Recitation ClassWork 1 solutions Physics 193 1. Active Learning Guide (ALG) 1.1.1.
• From the point of view of a person sitting in the backseat of the car, the person sitting in the passenger seat is at rest.
• From the point of view of a person sitting in the backseat of the car, the person standing on the sidewalk as the car passes appears to be moving closer to front of the car, then passing alongside the car, then moving away from the rear of the car.
• From the point of view of a person sitting in the backseat of the car, the driver of the second car is approaching from the rear of the car, then passing the car, then moving away from the front of the car.
2. ALG 1.1.3 a,b,e.
a. Yes. Just because the person sitting in the car is at rest with respect to the car doesn’t mean they are at rest with respect to everyone. The person standing on the sidewalk sees the person in the car moving. It seems that saying something is moving or not moving is a matter of perspective, and there is no way to say in an absolute sense that something is moving or not moving.
b. Yes. Anyone sitting in the car with the person would observer that person to be at rest.
e. When someone says an object is “moving” they really mean that they observe that the object is moving relative to them.
3.
a. If Eric walks in the same direction and at the same speed as Marta the basketball will appear to be at rest from Eric’s point of view.
b. If Joan walks in the same direction as Marta but at a greater speed, Marta will appear to be moving to the west from Joan’s point of view.
c. The idea that motion is relative is supported by the results of these experiments.
4. ALG 1.2.2
A car is moving to the right along a straight stretch of road. As soon as the car passes a speed limit sign indicating that it is okay to move faster, the car accelerates up to the new speed limit, then maintains the new greater speed.
5. ALG 1.2.4
6. ALG 2.1.3 a-d only.
b. The ball moves at a constant rate.
c. The ball slows down.
d. There is no pattern between the direction of the vector sum of the forces and the direction of �. Sometimes they are in the same direction, sometimes they aren’t. However, the vector sum of the forces and the direction of Δ� do always seem to be in the same direction.
8. ALG 3.2.4
