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Motion & ForcesMotion & Forces
Action and Reaction Newton’s Third Law Momentum Conservation of Momentum
A. NewtonA. Newton’’s Third Laws Third Law
Newton’s Third Law of Motion
When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.
A. NewtonA. Newton’’s Third Laws Third Law
“For every action there is an equal and opposite reaction.”
A. NewtonA. Newton’’s Third Laws Third Law
Action-Reaction Pairs
The rocket exerts a downward force on the exhaust gases.
The gases exert an equal but opposite upward force on the rocket.
FG
FR
A. NewtonA. Newton’’s Third Laws Third Law
Propulsion of fish through water A fish uses its fins
to push water backwards. In turn, the water reacts by pushing the fish forwards.
A. NewtonA. Newton’’s Third Laws Third Law
Action-Reaction Pairs
The hammer exerts a force on the nail to the right.
The nail exerts an equal but opposite force on the hammer to the left.
A. NewtonA. Newton’’s Third Laws Third Law
Consider the interaction between a baseball bat and baseball. Action: the baseball forces the
bat to the right. Reaction: ?
A. NewtonA. Newton’’s Third Laws Third Law
Using a diving board to spring into the air before a dive is a good example of Newton’s third law of motion. Explain.
A. NewtonA. Newton’’s Third Laws Third Law
Action-Reaction PairsBoth objects accelerate.The amount of acceleration
depends on the mass of the object.
a Fm
Small mass more accelerationLarge mass less acceleration
A. NewtonA. Newton’’s Third Laws Third Law
http://esamultimedia.esa.int/docs/issedukit/en/activities/flash/start_toolbar.html#ex03_gm01.swf
A. NewtonA. Newton’’s Third Laws Third Law
While driving, you observe a bug striking the windshield of your car.
Obviously, a case of Newton’s third law! The bug hits the windshield and the windshield hits the bug. Is the force on the bug or the force on the windshield greater?
B. MomentumB. Momentum
Momentum “mass in motion” Depends on object’s mass and
velocity. The more momentum an object
has, the harder it is to stop. It would require a greater amount
of force or a longer amount of time or both to bring an object with more momentum to a stop.
B. MomentumB. MomentumFind the momentum of a bumper car if it
has a total mass of 280 kg and a velocity of 3.2 m/s.
GIVEN:
m = 280 kg
v = 3.2 m/s
p = ?
WORK:
p = mv
p = (280 kg)(3.2 m/s)
p = 896 kg·m/s
m
p
v
C. Conservation of MomentumC. Conservation of Momentum
Law of Conservation of Momentum The total momentum in a group of
objects doesn’t change unless outside forces act on the objects.
pbefore = pafter
C. Conservation of MomentumC. Conservation of Momentum
If momentum is lost by one object, it must be gained by another object in the system so that the total momentum of the system is constant.
C. Conservation of MomentumC. Conservation of Momentum
Collision between 1-kg cart and 2-kg dropped brick
Momentum of the loaded cart-dropped brick system is conserved
C. Conservation of MomentumC. Conservation of Momentum
Elastic Collision (KE conserved)
Inelastic Collision (KE not conserved)
C. Conservation of MomentumC. Conservation of Momentum
A 5-kg cart traveling at 4.2 m/s strikes a stationary 2-kg cart and they connect. Find their speed after the collision.
BEFORECart 1:m = 5 kgv = 4.2 m/s
Cart 2 :m = 2 kgv = 0 m/s
AFTERCart 1 + 2:m = 7 kgv = ?
p = 21 kg·m/s
p = 0
pbefore = 21 kg·m/s pafter = 21 kg·m/s
m
p
vv = p ÷ mv = (21 kg·m/s) ÷ (7 kg)v = 3 m/s
C. Conservation of MomentumC. Conservation of Momentum
A 50-kg clown is shot out of a 250-kg cannon at a speed of 20 m/s. What is the recoil speed of the cannon?
BEFOREClown:m = 50 kgv = 0 m/s
Cannon:m = 250 kgv = 0 m/s
AFTERClown:m = 50 kgv = 20 m/s
Cannon:m = 250 kgv = ? m/s
p = 0
p = 0
pbefore = 0
p = 1000 kg·m/s
pafter = 0
p = -1000 kg·m/s