A box weighing 120 N rests on a floor, which pushes it up with 120 N.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A box weighing rests on a floor, which pushes it up with .
- Are the weight and the floor's push a third-law pair?
- Name the third-law partner of each.
- A person pushes the box sideways with and it does not move. How big is the friction, and what is its partner?
Need a reminder? The key ideaShow
Newton's third law: when A pushes B, B pushes A with a force of the same size in the opposite direction. The two forces act on different objects, so they never cancel each other.
The net force adds all the forces with their directions. If it is zero the forces are balanced and the motion does not change; if not, they are unbalanced and the object speeds up, slows down or turns. Forces at right angles are added separately.
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.
Step 1. (a) No. Both act on the box: they are balanced forces, not a pair.
Step 2. (b) The weight's partner is the box pulling the Earth up. The floor's push has as partner the box pushing down on the floor.
Step 3. (c) Friction is , opposite the push; its partner is the box's friction on the floor, the other way.
Answer: No; as above; .
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