Which of these bodies have zero net force on them?
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Which of these bodies have zero net force on them? Explain each.
- A lift moving up at a steady speed.
- A ball at the top of its flight, after being thrown straight up.
- A raindrop falling at its steady top speed.
- A cyclist going round a roundabout at a steady speed.
- A book at rest on a table.
Need a reminder? The key ideaShow
Newton's first law. A body keeps its velocity (stays at rest, or moves in a straight line at a steady speed) unless a net force acts on it. This reluctance to change velocity is inertia, and mass measures it.
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, ask whether the velocity is changing. If it is not, the net force is zero.
Step 1. (a) Steady speed in a straight line: zero net force.
Step 2. (b) The ball is still for an instant, but its velocity is changing (it will come down). Its acceleration is , so the net force is its weight, not zero.
Step 3. (c) Steady velocity: air drag balances the weight, so zero net force.
Step 4. (d) The direction of the velocity changes all the time, so there is an acceleration toward the centre. The net force is not zero.
Step 5. (e) At rest and staying at rest: zero net force.
Answer: (a), (c) and (e) have zero net force. (b) and (d) do not.
Watch out. "At rest for an instant" is not the same as "in equilibrium". What matters is whether the velocity is changing.
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