A volleyball of mass 0.27 kg hits a wall at 13.5 m/s and bounces straight back at 9.5 m/s.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A volleyball of mass hits a wall at and bounces straight back at . It touches the wall for .
- What is the size of the change in its momentum?
- What is the average force of the wall on the ball?
Need a reminder? The key ideaShow
Momentum is , a vector. A force acting for a time changes it. The change is the impulse:
Choose a positive direction and give every velocity its sign. For a force that changes with time, the impulse is the area under the force-time graph.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. , , , .
Plan. Find with signs, then .
Step 1. Change of momentum. . Its size is , away from the wall.
Step 2. Average force. , away from the wall.
Answer: (a) . (b) , pushing the ball away from the wall.
Check. A ball that bounces back has a bigger change than one that just stops (). Bouncing needs more force. That makes sense.
Watch out. Subtracting the speeds, , gives the wrong answer. The velocity reverses, so the change is the sum of the speeds.
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