NumericalModerate1 mark
6(iii)
Richa weighing 40 kgf leaves point P on her skateboard and reaches point Q on the ground with velocity 10 ms-1. Calculate.
the kinetic energy of Richa at point Q
Topic: Kinetic energy
Question wording verified against source paper
Answer
Exam answer
Given: Richa weighs 40 kgf, so her mass m = 40 kg; velocity at Q, v = 10 m s⁻¹
Kinetic energy = ½ m v² = ½ × 40 × (10)² = ½ × 40 × 100 = 2 000 J
Kinetic energy of Richa at Q = 2 000 J (2 kJ)
Explanation
A weight of 40 kgf corresponds to a mass of 40 kg — that is what the kgf unit means, the weight of a 40 kg mass. So use 40 in the formula, not 400.
Square the velocity: 10² = 100. Using 10 gives 200 J, a tenth of the right answer.
The ½ is easy to drop in a hurry, and doing so doubles the answer to 4 000 J.
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