Momentum and Collisions - Mission MC9 Detailed Help

A 120-kg red bumper car moving east at 2.40 m/s collides with a 150-kg blue car that is moving west at 1.20 m/s. After the collision, the red car moves west at 1.40 m/s. Assume the system is isolated. Fill in the momentum table and determine the final velocity of the blue car. Use the notation that east is the positive direction and west is the negative direction.
(Note: Your numbers are selected at random and likely different from the numbers listed here.)

The momentum (p) of an object can be calculated from knowledge of its mass (m) and velocity (v) using the formula: 

p = m • v

The ultimate goal of this analysis is to determine the post-collision velocity of the blue car. The following strategy will prove effective.
  • Since the mass and the before-collision velocity of each car is given, the momentum values can be calculated (see Formula Frenzy section). Since the blue car is moving west, its momentum will be a negative value (see Know the Law section).
  • The total momentum of the system is simply the sum of the individual momentum values of the two cars. Once individual values have been calculated, the total momentum of the system before the collision can be determined.
  • The post-collision velocity of the red car is known; thus, its momentum can be calculated. You will have to consider the fact that it's moving west (see Know the Law section).
  • The post-collision velocity of the blue car is not known. The momentum of this car must be written in terms of the unknown velocity v. Since its mass is 150 kg, the momentum can be represented as 150•v. Together, the total system momentum after the collision would be represented as RCM + 150•v, where RCM is the actual value of the red car's momentum (a negative value).
  • Since the system is said to be isolated (see Define Help section), the post-collision momentum of the system is the same as the pre-collision momentum. The post-collision momentum of the blue car is not known as an actual value; it is represented in terms of the blue car's unknown velocity v. Set the expression for the total momentum (written in terms of v) equal to the pre-collision momentum. Then solve for the variable v. This is the post-collision velocity of the blue car.
  • Now that v is known, the post-collision momentum of blue car can be calculated and entered into the cell of the table. The total momentum for the system can be calculated as well. This total system momentum should be the same as before the collision. If not, you will need to recheck your work.

Isolated System and External Forces:
A system of two colliding objects is considered to be an isolated system if the only momentum-changing forces exerted during the collision are the forces between the two objects themselves. If a third object exerts a force capable of changing one or both of the object's momentum during the collision, then this is considered an external force and the system is not isolated.

Momentum as a Vector:
Momentum is a vector and has a direction. The direction of an object's momentum is in the same direction that the object is moving. An eastward moving object has an eastward momentum (+); a westward moving object has a westward momentum (-).


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