Relative Motion

The motion of any object depends on the frame of reference – or point of view – of the observer. Pedestrian sees car moving along street. Driver sees car at rest. Both observers are correct about the motion of the car.

No absolute state of motion or rest – the ‘true’ state of an object is what you observe it to be. (Whether your moving car hits a stationary tree or a moving tree hits your stationary car, same damage results.) Is the sun going around the earth or is the earth going around the sun? Depends whose watching them.

Bob 3.0 m/s The velocity of an object in different frames of reference can be determined using the chain rule. escalator 1.0 m/s Paul (‘at rest’) What is Bob’s velocity relative to Paul?

Chain Rule Bob 3.0 m/s escalator 1.0 m/s Paul (‘at rest’) STEP 1: Identify object and observer for each velocity given.

Chain Rule Bob 3.0 m/s escalator 1.0 m/s Paul (‘at rest’) STEP 2: Add the velocities by “chaining” the reference frames together in the correct sequence. To find the velocity of Bob relative to observer Paul (who is in the ‘ground’ frame of reference:

Chain Rule Bob 3.0 m/s escalator 1.0 m/s Paul (‘at rest’) STEP 3: Sub and solve.

What is Bob’s velocity relative to a security guard riding west in a golf cart at 4.0 m/s? 1) 2) Security guard sees the ground moving east at 4.0 m/s. 3)