Which law answers
One cart, pulled by a rope. Will its motion change? By how much? What is the partner force?
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Step 1 of 4
- A cart of 60 kg. The rope pulls 110 N. Friction acts against it: 70 N.
- Net force: 110 − 70 = 40 N forward. Will it change? Yes.
- 40 ÷ 60 = 0.67 m/s². That is how much.
- The cart pulls the rope back with 110 N. That is the partner force.
Yes, the motion changes: the acceleration is about 0.67 m/s². The partner force is the cart's 110 N pull on the rope.
The rule
First law: no net force, no change in velocity. Second law: F = ma. Third law: forces come in equal, opposite pairs.
Each question needs its own law. The first question asks if the motion changes. The second asks by how much. The third asks about the partner force on the other object.
| Question | Law |
|---|---|
| Will it change? | First law: is the net force zero? |
| By how much? | Second law: a = F ÷ m |
| What is the partner force? | Third law: an equal, opposite force on the other object |