Question 1 Report
Fig. 1.1 shows a car of mass 1000 kg that speeds up from rest to 10 m/s in 5 s on a level road. Ignoring friction, what average power gives it this kinetic energy?
The correct answer is 10 000 W. First find the kinetic energy gained: \( KE = \tfrac{1}{2}mv^{2} = \tfrac{1}{2} \times 1000 \times 10^{2} = 50\,000 \) J. Then average power is \( P = \frac{E}{t} = \frac{50\,000}{5} = 10\,000 \) W. Choosing 50 000 W forgets to divide by time; 2000 W uses \( mv/t \) without the half; 5000 W halves the correct answer again.
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