Motion & Forces

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.


“For every action there is an equal and opposite reaction.”


Action-Reaction Pair


Action-Reaction Pair


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


Physics of walking


Propulsion of fish through water A fish uses its fins

to push water backwards. In turn, the water reacts by pushing the fish forwards.


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.


Consider the interaction between a baseball bat and baseball. Action: the baseball forces the

bat to the right. Reaction: ?


Using a diving board to spring into the air before a dive is a good example of Newton’s third law of motion. Explain.

NewtonNewton’’s Third Laws Third Law

Newton vs. Elephant

Who will move fastest?


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


http://esamultimedia.esa.int/docs/issedukit/en/activities/flash/start_toolbar.html#ex03_gm01.swf


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. Momentum

Momentum

p = mv

p: momentum (kg ·m/s)m: mass (kg)v: velocity (m/s)m

p

v

B. Momentum and ImpulseB. Momentum and Impulse

Newton’s 2nd Law

Impulse = Change of momentum

B. Momentum and ImpulseB. Momentum and Impulse

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


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.


Collision between 1-kg cart and 2-kg dropped brick

Momentum of the loaded cart-dropped brick system is conserved


Big fish in motion catches little fish

C. Conservation of MotionC. Conservation of Motion

Little fish in motion is caught by big fish


Elastic Collision (KE conserved)

Inelastic Collision (KE not conserved)


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


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


So…now we can solve for velocity.

GIVEN:

p = -1000 kg·m/s

m = 250 kg

v = ?

WORK:

v = p ÷ m

v = (-1000 kg·m/s)÷(250 kg)

v = - 4 m/s (4 m/s backwards)

m

p

v

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