Transcript Forces & Newton`s Laws

Chapter 4- Notes
Newton’s Laws
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So, what is a
“force”,
anyway?
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So, what is a “force”
Defn: a force is a push or a pull that
causes a change in motion, shape, or
direction.
 If
two forces are equal and opposite they
are called balanced forces.
 More than one force can combine to make
a net force (total force).
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Think About It...
Think of all the different
kinds of forces in the
universe... List as many
of them as you can.
How many different
forces are there?
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The Forces of Nature
I. Gravitational:
•attraction bet. masses
•tides, gravity, weight
II. Electromagnetic:
•friction
•adhesion
•electrostatic
•buoyant
•tension
•lift
•drag
•magnetic
II. Weak Nuclear:
•helps to explain
atomic collisions
III. Strong Nuclear:
•binds atomic nuclei
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Representing Forces...
 Forces
are vectors
Forces are drawn as arrows (vectors)
forces add like vectors.
the sum of all the forces is called the net
force.
A
picture of a body with arrows drawn
representing all the forces acting upon it
is called a FREE BODY DIAGRAM.
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Try it...
Draw a picture of your
book sitting on the
desk. Identify all the
forces acting on it.
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Free Body Diagrams...
T (table)
Fn= Normal Force
Book
Fw (weight)
Fg= Force gravity
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Free Body Diagrams...
What forces are acting on a
skier as she races down a hill?
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The Answer...
FN
Ff = friction force
between the ski’s
and the ice
W
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The Answer...
Normal force is
always to surface
Ff
W
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Forces & Newton’s Laws
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Gravitational Forces
Mass and weight
The Law of Inertia
The Law of Acceleration
The Law of Force-Counterforce
Normal force, Tension, and friction
The vector nature of forces
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Gravity and Weight...
• The force pulling on your mass (toward
the ground) is called your weight.
W
=mxg
 What
are the
units of weight
(force)?
1 Newton (N)
2
= 1 kg·m/s
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Conversions and Weights needed
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A mass of one slug will accelerate at one foot per
second squared when pushed by a one pound
force.
1 lb. = 32.17 slugs = 4.48 N
Avg. person = 150 lbs
Avg. Car = 2000 lbs
Bowling ball = 16 lbs
Quarter = 0.031 lbs
Bus = 40000 lbs.
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Some common Masses
you
a car
a bowling ball
a quarter
a bus
Weight
SI (N)
Eng (lb)
667
150
8,896
2,000
71.2
16.0
0.139
0.031
177,920
40,000
Mass
SI (kg) Eng (slug)
68
4.66
907
62.17
7.3
0.50
0.0
0.00
18,141
1,243
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Apparent Weight
•Is your weight always the same on the
Earth?
•How about in an elevator?
•What is this sky diver’s apparent
weight?
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Apparent Weight
•When accelerating up…
WA = mg + ma
Your rate of
acceleration!
•When accelerating down…
WA = mg - ma
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Apparent Weight Example
On the Mantis (at Cedar Point) at the
bottom of the first hill you experience an
acceleration of 27 m/s/s (about 3 g’s). What
is your apparent weight at that point?
 What is your apparent weight at the top of
the 2nd hill when you experience an
acceleration of 2.2 m/s/s?

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Free Fall
•In the absence of air resistance ALL
objects accelerate at 9.81 m/s2!
You remember this? Right???????
•So, d=1/2gt2
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True or False
1. The elephant and the feather each experience the
same force of gravity.
2. The elephant experiences a greater force of
gravity, yet both the elephant and the feather have the
same mass.
3. On earth, all objects (whether an elephant or a
feather) have the same acceleration.
4. The elephant clearly has more mass than the
feather, yet they each weigh the same.
5. The elephant has the greatest acceleration, yet the
amount of gravity is the same for each.
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Think about it...
What does “gravity” depend on?
 If
you’re standing on a scale and the
pressure in the room rises what
happens?
 If the Earth stopped spinning how
would your weight change?
 So, what is gravity caused by?
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Law of Universal Gravitation
• Any two bodies with mass will
attract each other with a
gravitational force!
• More mass ==> more force
• farther away ==> less force
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m1m2
Fg  G 2
d
G  6.67 10
11
Nm
2
kg
2
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Examples
 Find
the force of attraction between
you and your neighbor (estimate
distances and masses).
 Find
the force of gravitational
attraction between the Earth and
you.
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Think about it...
 When
a baseball is hit, what causes it to
slow down? What prevents it from
slowing down?
 When your car stops fast, why do you
lunge forward?
 Under low friction circumstances which
is harder to start moving, a large object or
a small object?
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Inertia...
Def: An object’s tendency to resist a
change in motion is called its inertia.
(Newton’s 1st law)
 inertia
is an intrinsic property that
depends only on mass
–more mass --> more inertia!
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The Law of Inertia...
An object will stay in a state of rest
or straight line motion unless acted
on by an unbalanced force.
 an
inertial reference frame (irf) is a
f.o.r. in which the law of inertia
holds true.
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Think about it...
A little girl sits on a skate
board and her friend
pulls her with a constant
force. Describe the
motion of the girl.
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The Law of Acceleration
When an unbalanced force is applied to an object
it will accelerate in the direction of the net force
with an acceleration proportional to the force
applied and inversely prop. to the mass. (Newton’s
2nd law)
∑F = m x a
 If
there is only one force then the equation
is simply: F=m x a
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Examples
 What
force is needed to decelerate a
2,000 lb car from 35 mph to rest in 5
seconds?
 ~2900N
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Examples
A
jet thruster applies a force of 200,000N
at maximum burn. If the jet has a mass of
15,500kg what is the acceleration of the
jet? How long will it take to increase
speed from 100 m/s to 150 m/s?
 a ~ 13 m/s2 and t ~ 4sec!!
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Think About it...
A
textbook rests on a table. What forces
act on the book? On the table?
 True or False. When you jump the Earth
accelerates.
 An 8 ton bus crashes into a 1200 lb VW
Rabbit. If the bus applies a force of
20,000 lb on the car, what is the force of
the car on the bus?
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The Law of Force-Counterforce
When one body exerts a force on
another body the second one exerts an
equal and opposite force on the first.
(Newton’s 3rd Law)
 unaccompanied
forces do not exist in nature.
 ‘action-reaction’ forces are not the same as
‘balanced’ forces.
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Questions to consider...

Identify the “counterforce”…
– The push of a rifle on your shoulder when you
shoot a gun.
– The hit of the lineman on the QB.
– The backwards force of the tires on the road.
– The gravity of the Earth on the Moon.

Why can you push harder on the pedals when
pulling up on the handle bars?
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Think About It...
A
feather rests on a strong, solid oak
table. Does the table exert any forces
on the feather?
 I lean on a solid cement wall. Does it
exert forces on me?
 What force causes a ball to bounce?
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Normal Forces...
 when
you stand on the ground your weight
pushes down on it. It pushes up on you
with the same force! That’s called a
normal force.
 a normal force is always perpendicular to
the surface in contact.
 Take a minute and identify all the normal
forces in the room.
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Think about it...
 What
is the reading on the scales in
the two diagrams below?
fig. 1
fig. 2
(weights are 200 N each)
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Tension...
Tension is a force that pulls things apart.
 Tension is an electromagnetic force.
 Tension is the force that pulls in BOTH
directions

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Tension…A Deeper Analysis

Mass less ropes and things...
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Think about it...
an ice skater coasts to a stop...
 a marble rolls across the table and comes to
rest…
 A boy in the back of a truck stays still while the
truck accelerates.
 a skydiver reaches terminal velocity...
 What force is responsible for the above?

 If
you said FRICTION, you are correct!
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Brainstorm...
 So
what exactly is friction and what
does it depend on?
 Friction is a resistive force:
– surface locking
– electrostatic attraction
– air drag
– fluid viscosity
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Friction...
 In
mechanics,
f = µ x FN
µ
(Greek “mu”) is a constant which is
different for each situation.
 N is the normal force
– What is “FN” for a book sliding across the
floor?
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Types of friction...
 Sliding
Friction (kinetic)
fk = µk x FN
 Static
Friction (static)
fs = µs x FN

In each case the only difference is the
coefficient of friction, µ.
µk ≤ µs
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Determining µ...
To determine the coefficient of friction
between two surfaces just pull a block at a
constant speed across the table top with a
spring scale. What force is necessary to do
this?
 Draw a Free Body Diagram. How can you
find µ (this is for kinetic friction)?
 How is it possible to find the coefficient of
static friction?

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Examples
What is the force of friction when a
150 lb boy (bike weight included) applies the
brakes w/o skidding while riding down the
street on his bike (µs=0.9)? How would it
change if he locked up his brakes (µk=0.7)?
 How many meters will the boy above take to
come to rest in each situation if his initial
speed is 26 mph?

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Forces are vectors
Forces can be resolved into vertical and
horizontal components.
 Ex. A boy pushes on a lawn mower with a
force of 400 N. If the mower weighs 65 lbs
and the handle makes an angle of 45° with the
ground what is the horizontal acceleration
(ignore friction)? How would the problem
change if you don’t ignore friction?

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Lawnmower Problem
Fx  Fcos(45)
Fx  400cos(45)  283N
Now, W  mg and 65lb  289N
So, 289N  m(9.81)
m  29.5kg
Finally,
Fx
283N
ax 

 9.59 m / s2
m 29.5kg
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Wrap-Up Questions
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What are the four basic forces of the universe? Which
is the strongest?
What is inertia? How does it relate to Newton’s first
Law?
What is equilibrium?
What kind of motion do forces cause?
How does the force of your car’s windshield on a
mosquito compare to the force of the mosquito on the
wind shield?
What is a tension? Normal force? Friction?
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