Two students are discussing how speed depends on the choice of reference frame. Alex stands on a flatcar that moves at constant speed along a straight, horizontal track. Bri jogs along the flatcar in the same direction that the flatcar is moving. A camera rigidly attached to the flatcar measures Bri’s speed to be 2.0 m/s. A stationary supervisor on the ground measures the flatcar’s speed to be 4.0 m/s. All motion is strictly along one straight line, and all speeds remain constant.
(1) Define, using an unambiguous two-subscript notation, the three distinct velocities that can be measured in this situation. State clearly which frame is the observer and which is the object for each velocity.
(2) Describe qualitatively and mathematically how the three velocities you named in part (a) are related. Explain why simple algebraic addition (with attention to sign) is sufficient to connect them in one-dimensional motion.
(3) If Bri maintains a constant jogging speed, would her acceleration be the same or different for observers on the flatcar and on the ground? Explain your reasoning.