A car travelling on a straight

PHYSICS · Multi-part · CBSE · Physics · Work, Energy and Power · Work, Energy and Power

Problem

A car travelling on a straight, level road experiences air resistance only. Its engine supplies a constant mechanical power output of 15 kW15\,\text{kW}. (i) Convert the car’s speed of 90 km h−190\,\text{km h}^{-1} to m s−1\text{m s}^{-1}. (ii) Calculate the magnitude of the resistive force acting on the car at this speed. (iii) The driver now reduces the speed to 60 km h−160\,\text{km h}^{-1} while the engine continues to deliver 15 kW15\,\text{kW}. Determine the new resistive force at this lower speed. (iv) Briefly explain, using a mathematical relationship, why the resistive force changes when the car’s speed changes while the engine power remains the same.

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