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š Revise, Reflect, Refine, Q13 (Page 138)
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. (Graph: speed constant at 35 m sā»¹ from A at t = 0 to B at t = 1 s, then decreasing to 0 at C at t = 3 s.)View answer
Answer-
(i) How does the car move between A and B?
- At constant speed of 35 m sā»¹.
- This is the reaction time — the driver has seen the obstruction but the brakes have not been applied yet.
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(ii) Kinetic energy of the car at A
- K = ½mv²
- K = ½ × 1000 × (35)²
- K = ½ × 1000 × 1225
- K = 612500 J
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(iii) Work done by the brakes between B and C
- The car goes from 612500 J of kinetic energy to 0.
- Work done by brakes = change in kinetic energy = 0 − 612500
- Work done by brakes = − 612500 J (negative — brake force is opposite to motion)
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(iv) What does the kinetic energy transform into?
- Mainly thermal energy (heat) — due to friction between the brakes and wheels, and tyres and road.
- Some of it becomes sound energy.
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(i) How does the car move between A and B?
Q13 - A 1000 kg car is
Last updated at September 16, 2026 by Teachoo
