# Is resistance to motion directly proportional to the speed of a moving object?

Power is known to be equal to the force x velocity (P=FV).

Im solving a question that states the following :

Car with engine working at 32 kW, mass of 1 tonne, travels at a constant speed of 40m/s along a level road.

Given that the resistance to motion is directly proportional to the speed at which the car is travelling, find the resistance experienced when the car is travelling at 30m/s

How is the text in Bold correct ?? P=FV .. Which means F=P/V, Velocity(speed) of car is indirectly proportional to the Force !! Can someone please explain this ?

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## 1 Answer

They are stating that as part of the question, not stating that it's necessarily true. You often see things like "ignoring air resistance", a "frictionless plane" or "massless spring" as part of questions to allow a simple analytical answer.

in reality for a car moving at 40 m/s air resistance is likely to be the major source of drag and this is proportional to velocity^2, but this makes the resulting equations trickier

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Oh okay, makes sense then. Thank you. –  Fendi Dec 30 '12 at 22:40