Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
A car moves at a steady against a resisting force of .
Power is the rate of doing work: , in watts. For a force at speed , . Efficiency useful power out power in. One kilowatt-hour is .
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. , with the right force at each speed.
Step 1. (a) .
Step 2. (b) .
Step 3. (c) The force becomes , so .
Answer: (a) . (b) . (c) .
Check. In (c) the power grows as : , and . Same.
Watch out. At highway speeds the air dominates, so a modest rise in speed needs a large rise in power.
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