AP Physics C: Mechanics : Understanding Conservation of Momentum
Study concepts, example questions & explanations for AP Physics C: Mechanics
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Example Questions
Example Question #1 : Understanding Conservation Of Momentum
In an inelastic collision, which combination of quantities is conserved?
Mass, kinetic energy, and momentum
Mass and momentum
Mass and potential energy
Kinetic energy and potential energy
Momentum and kinetic energy
Mass and momentum
In a perfectly inelastic collision, the two colliding objects stick together; the two colliding objects deform, but mass is still conserved. Momentum is conserved during collisions of any sort, including inelastic collisions.
Kinetic energy is reduced during an inelastic collision, and is only conserved in elastic collisions. During inelastic collisions, some kinetic energy is lost to the environment in the form of heat or sound.
The problem does not give any information regarding position, and thus we cannot comment on any changes or lack of changes in potential energy.
Example Question #2 : Understanding Conservation Of Momentum
In an elastic collision, what combination of quantities is conserved?
Momentum, kinetic energy, and potential energy
Momentum, kinetic energy, and potential energy
Mass and momentum
Mass, momentum, and kinetic energy
Potential energy and kinetic energy
Mass, momentum, and kinetic energy
The primary difference between elastic and inelastic collisions is the conservation of kinetic energy. Kinetic energy is conserved in elastic collisions, but is not conserved in inelastic collisions. Momentum is always conserved, regardless of collision type. Mass is conserved regardless of collision type as well, but the mass may be deformed by an inelastic collision, resulting in the two original masses being stuck together.
There is no description in the problem reagrding position, so we cannot comment on potential energy.
Example Question #1 : Momentum
A 200kg car is traveling west at 
The collision is assumed to be elastic, so both momentum and kinetic energy are conserved. Use the law of conservation of momentum:
Momentum is the product of velocity and mass:
We can expand the summation for the initial and final conditions:
Use the given values to fill in the equation and solve for 
Since the final velocity is positive, we know that the car is still traveling toward the west.
Example Question #1 : Momentum
Two train cars, each with a mass of 2400 kg, are traveling along the same track. One car is traveling with a velocity of 
Use the law of conservation of momentum:
Momentum is the product of velocity and mass:
We can expand the summation for the initial and final conditions:
Note that we are working with an inelastic collision, meaning that the two masses stick together after the collision. Because of this, they will have the same final velocity:
Use the given values to fill in the equation and solve for 
Since the final velocity is positive, we can determine that they train cars are traveling toward the east.
Example Question #1 : Momentum
A 
What is the mass of the block?
This problem is a conservation of momentum problem. When doing these types of problems, the equation to jump to is:
It is given to us that 
With all this information given, the only unknown is 
Plugging everything in, we get:
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