Momentum and Collisions - Mission MC9 Detailed Help

A 0.600-kg red ball moving east at 72.0 cm/s collides with a 0.850-kg green ball that is moving east at 32.0 cm/s. After the collision, the red ball moves east at 28.0 cm/s. Assume the system is isolated. Fill in the momentum table and determine the final velocity of green ball. 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 green ball. The following strategy will prove effective.
  • Since the mass and the before-collision velocity of each ball is given, the momentum values can be calculated (see Formula Frenzy section).
  • The total momentum of the system is simply the sum of the individual momentum values of the two balls. 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 ball is known; thus, its momentum can be calculated.
  • The post-collision velocity of the green ball is not known. The momentum of this ball must be written in terms of the unknown velocity v. Since its mass is 0.850 kg, the momentum can be represented as 0.850•v. Together, the total system momentum after the collision would be represented as RBM + 0.850•v, where RBM is the actual value of the red ball's momentum.
  • 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 green ball is not known as an actual value; it is represented in terms of the green ball'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 green ball.
  • Now that v is known, the post-collision momentum of green ball 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.


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