AP Physics C: Mechanics / Unit 1: Kinematics
1.4 Reference Frames and Relative Motion
Notes
Velocity depends on the reference frame:
$$\vec v_{A\text{ rel }C}=\vec v_{A\text{ rel }B}+\vec v_{B\text{ rel }C}$$
Practice
You walk 1 m/s forward on a train moving 20 m/s. Your speed relative to the ground:
Velocities add.
Two cars drive toward each other at 25 m/s each. Their relative speed:
Opposite directions add.
A passenger sitting on a moving bus is at rest relative to:
Frames matter.
A boat heads north at 4 m/s in a river flowing east at 3 m/s. Its speed relative to the shore:
$\displaystyle \sqrt{16+9}$.
Ready for more? Take the Unit 1 test.