A freight car in a park moves along a frictionless level railroad track at constant speed. The freight car is open on top. A kid who was hanging on a branch of a tree that is over the track. As soon as the car passes under the branch, the kid let go of the branch and landed on the car. What happens to the speed of the freight car? It becomes zero. It remains the same. It decreases. It increases. Cannot be determined from the information given.

College Physics
11th Edition
ISBN:9781305952300
Author:Raymond A. Serway, Chris Vuille
Publisher:Raymond A. Serway, Chris Vuille
Chapter1: Units, Trigonometry. And Vectors
Section: Chapter Questions
Problem 1CQ: Estimate the order of magnitude of the length, in meters, of each of the following; (a) a mouse, (b)...
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**Question:**

A freight car in a park moves along a frictionless level railroad track at constant speed. The freight car is open on top. A kid who was hanging on a branch of a tree that is over the track. As soon as the car passes under the branch, the kid let go of the branch and landed on the car. What happens to the speed of the freight car?

**Options:**

- It becomes zero.
- It remains the same.
- It decreases.
- It increases.
- Cannot be determined from the information given.

**Explanation:**

When the child lands on the moving freight car, their mass is added to the system of the car. According to the conservation of momentum, the total momentum before and after the event must be the same if no external forces are acting on the system. Since the track is frictionless, the system is isolated in terms of horizontal forces.

Initially, the momentum is only due to the moving freight car. After the child lands, the combined mass now moves with a new velocity. Because the total momentum must remain constant, the increased mass typically results in a decreased speed, assuming mass increases while maintaining momentum.

Therefore, the correct answer is that the speed of the freight car *decreases*.
Transcribed Image Text:**Question:** A freight car in a park moves along a frictionless level railroad track at constant speed. The freight car is open on top. A kid who was hanging on a branch of a tree that is over the track. As soon as the car passes under the branch, the kid let go of the branch and landed on the car. What happens to the speed of the freight car? **Options:** - It becomes zero. - It remains the same. - It decreases. - It increases. - Cannot be determined from the information given. **Explanation:** When the child lands on the moving freight car, their mass is added to the system of the car. According to the conservation of momentum, the total momentum before and after the event must be the same if no external forces are acting on the system. Since the track is frictionless, the system is isolated in terms of horizontal forces. Initially, the momentum is only due to the moving freight car. After the child lands, the combined mass now moves with a new velocity. Because the total momentum must remain constant, the increased mass typically results in a decreased speed, assuming mass increases while maintaining momentum. Therefore, the correct answer is that the speed of the freight car *decreases*.
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