EasyPhysicsHigh School (11–12)Mechanics › Momentum and CollisionsMechanics › Newton's Laws of MotionConceptual
A lamp hangs at rest from the ceiling on a cord.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A lamp hangs at rest from the ceiling on a cord.
- The Earth pulls the lamp down. What is the third-law partner of this force?
- The cord pulls the lamp up. What is the third-law partner of this force?
- The lamp's weight and the cord's pull are equal and opposite. Are they a third-law pair?
Need a reminder? The key ideaShow
Newton's third law. If A pushes or pulls on B, then B pushes or pulls on A with a force equal in size and opposite in direction. The two forces act on different bodies, and they are always the same kind of force.
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. For each force "A on B", the partner is "B on A", of the same kind.
Step 1. (a) The Earth pulls the lamp down by gravity. The partner is the lamp's gravitational pull on the Earth, upward.
Step 2. (b) The cord pulls the lamp up. The partner is the lamp pulling the cord down.
Step 3. (c) Both forces act on the lamp, and they are different kinds (gravity and tension). So they are not a third-law pair. They balance because the lamp is at rest: that is the first law.
Answer: (a) The lamp pulls the Earth up. (b) The lamp pulls the cord down. (c) No. They act on the same body; they balance only because the lamp is in equilibrium.
Watch out. "Equal and opposite" is not enough for a third-law pair. If the cord were jerked upward, the cord's pull would exceed the weight, yet each third-law pair would still be equal.
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