Reference Points

1. A mother is riding a bike, pulling her children in a cart behind the bike. Is the mother moving relative to a crack in the sidewalk?

Yes

No

The mother's distance from the crack in the sidewalk is changing, so she is moving when the crack is her reference point.

Explanation

The mother's distance from the crack in the sidewalk is changing, so she is moving when the crack is her reference point.

2. You are sleeping in class. Are you moving?

Yes, if your reference point is the chair.

Yes, if your reference point is the table.

Yes, if your reference point is the Sun.

Your distance from the chair and the table aren't changing, so you are not moving relative to them. You are moving relative to the Sun because you are on the Earth, and the Earth is rotating and revolving. Therefore, your distance from the Sun is changing.

Explanation

Your distance from the chair and the table aren't changing, so you are not moving relative to them. You are moving relative to the Sun because you are on the Earth, and the Earth is rotating and revolving. Therefore, your distance from the Sun is changing.

3. A mother is riding a bike, pulling her children in a cart behind the bike. Is the mother moving relative to the cart?

Yes

No

The cart is moving at the same rate as the bike, so her distance from the cart is not changing. Therefore, she is not moving if the cart is her reference point.

Explanation

The cart is moving at the same rate as the bike, so her distance from the cart is not changing. Therefore, she is not moving if the cart is her reference point.

4. You are riding on a bus going 40 miles/hour. Are you moving?

No, not if the reference point is the bus.

Yes, if the reference point is someone standing on the sidewalk.

No, not if the reference point is the bus driver.

All of the above.

You are moving if your distance from a reference point is changing. Since you are moving away from the person on the sidewalk, you are moving relative to that person. However, since you are not changing the distance between you and the bus or the bus driver, you are not moving relative to them.

Explanation

You are moving if your distance from a reference point is changing. Since you are moving away from the person on the sidewalk, you are moving relative to that person. However, since you are not changing the distance between you and the bus or the bus driver, you are not moving relative to them.