Question 12
A car of mass 500 kg
is at rest at point X on a slope, as shown.
The car’s brakes are
released and the car rolls down the slope with its engine switched off. At point
Y the car has moved through a vertical height of 30 m and has a speed of 11 m s-1.
What is the energy
dissipated by frictional forces when the car moves from X to Y?
A 3.0 × 104 J B 1.2 × 105 J C 1.5 × 105 J D 1.8 × 105 J
Reference: Past Exam Paper – November 2015 Paper 12 Q16 & June 2018 Paper 12 Q19
Solution:
Answer: B.
As the car rolls down the
slope, its gravitational potential energy is being converted to kinetic energy
AND work done against friction.
Loss in GPE = Gain in KE +
Work down against friction
mgh = ½ mv2 + Work
down against friction
Work down against friction
= mgh – ½ mv2
Work down against friction
= (500×9.81×30) – (½ × 500 × 112)
Work down against friction = 116 900 J = 1.2 × 105 J
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