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A 55 kg boy stands at the front of a 165 kg canoe on still water.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A boy stands at the front of a canoe on still water. He walks back along the canoe. The water gives no horizontal force.
- How far does the canoe move over the water?
- How far does the boy move over the water?
Need a reminder? The key ideaShow
Only external forces move the centre of mass: . With no external force, the total momentum is conserved, and the centre of mass stays at rest or moves steadily.
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. , , walk along the canoe.
Plan. No external horizontal force, so the centre of mass does not move.
Step 1. Balance. , so .
Step 2. The canoe. , forward.
Step 3. The boy. , back.
Answer: (a) . (b) .
Check. and . They balance.
Watch out. The is measured along the canoe, which itself slides. Over the water he moves less.
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