Work and Power
5 marks · NormalA 400 kg rollercoaster car starts from rest at the top of a track 15 m above the ground. The car descends to the lowest point with negligible friction. Use g = 10 N/kg.
Calculate the gravitational potential energy of the car at the top of the track.
Calculate the speed of the car at the lowest point, assuming all the potential energy changes to kinetic energy.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) 60,000 J
(b) 17 m/s
Common mistakes: Wrong use of mass/weight for energy; Not taking square root
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Uses GPE = mgh correctly
A1 (a) · 1 mark · Substitutes correctly and calculates GPE (accept rounding) (follow-through)
M2 (b) · 1 mark · Equates GPE and KE (follow-through)
M3 (b) · 1 mark · Rearranges for v and substitutes values correctly (follow-through)
A2 (b) · 1 mark · Correct speed found (to reasonable precision) (follow-through)
Worked solution
a) GPE = mgh = 400 × 10 × 15 = 60,000 J
b) Equate GPE lost to KE gained: