EasyPhysicsHigh School (11–12)Mechanics › Rotational MotionMechanics › Momentum and CollisionsConceptualBoard ExamAP
Answer, and give a reason. A firework rocket bursts in the air…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Answer, and give a reason.
- A firework rocket bursts in the air. Ignoring air, does the burst change the path of the centre of mass of the pieces?
- A person stands still on perfectly smooth ice. Can she get herself moving?
- A boat is at rest. Can the people on it make it move by walking about, without touching 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.
Plan. Only external forces change the motion of a centre of mass.
Step 1. (a) No. The burst is internal; gravity, the only external force, is unchanged. The centre of mass keeps the old path until a piece lands.
Step 2. (b) Not by moving her body alone, since the ice gives no horizontal force. But she can throw something, a shoe or a phone: it goes one way and she goes the other, and their centre of mass stays still.
Step 3. (c) The boat can move, but the centre of mass of boat and people cannot. As people walk one way, the boat slides the other way.
Answer: (a) No. (b) Only by throwing something away. (c) The boat moves, the centre of mass does not.
Watch out. "The centre of mass does not move" does not mean "nothing moves". Parts can move, as long as they balance.
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