Leilehua High School Physical Science - Home - For...
Transcript of Leilehua High School Physical Science - Home - For...
9/16/14
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Newton’s Third Law of Motion & Momentum
12-3 pg 372-377
Newton’s 3rd Law of Motion
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object
For every action there is an equal and opposite reaction
Action and Reaction forces
Action force = The force exerted by the first object
Reaction force = The equal and opposite force the second object exerts back on the first object
Bumper Cars Action force - your car
hitting the other car
Reaction force - the force the other car exerts on your car
These are equal in size and opposite in direction
Hammer hitting a nail Hammer exerts a
force on the nail driving it into the wood (action)
The nail exerts a force on the hammer causing it to stop moving (reaction)
Action-Reaction Forces and Motion Swimmers use their arms to push against
the water to create an action force
This action force pushes the water in the same direction of the action force
The reaction force of the water acts on the swimmer and pushes them forward
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Action-Reaction Force The example of the
swimmer shows that not all action-reaction forces cancel each other out
The reason these forces do not cancel is because they act on different objects
The action force acts on the water. The reaction force acts on the swimmer.
Momentum The product of an object’s mass and its velocity p = m x v Use p to represent momentum
because m = mass
An object has a large momentum if the product of the mass and velocity is large.
Momentum (p)
Momentum = mass x velocity
Units for momentum = kgm/s
Units are not the same as units for force which is kgm/s2
Momentum (p) = m x v Momentum = Mass x velocity The truck has much more mass than the mini cooper. Therefore, its momentum will be much larger and the mini cooper would be destroyed if they were to collide.
p = 13,600 kg x 2 m/s = 27,000 kgm/s
p = 1,150kg x 2m/s = 2,300 kgm/s
Find the momentum of an object if its mass is 7 kg and its
velocity is 6 m/s
Given information m = 7 kg v = 6 m/s
p = m x v
p = 7 kg x 6 m/s
p = 42 kgm/s
Law of Conservation of Momentum
If no net force acts on a system, then the total momentum of the system does not change.
In a closed system, the loss of momentum of one object equals the gain in momentum of another object, momentum is conserved