Question

Bob is running on a train at 3m/s [E] relative to the train. The train is moving at 5 m/s [E] relative to the ground. What is Bob's velocity related to Ernie who is sitting stationary on the ground as the train passes by?

Answer 1

Since Bob and the train are moving in the same direction, and Bob is on the train, the relative speed between Bob and Ernie will be the sum of velocities:

$\begin{gathered} \text{velocity}=\text{trainvelocity}+\text{bobvelocity} \\ \text{velocity}=5+3 \\ \text{velocity}=8\text{ m/s (E)} \end{gathered}$

Therefore Bob's velocity related to Ernie is 8 m/s [E].