An automobile accelerates westward along a straight road
PHYSICS · Single correct · AP · Physics C: Mechanics · Kinematics · Kinematics
Problem
An automobile accelerates westward along a straight road. Observer O stands at rest on the road, while observer O′ rides in a bus that travels westward at a steady 15m/s. Both observers start their stopwatches when the automobile passes them. Which statement best explains why both observers calculate the same numerical value for the automobile’s acceleration?
(a) Both observers begin timing when the automobile is next to them, so they record identical velocities at every instant; therefore their calculated accelerations must match.
(b) Because the bus maintains a constant speed, its zero acceleration can simply be subtracted from the automobile’s acceleration, leaving the same value for both observers.
(c) Subtracting the bus’s constant velocity from each of the automobile’s velocity readings shifts every value by the same amount, leaving the rate of change unchanged.
(d) Distances in the two frames differ only by the bus’s displacement, which is proportional to time; dividing by time twice to obtain acceleration eliminates that difference.