Train A moves due north at 54 km h⁻¹ and train B moves due south at 90 km h⁻¹ on parallel tracks. A monkey runs on the roof of train A at 18 km h⁻¹ relative to A, opposite to A's motion. Which statements are correct?
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Question

Train A moves due north at 54 km h⁻¹ and train B moves due south at 90 km h⁻¹ on parallel tracks. A monkey runs on the roof of train A at 18 km h⁻¹ relative to A, opposite to A's motion. Which statements are correct?

Select all that apply

G1 Physics · Accepted Answer

Take north as positive. Train A: vₐ = +54 km h⁻¹. Train B: vᵦ = -90 km h⁻¹. (a) Velocity of B relative to A: vᵦₐ = vᵦ-vₐ = -90-54 = -144 km h⁻¹. So B moves at 144 km h⁻¹ south relative to A. (b) Ground relative to B: vgᵦ = 0-(-90)=+90 km h⁻¹. So ground appears to move north at 90 km h⁻¹. (c) Monkey relative to ground: Monkey runs at 18 km h⁻¹ south relative to A. vₘ = 54-18 = 36 km h⁻¹ north. (d) Monkey relative to B: vₘᵦ = 36-(-90)=126 km h⁻¹ north.