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A 1000 kg car is moving along a road at a constant speed. Suddenly, the driver notices some obstruction ahead and applies the brakes to come to a complete stop. The graphical representation of motion of the car (Fig. 7.38) starts from the instant the driver spots the traffic. (i) Describe how the car moves between positions A and B. (ii) Calculate the kinetic energy of the car at A. (iii) State the work done by the brakes in bringing the car to a halt between B and C. (iv) What does the kinetic energy of the car transform into?
Show Answer
(i) Between A and B the car moves at constant speed (35 m s⁻¹) — this is the driver's reaction time before braking.
(ii) KE at A
=
1
2
× 1000 × 35
2
= 612500
J
.
(iii) The brakes bring the car from 35 m s⁻¹ to rest, so work by the brakes = 0 - 612500 = -612500 J .
(iv) The kinetic energy transforms mainly into heat (in the brakes, tyres and road) and some sound .
= 1 2 (1000)(35) 2
= 612500 J
