weakest but most dominant
• Electromagnetic
– Forces between any two bodies w/ charge
– Attractive or repulsive
• Weak nuclear force – responsible for
radioactive decay
• Strong nuclear force – holds quarks
together (constituents of protons and
neutrons)
Newton’s Laws of Motion
1. Inertia: “An object in motion
tends to stay in motion. An object
at rest tends to stay at rest.”
2. Fnet = ma
3. Action – Reaction: “For every
action there is an equal but
opposite reaction.”
st
1
Law: Inertia
“An object in motion tends to stay in motion;
an object at rest tends to stay at rest.”
• A moving body will continue moving
in the same direction with the same
speed until some net force acts on it.
• A body at rest will remain at rest
unless a net force acts on it.
• Summing it up: It takes a net force
to change a body’s velocity.
Inertia Example 1
An astronaut in
outer space will
continue drifting
in the same
direction at the
same speed
indefinitely, until
acted upon by an
outside force.
Inertia Example 2
If you’re driving at 65 mph and have an
accident, your car may come to a stop in
an instant, while your body is still moving
at 65 mph. Without a seatbelt, your inertia
could carry you through the windshield.
nd
2
Law: Fnet = m a
• The acceleration an object undergoes is
directly proportion to the net force acting on it.
• Mass is the constant of proportionality.
• For a given mass, if Fnet doubles, triples, etc.
in size, so does a.
• For a given Fnet if m doubles, a is cut in half.
• Fnet and a are vectors; m is a scalar.
• Fnet and a always point in the same
direction.
• The 1st law is really a special case of the 2nd
law (if net force is zero, so is acceleration).
What is Net Force?
F1
F2
F3
Fnet
When more than one
force acts on a body,
the net force
(resultant force) is the
vector combination of
all the forces, i.e., the
“net effect.”
Net Force & the 2nd Law
For a while, we’ll only deal with forces that
are horizontal or vertical.
When forces act in the same line, we can just
add or subtract their magnitudes to find the
net force.
32 N
15 N
2 kg
10 N
Fnet = 27 N to the right
a = 13.5 m/s2
Units
Fnet = m a
1N
= 1 kg
2
m/s
The SI unit of force is the Newton.
A Newton is about a quarter pound.
1 lb = 4.45 N
Graph of F vs. a
In the lab various known forces are applied—
one at a time, to the same mass—and the
corresponding accelerations are measured.
The data are plotted. Since F and a are
directly proportional, the relationship is linear.
F
a
Slope
Since slope = rise / run = F / a, the slope is
equal to the mass. Or, think of y = mx + b,
like in algebra class. y corresponds to force,
m to mass, x to acceleration, and b (the
y-intercept) is zero.
F
F
a
a
W = mg
• Weight = mass  acceleration due to gravity.
• This follows directly from F = m a.
• Weight is the force of gravity on a body.
• Near the surface of the Earth,
g = 9.8 m/s2.
Two Kinds of Mass
• Inertial mass: the net force on an object
divided by its acceleration. m = Fnet / a
• Gravitational mass: Compare the
gravitational attraction of an unknown
mass to that of a known mass, usually
with a balance. If it balances, the
masses are equal.
?
m
Balance
Einstein asserted that
these two kinds of masses
are equivalent.
Action - Reaction
“For every action there’s an
equal but opposite
reaction.”
• If you hit a tennis ball with a racquet,
the force on the ball due to the racquet
is the same as the force on the racquet
due to the ball, except in the opposite
direction.
• If you drop an apple, the Earth pulls on
the apple just as hard as the apple pulls
on the Earth.
• If you fire a rifle, the bullet pushes the
rifle backwards just as hard as the rifle
pushes the bullet forwards.
Earth / Apple
How could the forces on the tennis ball, apple, and
bullet, be the same as on the racquet, Earth, and rifle?
The 3rd Law says they must be, the effects are different
because of the 2nd Law!
apple
0.40 kg
3.92 N
Earth
3.92 N
5.98  1024 kg
A 0.40 kg apple weighs 3.92 N
(W = mg). The apple’s weight
is Earth’s force on it. The
apple pulls back just as hard.
So, the same force acts on
both bodies. Since their
masses are different, so are
their accelerations (2nd Law).
The Earth’s mass is so big,
it’s acceleration is negligible.
Earth / Apple
(cont.)
The products are the same, since the forces are the same.
m
Apple’s
little mass
a
=
Apple’s big
acceleration
m
Earth’s
big mass
a
Earth’s little
acceleration
Lost in Space
Suppose an International Space Station
astronaut is on a spacewalk when her tether
snaps. Drifting away from the safety of the
station, what might she do to make it back?
Swimming
Due to the 3rd Law, when you swim you push the water
(blue), and it pushes you back just as hard (red) in the
forward direction. The water around your body also
produces a drag force (green) on you, pushing you in the
backward direction. If the green and red cancel out, you
don’t accelerate (2nd Law) and maintain a constant velocity.
Note: The blue vector is a force on the water, not the on
swimmer! Only the green and red vectors act on the swimmer.
Demolition Derby
When two cars of
different size collide,
the forces on each are
the SAME (but in
opposite directions).
However, the same
force on a smaller car
means a bigger
acceleration!
Free fall
• An object is in free fall if the only force
acting on it is gravity.
• It doesn’t matter which way it’s moving.
• A shell in a cannon is not in freefall until
it leaves the barrel of the cannon. (There
are other forces acting on it while inside
the barrel.)
• For an object in free fall, a = -g, if:
– we ignore air resistance.
– don’t stray too far from Earth.
Freefall
(cont.)
• Any launched object is in freefall the
entire time it’s in the air, if:
– we ignore air resistance.
– it has no propulsion system.
• With the previous condition met,
a = -g = -9.8 m/s2 everywhere:
– on the way up
– at its peak
– on the way down
Hippo & Ping Pong Ball
In a vacuum, all bodies fall at the same rate.
If a hippo and a
ping pong ball
were dropped
from a helicopter
in a vacuum
(assuming the
copter could fly
without air),
they’d land at the
same time.
When there’s no air resistance, size and shape don’t
matter!
Misconceptions
• If an object is moving, there must be
some force making it move. Wrong! It
could be moving without accelerating.
• If v = 0, then
a and Fnet must be zero.
Wrong! Think of a projectile shot straight up at
its peak.
• An object must move in the direction of
the net force. Wrong! It must accelerate that
way but not necessarily move that way.
Misconceptions
(cont.)
• Heavy objects must fall faster than light
ones. Wrong! The rate is the same in a vacuum.
• When a big object collides with a little one,
the big one hits the little one harder than
the little one hits the big one. Wrong! The 3rd
Law says they hit it each other with the same force.
• If an object accelerates, its speed must
change. Wrong! It could be turning at constant
speed.
Projectile confusion
a  0 at the vertex (peak) of a projectile’s
trajectory. Velocity can be zero there, but not
acceleration!
If a were zero at the vertex, Fnet would have
to be zero as well (by the 2nd law), which
means gravity would have to be turned off!
a = -g throughout the whole trip, including
the high point !
Forces & Kinematics
To solve motion problems involving forces:
1. Find net force (by combining vectors).
2. Calculate acceleration (using 2nd law).
3. Use kinematics equations:
vf = v0 + a t
1
x = v0 t + 2 a t2
vf2 – v02 = 2 a x
Sample Problem 1
Goblin
400 N
Ogre 1200 N
Treasure 300 kg
Troll 850 N
A troll and a goblin are fighting with a big, mean ogre
over a treasure chest, initially at rest. Find:
1. Fnet = 50 N left
2. a = 0.167 m/s2 left
3. v after 5 s = 0.835 m/s left
4. x after 5 s = 2.08 m left
A 3 kg watermelon is launched straight up by applying a
70 N force over 2 m. Find its max height. Hints:
Phase I: the launch
1. Draw pic and find net force.
2. Calculate a during launch.
40.6 N up
+13.5333 m/s2
3. Calculate vf at the end of the launch (after 2 m).
+7.3575 m/s
Phase II: freefall
4. Draw pic and think about what a is now. -9.8 m/s2
5. vf from phase I is v0 for phase II.
6. What is vf for phase II?
-9.8 m/s2
zero
7. Calculate max height & add 2 m.
4.76 m
Normal force
• When an object lies on a table or on the
ground, the table or ground must exert
an upward force on it, otherwise gravity
would accelerate it down.
• This force is called the normal force.
N
In this particular case,
N = mg.
m
mg
So, Fnet = 0; hence a = 0.
Normal forces aren’t always up
“Normal” means perpendicular. A normal
force is always perpendicular to the contact
surface.
N
For example, if a
mg
flower pot is
setting on an
incline, N is not
vertical; it’s at a
right angle to the
incline. Also, in
this case, mg > N.
Normal force directions
• Up
– You’re standing on level ground.
– You’re at the bottom of a circle while flying a loopthe-loop in a plane.
• Sideways
– A ladder leans up against a wall.
– You’re against the wall on the “Round Up” ride
when the floor drops out.
• At an angle
– A race car takes a turn on a banked track.
• Down
– You’re in a roller coaster at the top of a loop.
Cases in which N  mg
1. Mass on incline
2. Applied force acting on the mass
3. Nonzero acceleration, as in an elevator or
launching space shuttle
FA
N
N
a
N
mg
mg
mg
When does N = mg ?
If the following conditions are satisfied,
then N = mg:
• The object is on a level surface.
• There’s nothing pushing it down or
pulling it up.
• The object is not accelerating vertically.
N and mg are NOT an Action-Reaction Pair!
N
“Switch the nouns to find the reaction partner.”
The dot represents the man.
m
mg, his weight, is the force on
the man due to the Earth.
mg
Fg
FE
Earth
FE is the force on the
Earth due to the man.
N, the normal force, is the force
on the man due to the ground.
Fg is the force on the
ground due to the man.
The red vectors are an action-reaction pair. So are the blue
vectors. Action-reaction pairs always act on two different bodies!
Box / Tension Problem
38 N
8 kg
T1
5 kg
T2
6 kg
frictionless floor
A force is applied to a box that is connected
to other boxes by ropes. The whole system
is accelerating to the left.
 The problem is to find the tensions in the
ropes.
 We can apply the 2nd Law to each box
individually as well as to the whole system.

Box / Tension Analysis
38 N
8 kg
T1
5 kg
T2
6 kg
frictionless floor
T1 pulls on the 8-kg box to the right just as
hard as it pulls on the middle box to the left.
 T1 must be < 38 N, or the 8-kg box couldn’t
accelerate.
 T2 pulls on the middle box to the right just as
hard as it pulls on the 6-kg box to the left.
 T1 must be > T2 or the middle box couldn’t
accelerate.

Free Body Diagram – system
N
For convenience, we’ll choose
left to be the positive direction.
The total mass of all three
boxes is 19 kg.
38 N
19 kg
N and mg cancel out.
Fnet = m a implies
a = 2.0 m/s2
mg
Since the ropes don’t
stretch, a will be 2.0 m/s2
for all three boxes.
Free Body Diagram – right box
N and mg cancel out.
For this particular box,
Fnet = ma implies:
N
T2
6 kg
T2 = 6a = 6(2) = 12 N.
(Remember, a = 2 m/s2 for all
three boxes.)
T1
38 N
8 kg
mg
5 kg
frictionless floor
T2
6 kg
Free Body Diagram – middle box
N and mg cancel
out again.
Fnet = m a implies:
N
T1
T1 – T2 = 5a. So,
T1 – 12 = 5(2), and
T1 = 22 N
38 N
8 kg
T1
T2 = 12 N
5 kg
mg
5 kg
frictionless floor
T2
6 kg
Free Body Diagram – left box
Let’s check our work
using the left box.
N
38 N
8 kg
T1 = 22 N
N and mg cancel out
here too.
Fnet = ma implies:
mg
38 - 22 = ma = 8(2).
16 = 16.
38 N
8 kg
T1
5 kg
T2
6 kg
Atwood Device
Assume m1 < m2 and that
the clockwise direction is +.
T
T
m1
m1g
m2
m2g
If the rope & pulley have
negligible mass, and if the
pulley is frictionless, then T is
the same throughout the
rope.
If the rope doesn’t stretch, a
is the same for both masses.
Atwood Analysis
Remember, clockwise has been defined as +.
2nd Law on m1: T - m1g = m1a
2nd Law on m2: m2g - T = m2 a
T
Add equations:
T
m1
m1g
m2
m2g
m2g – m1g = m1a + m2 a
(The T’s cancel out.)
Solve for a:
m2 – m1
a=m +m g
1
2
Atwood as a system
Treated as a system (rope & both
masses), tension is internal and
the T’s cancel out (one clockwise, one counterclockwise).
T
T
m1
m1g
m2
m2g
Fnet = (total mass)  a implies
(force in + direction) (force in - direction)
= m2g - m1g = (m1 + m2) a.
Solving for a gives the same
result. Then, knowing a, T can
be found by substitution.
Atwood: Unit Check
m2 – m1
a=m +m g
1
2
units:
kg - kg
kg + kg
m
m
= 2
2
s
s
Whenever you derive a formula you should check
to see if it gives the appropriate units. If not, you
screwed up. If so, it doesn’t prove you’re right,
but it’s a good way to check for errors.
Remember, you can multiply or divide scalar
quantities with different units, but you can only add
or subtract quantities with the same units!
Atwood: Checking Extremes
Besides units, you should
m2 – m1
also check a formula to see
a=m +m g
if what happens in extreme
1
2
& special cases makes sense.
m2 >> m1 : In this case, m1 is negligible compared
to m2. If we let m1 = 0 in the formula, we get
m1
a = (m2 / m2 )g = g, which makes sense, since with
only one mass, we have freefall.
m2 << m1 : This time m2 is negligible compared to
m2
m1, and if we let m2 = 0 in the formula, we get
a = (-m1 / m1 )g = -g, which is freefall in the negative
(counterclockwise) direction.
m2 = m1 : In this case we find a = 0 / (2m1)g = 0, which is
what we would expect considering the device is balanced.
Note: The masses in the last case can still move but only
with constant velocity!
Force Vectors
• Force is a vector.
• A block sliding on an inclined plane has
forces acting on it.
• We know there is a force of gravity and
normal force.
q
Force Diagram
• Draw the forces acting on
the block.
FN
q
• The force of gravity points
down with magnitude Fg=mg.
Fg  mg
• The normal force points
away from the surface of the
inclined plane.
Force Diagram
iˆ
FN
m gsin q
q
m g cosq
Fg  mg
• Draw components of the
forces so the vectors can be
added.
• The coordinates should point
along the surface.
– The normal force is
unknown
– The motion will be along the
surface
• The components of Fg are
compared to the surface.
Net Force
• The block stays along the
plane.
FN
m gsin q
q
m g cosq
Fg  mg
– No motion or acceleration
away from the surface
– No force in that component
• Solve for the normal force:

FN  mgcosqˆj
• Net force:
 
Fg  FN  mgsin qiˆ
Motion from Force
• The net force causes the block to
accelerate.
• The amount of acceleration is force
divided by mass.
• This is a constant acceleration moving
F
a   g sin q
the block.
m
q
Inclined Plane
• Pushing a block up an
incline is easier than
lifting.
Fapp  mgsinq
• To get constant velocity
a force equal to the
downward acceleration
must be applied.
FN
m gsin q
q
m g cosq
Fg  mg
Mechanical Advantage
• The inclined plane has been
used as a machine for
centuries.
• Pushing a block up an
incline is easier than lifting.
mg
m gsin q
q
• The ratio of the force to lift
directly compared to the
force using the incline is the
mechanical advantage.
mg
1

m gsin q sin q
Friction
Friction is the force bodies can impart on each
other when they’re in contact.
The friction forces are parallel to the contact
surface and occur when…
• One body slides over the other, or…
• They cling together despite and external force.
The forces shown are an action-reaction pair.
(force on box
due to table)
f
Acme Hand
Grenades
f
v
(force on table due to box)
Friction Facts
• Friction is due to electrostatic attraction between
the atoms of the objects in contact.
• It can speed you up, slow you down, or make you
turn.
• It allows you to walk, turn a corner on your bike,
warm your hands in the winter, and see a meteor
shower.
• Friction often creates waste heat.
• It makes you push harder / longer to attain a
given acceleration.
• Like any force, it always has an action-reaction
pair.
Two Kinds of Friction
• Static friction
fs
– Must be overcome in order
to budge an object
– Present only when there is
no relative motion between
the bodies, e.g., the box &
table top
• Kinetic friction
– Weaker than static friction
– Present only when objects
are moving with respect to
each other (skidding)
FA
Objects are still or
moving together.
Fnet = 0.
fk
FA
Fnet is to the right.
a is to the right.
v is left or right.
Friction Strength
The magnitude of the friction force is
proportional to:
• how hard the two bodies are pressed
together (the normal force, N ).
• the materials from which the bodies are
made (the coefficient of friction,  ).
Attributes that have little or no effect:
• sliding speed
• contact area
Coefficients of Friction
• Static coefficient … s.
• Kinetic coefficient … k.
• Both depend on the materials in
contact.
– Small for steel on ice or scrambled egg on
Teflon frying pan
– Large for rubber on concrete or cardboard
box on carpeting
• The bigger the coefficient of friction, the
bigger the frictional force.
Static Friction Force
fs  s N
static
frictional
force
coefficient of
static friction
normal
force
fs, max = s N
maximum
force of static
friction
fs, max is the force you
must exceed in order to
budge a resting object.
Static friction force varies
• fs, max is a constant in a given problem, but fs
varies.
• fs matches FA until FA exceeds fs, max.
• Example: In the picture below, if s for a
wooden crate on a tile floor is 0.6,
fs, max = 0.6 (10 ) (9.8) = 58.8 N.
fs = 27 N
FA = 27 N
10 kg
fs = 43 N
FA = 43 N
10 kg
fk
FA = 66 N
10 kg
The box finally budges when FA
surpasses fs, max. Then kinetic
acts on the box.
Kinetic Friction
fk = k N
kinetic
frictional
force
coefficient of
kinetic friction
normal
force
• Once object budges, forget about s.
• Use k instead.
• fk is a constant so long as the materials
involved don’t change.
• There is no “maximum fk.”
 values
• Typically, 0 < k < s < 1.
• This is why it’s harder to budge an object
than to keep it moving.
• If k > 1, it would be easier to lift an object
and carry it than to slide across the floor.
• Dimensionless (’s have no units, as is
apparent from f =  N).
Friction Example 1
You push a giant barrel o’ monkeys setting
on a table with a constant force of 63 N. If
k = 0.35 and s =0.58, when will the barrel
have moved 15 m?
answer:
Never, since this force won’t even budge it!
Barrel o’
Monkeys
63 < 0.58 (14.7) (9.8)  83.6 N
14.7 kg
Friction Example 2
Same as the last problem except with a bigger FA: You push
the barrel o’ monkeys with a constant force of 281 N.
k = 0.35 and s =0.58, same as before. When will the barrel
have moved 15 m?
step 1: fs, max = 0.58 (14.7) (9.8)  83.6 N
step 2: FA= 281N > fs max. Thus, it budges this time.
,
step 3: Forget fs and calculate fk:
fk = 0.35 (14.7) (9.8) = 50.421 N
Barrel o’
Monkeys
14.7 kg
(continued on next slide)
step 4: Free body diagram while sliding:
Friction Example 2 (continued)
N
FA
fk
mg
step 5: Fnet = FA – fk = 281 - 50.421 = 230.579 N
Note: To avoid compounding of error, do not round until the
end of the problem.
step 6: a = Fnet / m = 230.579 / 14.7 = 15.68564 m/s2
step 7: Kinematics: x = +15 m, v0 = 0,
a = +15.68564 m/s2, t = ?
x = v0 t + ½ a t 2  t = 2 x / a  1.38 s
Friction as the net force
A runner is trying to steal second base. He’s
running at a speed v; his mass is m. The
coefficient of kinetic friction between his uniform
and the base pass is . How far from second base
should he begin his slide in order to come to a stop
right at the base?
Note: In problems like these where no numbers are
given, you are expected to answer the questions in
terms of the given parameters and any constants.
Here, the given parameters are m, , and v.
Constants may include g, , and “regular” numbers
like 27 and –1.86.
(continued on next slide)
N
fk
mg
Once the slide begins, there is no
applied force. Since N and mg cancel
out, fk is the net force. So Newton’s
2nd Law tells us:
Friction as the net force (cont.)
fk = ma. But the friction force is also
given by fk =  N =  mg.
Therefore,  mg = m a. Mass cancels out, meaning
the distance of his slide is completely independent of
how big he is, and we have a =  g. (Note that the
units work out since  is dimensionless.) This is just
the magnitude of a. If the forward direction is
positive, his acceleration (which is always in the
direction of the net force) must be negative.
(continued on next slide)
So, a = - g.
Since he comes to rest at 2nd base, vf = 0.
Friction as the net force (last)
vf 2 - v02 = 2 a x
 0 - v 2 = -2  g x
 x = v 2 / (2  g)
Unit check: (m/s)2 / (m/s2) = m2 / m = m
Note the slide distance is inversely proportional
to the coefficient of friction, which makes sense,
since the bigger  is, the bigger f is.
Note also that here v and Fnet are in opposite
directions, which is perfectly fine.
Scales
• A scale is NOT necessarily a weight meter.
• A scale is a normal force meter.
• A scale might lie about your weight if
– you’re on an incline.
– someone pushes down or pulls up on you.
– you’re in an elevator.
• You’re actual weight doesn’t change in the
above cases.
Weight in a Rocket
U
S
A
You’re on a rocket excursion
standing on a purple
bathroom scale. You’re still
near enough to the Earth so
that your actual weight is
unchanged.
The scale, recall, measures
normal force, not weight.
Your apparent weight
depends on the acceleration
of the rocket.
Rocket:
At rest on the launch pad
U
S
A
a=0
v=0
N
m
mg
During the countdown
to blast off, you’re not
accelerating. The scale
pushes up on you just
as hard as the Earth
pulls down on you. So,
the scale reads your
actual weight.
Rocket:
Blasting Off
a 
U
S
A
v 
N
During blast off your
acceleration is up, so the
net force must be up (no
matter which way v is).
Fnet = m a
 N - mg = m a
 N = m (a + g) > mg
 Apparent weight > Actual weight
mg
Rocket:
Conversion trick
Here’s a useful trick to avoid having to convert
between pounds, newtons, and kg. Suppose you
weigh 150 lb and you’re accelerating up at 8 m/s2.
N - mg = m a 
N
N = m a + mg = m a + 150 lb
But to find m, we’d have to convert your weight to
newtons and  by 9.8 m/s2 (a pain in the butt).
The trick is to multiply and divide ma by g and
replace mg with 150 lb again:
Apparent weight = N = mga / g + mg
= (150 lb) (8 m/s2) / 9.8 m/s2 + 150 lb = 272.44 lb
Note that all units cancel out except for pounds,
and no conversions are required.
mg
Rocket:
Cruising with constant velocity
U
S
A
a=0
v
N
m
mg
If v = constant, then a = 0.
If a = 0, then Fnet = 0 too.
If Fnet = 0, then N must be
equal in magnitude to mg.
This means that the scale
reads your normal weight
(same as if you were at rest)
regardless of how fast you’re
going, so long as you’re not
accelerating.
Rocket:
Engines on low
As soon as you cut way back on the
engines, the Earth pulls harder on
you than the scale pushes up. So
you’re acceleration is down, but
you’ll still head upward for a while.
Choosing down as the positive
direction,
Fnet = m a
 mg - N = m a
 N = m (g - a) < mg
 Apparent weight < Actual weight
U
S
A
a
v
N
m
mg
Air Resistance
• Although we often ignore it, air
resistance, R, is usually
significant in real life.
• R depends on:
R
m
mg
– speed (approximately
proportional to v 2 )
– cross-sectional area
– air density
– other factors like shape
• R is not a constant; it changes
as the speed changes
Volume & Cross-sectional Area
2z
z
Area
y
x
Volume = xyz
Area = xy
Area
2x
2y
Volume = 8 xyz
Area = 4 xy
If all dimensions of an object are doubled the
cross-sectional area gets 4 times bigger, but
the volume goes up by a factor of 8.
R
Falling in Air
m
A
mg
With all sides doubled, the area exposed
to air is quadrupled, so the resistance
force is 4 times greater. However, since
the volume goes up by a factor of 8, the
weight is 8 times greater (as long as
we’re dealing with the same materials).
Conclusion: when the only difference is
size, bigger objects fall faster in air.
4R
8m
4A
8 mg
Terminal Velocity
Suppose a daredevil frog jumps out of a
skyscraper window. At first v = 0, so R = 0 too,
and a = -g. As the frog speeds up, R increases,
and his acceleration diminishes. If he falls long
enough his speed will be big enough to make R
as big as mg. When this happens the net force
is zero, so the acceleration must be zero too.
R
This means this frog’s velocity
can’t change any more. He has
reached his terminal velocity.
Small objects, like raindrops and
insects, reach terminal velocity
mg
more quickly than large objects.
Biophysics
F
e
m
u
r
The strength of a bone, like a femur, is
proportional to its cross-sectional area, A. But
the animal’s weight is proportional to its volume.
Giant ants and rats from sci-fi movies couldn’t
exist because they’d crush themselves!
Here’s why: Suppose all dimensions are
increased by a factor of 10. Then the volume
(and hence the weight) becomes 1000 times
bigger, but the area (and hence the strength)
only becomes 100 times bigger.
Consequences: Basketball players, because
of their height, tend to suffer lots of stress
fractures; and elephants have evolved
proportionally bigger femurs than deer.
Early Astronomy
• As far as we know, humans have always
been interested in the motions of objects
in the sky.
• Not only did early humans navigate by
means of the sky, but the motions of
objects in the sky predicted the changing
of the seasons, etc.
Early Astronomy
• There were many early attempts both to
describe and explain the motions of stars
and planets in the sky.
• All were unsatisfactory, for one reason or
another.
The Earth-Centered Universe
• A geocentric (Earth-centered) solar
system is often credited to Ptolemy, an
Alexandrian Greek, although the idea is
very old.
Image from:
http://abyss.uoregon.edu/~js/ast123/lectures/lec02.html
Ptolemy’s Solar System
• Ptolemy’s solar system could be made to
fit the observational data pretty well, but
only by becoming very complicated.
Image from:
http://abyss.uoregon.edu/~js/ast123/lectures/lec02.html
Copernicus’ Solar System
• The Polish cleric Copernicus proposed a
heliocentric (Sun centered) solar system in
the 1500’s.
Image from:
Objections to Copernicus
• How could Earth be moving at enormous
speeds when we don’t feel it?
– (Copernicus didn’t know about inertia.)
• Why can’t we detect Earth’s motion
against the background stars (stellar
parallax)?
not
• Copernicus’ model did
fit the
observational data very well.
Galileo & Copernicus
• Galileo became convinced that Copernicus was
correct by observations of the Sun, Venus, and
the moons of Jupiter using the newly-invented
telescope.
• Perhaps Galileo was motivated to understand
inertia by his desire to understand and defend
Copernicus’ ideas.
Tycho and Kepler
• In the late 1500’s, a Danish nobleman
named Tycho Brahe set out to make the
most accurate measurements of
planetary motions to date, in order to
validate his own ideas of planetary motion.
Tycho and Kepler
• Tycho’s data was successfully interpreted
by the German mathematician and
scientist Johannes Kepler in the early
1600’s.
Kepler’s Laws
• Kepler determined that the orbits of the
planets were not perfect circles, but
ellipses, with the Sun at one focus.
Planet
Sun
Kepler’s Second Law
• Kepler determined that a planet moves
faster when near the Sun, and slower
when far from the Sun.
Planet
Faster
Sun
Slower
Why?
• Kepler’s Laws provided a complete
kinematical description of planetary
motion (including the motion of planetary
satellites, like the Moon) - but why did
the planets move like that?
The Apple & the Moon
• Isaac Newton realized that the motion of a
falling apple and the motion of the Moon
were both actually the same motion,
caused by the same force - the
gravitational force.
Universal Gravitation
• Newton’s idea was that gravity was a
universal force acting between any two
objects.
At the Earth’s Surface
• Newton knew that the gravitational force
on the apple equals the apple’s weight,
mg, where g = 9.8 m/s2.
W = mg
Weight of the Moon
• Newton reasoned that the centripetal force
on the moon was also supplied by the
Earth’s gravitational force.
?
Fc = mg
Weight of the Moon
• Newton’s calculations showed that the
centripetal force needed for the Moon’s
motion was about 1/3600th of Mg, however,
where M is the mass of the Moon.
Weight of the Moon
• Newton knew, though, that the Moon was
about 60 times farther from the center of
the Earth than the apple.
• And 602 = 3600
Universal Gravitation
• From this, Newton reasoned that the
strength of the gravitational force is not
constant, in fact, the magnitude of the
force is inversely proportional to the
square of the distance between the
objects.
Universal Gravitation
• Newton concluded that the gravitational
force is:
– Directly proportional to the masses of both
objects.
– Inversely proportional to the distance
between the objects.
Law of Universal Gravitation
• In symbols, Newton’s Law of Universal
Gravitation is:
– Fgrav = G
Mm
r2
– Where G is a constant of proportionality.
– G = 6.67 x 10-11 N m2/kg2
Inverse Square Law
• Newton’s Law of Universal Gravitation is
often called an inverse square law, since
the force is inversely proportional to the
square of the distance.
An Inverse-Square Force
Experimental Evidence
• The Law of Universal Gravitation allowed
extremely accurate predictions of
planetary orbits.
• Cavendish measured gravitational forces
between human-scale objects before
1800. His experiments were later
simplified and improved by von Jolly.
Action at a Distance
• In Newton’s time, there was much
discussion about HOW gravity worked how does the Sun, for instance, reach
across empty space, with no actual
contact at all, to exert a force on the
Earth?
• This spooky notion was called “action at a
distance.”
The Gravitational Field
• During the 19th century, the notion of the
“field” entered physics (via Michael
Faraday).
• Objects with mass create an invisible
disturbance in the space around them
that is felt by other massive objects - this
is a gravitational field.
The Gravitational Field
• So, since the Sun is very massive, it
creates an intense gravitational field
around it, and the Earth responds to the
field. No more “action at a distance.”
Gravitational Field Strength
• To measure the strength of the
gravitational field at any point, measure
the gravitational force, F, exerted on any
“test mass”, m.
• Gravitational Field Strength, g = F/m
Gravitational Field Strength
• Near the surface of the Earth, g = F/m =
9.8 N/kg = 9.8 m/s2.
• In general, g = GM/r2, where M is the
mass of the object creating the field, r is
the distance from the object’s center, and
G = 6.67 x10-11 Nm2/kg2.
Gravitational Force
• If g is the strength of the gravitational field
at some point, then the gravitational force
on an object of mass m at that point is
Fgrav = mg.
• If g is the gravitational field strength at
some point (in N/kg), then the free fall
acceleration at that point is also g (in
m/s2).
Gravitational Field Inside a
Planet
• If you are located a distance r from the
center of a planet:
– all of the planet’s mass inside a sphere of
radius r pulls you toward the center of the
planet.
– All of the planet’s mass outside a sphere of
radius r exerts no net gravitational force on
you.
Gravitational Field Inside a
Planet
• The blue-shaded part
of the planet pulls you
toward point C.
• The grey-shaded part
of the planet does
not pull you at all.
Gravitational Field Inside a
Planet
• Half way to the center of the planet, g has
one-half of its surface value.
• At the center of the planet, g = 0 N/kg.
Black Holes
• When a very massive star gets old and
runs out of fusionable material,
gravitational forces may cause it to
collapse to a mathematical point - a
singularity. All normal matter is crushed
out of existence. This is a black hole.
Black Hole Gravitational Force
Black Hole Gravitational Force
• The black hole’s gravity is the same as
the original star’s at distances greater
than the star’s original radius.
• Black hole’s don’t magically “suck
things in.”
• The black hole’s gravity is intense
because you can get really, really close to
it!
Earth’s Tides
• There are 2 high tides and 2 low tides per
day.
• The tides follow the Moon.
Why Two Tides?
• Tides are caused by the stretching of a planet.
• Stretching is caused by a difference in forces on
the two sides of an object.
• Since gravitational force depends on distance,
there is more gravitational force on the side of
Earth closest to the Moon and less gravitational
force on the side of Earth farther from the Moon.
Why Two Tides?
• Remember that
Why the Moon?
• The Sun’s gravitational pull on Earth is
much larger than the Moon’s gravitational
pull on Earth. So why do the tides follow
the Moon and not the Sun?
Why the Moon?
• Since the Sun is much farther from Earth
than the Moon, the difference in distance
across Earth is much less significant for
the Sun than the Moon, therefore the
difference in gravitational force on the two
sides of Earth is less for the Sun than for
the Moon (even though the Sun’s force on
Earth is more).
Why the Moon?
• The Sun does have a small effect on
Earth’s tides, but the major effect is due to
the Moon.