Newton’s 3rd Law

PAGES 412-417

Newton’s 3rd law of motion:

• Forces always occur in pairs

• For every action force…there is an equal and opposite reaction force

• Forces in a force pair do not act on the same object

• Equal forces do not always have equal effects

ACTION FORCE REACTION

FORCE

Also applies to skateboards… cars…etc…

EX 1:

Swimmer pushes backward on wall…wall pushes forward on swimmer

Swimmer pushes backward on water…water pushes forward on swimmer

EX 2:

Bird flying

Given the action force, identify the reaction force:

ACTION:

REACTION:

The wing of the bird pushes the air downward.

The air pushes the wing of the bird upward.

Given the action force, identify the reaction force:

Rocket- p 416 ACTION: The rocket pushes hot gases backwards (from the combustion chamber through the nozzle)

REACTION:

The hot gases push the rocket forward (up)

A. Momentum

• ___________ is a property of all moving objects

• For movement along a straight line, momentum is calculated by multiplying an object’s _____ and ________:

momentum = mass x velocity

p = mv

Momentum

mass velocity

momentum = mass x velocityp = mv

• Momentum is expressed in ________

• Like velocity, momentum has _______. An object’s momentum and velocity are in the same direction.

kgm/s

direction

P

m v

The greater the mass of an object, the ________ the momentum of the object.

• Ex. a tractor trailer truck has much more momentum than a sports car moving at the same speed.

greater

The faster an object is the ________ the momentum of the object.

If an object is not moving its momentum is ______.

greater

zero

B. Practice Problems

1. Calculate the momentum of a ____ kg speed skater moving forward at ____ m/s

p = mv = (75 kg)(16 m/s) = 1,200 kgm/s forward

75 16

3. Calculate the momentum of a ____ kg passenger seated on a train that is __________

p = mv = (48.5 kg)(0 m/s) = 0 kgm/s

48.5

stopped

4. Calculate the velocity of a ____ kg kitten with a forward momentum of ____ kgm/s

v = p/m = (5 kgm/s)/(0.8 kg) = 6m/s forward

0.8

5

C. Conservation of Momentum

• When a moving object hits a second object, some or all of the momentum of the first object is ___________ to the second object

• The total momentum of two or more objects after a collision is the _____ as it was before the collision

transferred

same

• Ex. Imagine hitting a billiard ball with a cue ball so that the billiard ball starts moving. During the collision, the cue ball transfers some of its momentum to the billiard ball

• The cue ball hits the billiard ball with a force – the _______ force. The equal but opposite force exerted by the billiard ball on the cue ball is the ________ force

action

reaction

Law of Conservation of Momentum

• Law of conservation of momentum states the total amount of momentum in an isolated system is conserved

• Anytime two or more objects interact, they may exchange momentum, but the total momentum of the system stays the ______. same