WAEC 2024 Physics · Question 32

A boy of mass 40 kg jumps off a stationary skateboard and moves horizontally with a speed of 1.5 m/s while the…

Question

A boy of mass 40 kg jumps off a stationary skateboard and moves horizontally with a speed of 1.5 m/s while the skateboard moves in the opposite direction with a speed of 20 m/s. Calculate the mass of the skateboard.

A.60 kg
B.40 kg
C.3 kg
D.2 kg

Answer: C. 3 kg

Explanation written with AI and checked against the answer key

Analyze the question: A boy and a skateboard are initially at rest (stationary). The boy jumps off, so the system's total momentum is conserved. Before the jump, total momentum = 0. Concept: Conservation of linear momentum: Total momentum before = Total momentum after Solution: Let the skateboard's mass be M. Before: (40 + M)(0) = 0 After: (40)(1.5) + M(−20) = 0 (the skateboard moves in the opposite direction) 60 − 20M = 0 M = 60 / 20 = 3 kg Answer: The mass of the skateboard is 3 kg, which is option C. Tip: For "explosion" or "jump-off" problems starting from rest, always set the total momentum after equal to zero. That gives m₁v₁ = m₂v₂, so the two momenta are equal in size and opposite in direction. Always check that the sign of the opposite direction is handled correctly.

AI can make mistakes. Report anything that looks wrong.