AP Physics C: Mechanics : Interpreting Collision Diagrams
Study concepts, example questions & explanations for AP Physics C: Mechanics
All AP Physics C: Mechanics Resources
Example Questions
Example Question #1 : Interpreting Collision Diagrams
A 5000kg pickup truck with a 500kg load is traveling at a velocity of 
We need to use conservation of momentum to solve this problem.
Initially, the pickup truck moves at 
We can calculate the truck-load momentum as follows:
Note that this is the sum of the truck's momentum AND the load's momentum. We are looking at is as a whole since they are moving together.
The car moves at 
Therefore, we have that the momentum for the car as:
We can find the initial momentum of the system (truck, load, and car) via simple addition:
After the collision, we know that all three objects move together, so they all move with the same velocity. Therefore we can express the momentum of the system after the collision as follows:
By conservation of momentum we know that the momentum of the system before the collision is equal to the momentum of the system after the collision.
Use this to solve for the final velocity.
Example Question #1 : Interpreting Collision Diagrams
Two identical atoms, A and B, are at rest some distance apart in a vacuum.
If atom A moves at a constant velocity, 
The velocity of atom A is 
Atom A moves with velocity 
The velocities of the identical atoms are the same, but in opposite directions after the collision
The velocity of atom B after the collision is
The speed of both atoms is the same after collision
The velocity of atom A is
Assuming the atoms have the same mass, a collision between atoms in a vacuum is most nearly an elastic collision. This means the kinetic energy of the system is conserved and when they collide, all of the kinetic energy in atom A will be transferred perfectly to atom B. So just after collision, atom A will have velocity 
Example Question #11 : Momentum
A pickup truck with a mass of 
It turns out the truck can only take on an additional 
The equation for conservation of momentum is:
Given:
Plug in given values into the conservation of momentum equation.
Simplify.
Subtract the final velocity from the initial velocity to find change in speed (since these velocities are in the same direction).
Example Question #1 : Interpreting Collision Diagrams
A pickup truck with a mass of 
Assuming no outside forces are acting on the system, how many 
Because of conservation of momentum, the product of the initial mass and velocity should equal the final mass and velocity. The equation for conservation of momentum is:
The initial mass of the truck is
The initial velocty is
The final velocity is 
Plug in known values to the conservation of momentum equation.
Simplify.
Note that while necessary, the final mass is not our final answer. The final mass, 
The mass of the truck is known so set up an equation and solve for the mass of sand added to the truck.
Certified Tutor
All AP Physics C: Mechanics Resources
