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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
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