Transcript PHY_Forces Newton`s Laws

Forces
Force-
A push or a pull
 Long Range forces (four fundamental forces) “Jedi – Forces”
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Gravity
Magnetism
Weak force
Strong force
 Contact forces
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Forces that result from direct contact between objects
Friction
Normal force
Any force that’s not a Long-Range force
Past notions on forces
 Objects needed a force in order to move
 And object could not possibly move without a force pushing
it
 Aristotle (~300 BC) first formulated these laws and were
not proven wrong until much later by Galileo
Galileo
 force is not necessary to keep objects in motion
 Galileo argued, that only when friction is present (which it
almost always is) is a force necessary to keep an object in motion
 Force
- A push or a pull
 Friction
- a force that acts between materials that touch as they move
past each other
o Acts opposite the direction of motion
 He tested this with balls rolling up and own inclines
 hypothesized ….if no friction, a ball rolling on a level surface
would stay in motion forever w/ no force pushing it…. It just had a
natural tendency to stay in motion
 This natural tendency of objects to stay in motion and resist changes
in its motion is called INERTIA
Newton’s 1st Law - The Law of Inertia
 Isaac Newton … was Galileo’s successor in the world of Physics.
 formulated Three Laws of Motion
 replaced 2000 year old Aristotelian ideas
 NEWTON’s 1st Law - The Law of Inertia
 “An object in motion will stay at a constant
speed in a straight line, an object at rest will
stay at rest unless acted upon by outside forces”
Inertia -- property of matter that resists changes in its motion.
Examples of inertia
 Why seatbelts are necessary
 Astronauts in space… get back to spaceship?
 The more mass something has the more inertia it has
 Large boats not being able to stop their motion
 A force is the only thing that can put an object in motion OR stop a
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object from moving. Objects cannot change their own motion
w/out an outside force
Nickel on a piece of paper
Rollercoaster
1994 Nissan Maxima
Car with cinder blocks
Car going around a circle (stuff on dashboard?)
Satellite
Example--Mr. Schober goes to the shooting range …
Which gun to choose??
Heavy gun, w/ small caliber bullets=
means small kick back because gun has
more inertia– Equals easier for smaller
person to shoot Video
The moving Earth
 Throw a coin upward in a moving car
 Bird catching a worm
 Helicopter hovering above Earth…. Does the Earth move
under it??
 No, helicopters inertia keeps it moving with the Earth
Net Force (∑F) (Fnet)
 Net Force  Total force acting on an object
 ∑F = F1 + F2 + F3…
 force is a vector quantity
 Sign indicates direction
 Forces in opposite directions will cancel out
Free Body
Diagrams.
Simple
drawings
showing all
force vectors
acting on an
object
 A 5 N force to the right would cancel out a 5 N force to the left
-7 N
 ∑F =
13 N
6N
 Only the NET force acting on an object affects its motion
Skydiver (m=75 kg) falling towards Earth, if
the force of air resistance is 100 N what is
the Net Force on the skydiver?
100 N
75 kg x -9.8 m/s2=
-735 N
Force from Air
Resistance
directed upwards
Force of Gravity (aka
Weight) pulling skydiver
towards Earth.
Fg=mg
∑F = -735N + 100N = -635 N
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We learned w/ the 1st Law that objects do not
like changes in their motion…
 if a net force is applied to an object the motion of
an object will change…
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‘changing motion’ basically means the same
thing as Acceleration
So then we can say…. ‘force causes
acceleration’
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All forces do not accelerate all objects equally
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Objects with more mass are more difficult to
accelerate, and thus require more force
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‘The acceleration of an object is directly proportional
to the net force on it, and inversely proportional to
its mass’
This means…. The larger the force, the larger the
acceleration
 AND …. The larger the mass, the smaller the
acceleration
 This can be summed up w/ the equation
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 ∑F=ma
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a acceleration (m/s2); ∑FNet Force (N); mmass (kg)
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∑F=F1 + F2 + F3..
∑F=ma
Soo we can say
AND
 ∑F = ma = F1 + F2 + F3..
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This also works in 2-D
 ∑Fx = max =F1x + F2x + F3x..
 ∑Fy = may = F1y + F2y + F3y..
Mass & Weight
 Mass (m)
 How much matter is in an object
 A measure of an object’s inertia
 Mass never changes depending on location… you have the
same mass in space as you do here on Earth
 Weight (W)
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How much gravity is pulling on an object
Aka… The FORCE of gravity acting on an object
Is a function of how strong gravity is
can change from place to place…. You have a much different
weight in space than you do here on Earth
 B/c mass is a measure of inertia it is just as hard to move a
large boulder in space as it is here on Earth
How to determine an object’s weight?
 Mass and weight are always directly proportional
 We find weight by using the equation
 Weight = mass x acc. of gravity
 Or..
W = mg
 g = -9.8 m/s2
 So if an object has a mass of 1 kg, then its weight will be
9.8 N.
 The “Newton” is the standard unit for force, and since
Weight is a force, the Newton is used here.
 1 N = 1 kg*m/s2
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Newton’s 2nd says that the more force
an object has on it the faster it should
accelerate….. So then if heavier
objects have a larger weight (force of
gravity), why do they not accelerate
faster?
 Ans. True, heavier objects have
more force, but they also have more
mass, which means that it requires
more force to accelerate as fast as
lighter objects….
 Essentially…. The larger force and
larger mass cancel each other out
 All falling objects accelerate
downward at 9.8 m/s2
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You push a 10 kg cart with a force of 8 N.
There is a frictional force of 3 N that acts
between the cart and ground. What is the
acceleration of the cart?
∑F = ma = FA + FF
10kg (a) = 8 N + -3 N
10kg (a) = 5 N
a=.5 m/s2
NEWTON’S THIRD LAW
Equal & Opposite Forces
3rd Law
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Forces are not a thing of themselves, but are part
of a mutual interaction
Forces only exist in pairs
Third Law says….
 “For
every force there is exists another force that is
equal in magnitude but opposite in direction”
 You cannot push on something without it pushing on you
by the same amount in the opposite direction
Action & Reaction
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Action forces produce the interaction, and the
reaction force must be in existence at the same time,
have the same size, and be opposite in direction
You apply a 10 N eastward force to wall, wall
applies a 10 N westward force to you
How do objects move? What pushes
them?
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Car??
Rocket??
Person??
What is the reaction to the Force of
Gravity?
ACTION- Earth’s
Gravity pulls person
down
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Earth’s gravity pulls objects towards it
So if all forces have are part of a pair, what is the
reaction force to this??
Object’s gravity pulling Earth up!
REACTIONPerson’s gravity
pulls Earth up
An object pulls upward on the Earth as much as the
Earth pulls it downward
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Object’s mass is much less than Earths, so object falls down
rather than Earth rising up to object
Examples
Draw all forces on the car/Earth system
as it is driving down the road
The road
pushing the
car forward
The car’s
tire
pushing
on the
road
Normal
force of
the Earth
pushing
up on the
car
Contact force
of the car
pushing down
on the Earth
The road’s
friction on
the car’s
tires
**Action/Reaction pairs
have matching colors in
drawing
Earth’s
Gravity
pulling the
car down
The cars
friction on
the road
The car’s
gravity
pulling the
Earth up
Astronauts & Reaction Forces
Stuck in Space??
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An astronaut in space…not moving with just a spacesuit on
and he’s not tethered to the ship. If he is only a few
meters away from his ship and there are no other people
to help him how could you get back to his spaceship?
Oh No!!
Forces on different masses
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Cannon and cannonball
Do both the cannon and the cannonball experience
the same force the cannon is fired?
 Yes,
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the force on each are the Action/Reaction pairs
Does the cannon or the cannon ball have a larger
acceleration?
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ball, b/c it has a small mass. It is easier to accel. a
small mass
 a = ∑F/m
5.2 - Applying Newton’s Laws
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Different types of Forces
 Normal
Force (FN)
 Weight (W)
 Apparent Weight (Wa)
 Force from a Spring
 Tension
 Friction (covered in section 5.3)
Normal Force (FN)
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“Normal” means perpendicular
Force of the Earth (ground) pushing up on an object when they are in
contact with each other
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IF object is….
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1) on a horizontal surface and
2) no other vertical forces are applied and
3) no vertical acceleration.
THEN Normal Force is Equal to the weight (force of gravity) of the object.
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Normal Force is NOT the reaction to the force of gravity acting on an
object
Responsible for how heavy we “feel”
AKA “Apparent Weight” --- Wa
Elevators & Normal Force
Elevator Accelerating Down
Elevator at rest OR
Moving at constant
velocity
FN=890 N
FN=900 N
W=900 N
FN=910 N
W=900 N
FNET=-10 N
Elevator accelerating Up
W=900 N
FNET=0 N
FNET=+10 N
Equilibrium
 Equilibrium occurs when the Net Force acting on an object is
equal to zero (∑F = 0 N)
 If an object is in equilibrium 1 of 2 things must be true
 The object is moving at a constant speed in a straight
line, OR
 The object is at rest
13 N
13 N
 ∑F= 0
Equilibrium in 2-D
 If forces are acting in more than one dimension then the
equilibrium equation holds true separately for the x and the y
 ΣFx= 0 = F1x + F2x …
 ΣFy= 0 = F1y + F2y…
A stretched or compressed spring exerts a force
Hooke’s Law
F = kx
K – spring constant
x- the stretch or
compression distance
5.3 - More about Friction
 Always opposes motion
 If direction of motion is positive, Force of Friction is negative
 Two types of friction
 Force of Static Friction (Fs)
 Friction force that opposes the START of motion
 Equation
Fs,max = μsFN
 If no pushing force, no Fs. To start an object moving the applied force must
be larger than the maximum Fs possible.
 Force of Kinetic Friction (Fk)
 Friction force that opposes the continuation of motion, exists only on objects that are
moving
 Equation
Fk = μkFN
 Fk = force of kinetic friction (N)
FN = Normal Force (N)
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μk= coefficient of kinetic friction (no units)
Static Friction is always larger than kinetic friction
 Harder to start motion than it is to sustain it
More on Friction Equations
 Fk = force of kinetic friction (N)
 μk= coefficient of kinetic friction (no units)
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Constant that depends on the nature of the two surfaces in
contact
No units
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Ratio of Friction force to Normal force
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(Fk/ FN)
 μs- coefficient of static friction is a different value for two
surfaces (will be larger) but same concept
 FN = Normal Force (N)
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Force pushing two surfaces together
“normal” means perpendicular
If object is on horizontal surface AND no other vertical forces are
being applied Normal Force is equal to object’s Weight (mg)
More on Normal Force Later
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A 40 kg cart is
getting pulled by a
140 N Force, if
there exists a
coefficient of
kinetic friction
between the cart
and road of .10
what will be the
acceleration of the
cart?
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