## Since Earth is spinning, why do we land in the same place when we jump or fall? (Intermediate)

I have a question that has been bothering me... I was on the bus earlier, and I was throwing an apple. The bus was moving, but the apple always fell back into my hand. Why didn't the bus move around the apple so that the apple landed further back?

That got me thinking, Earth is spinning around hundreds of meters per second, so when we jump, even if it is for a half a second, shouldn't we land many meters away? (Though we may crash into a building or something.)

I read a question that was about what would happen if Earth stopped spinning, and the answer said that anything that wasn't fixed to the ground would continue spinning, so we would crash against buildings and such. So if that would happen, why do we jump or fall and land pretty much in the same place?

Sorry it took me so long to respond to your question - hopefully it's not too late for an answer!

I guess I'll start out by talking about the bus part of the question. If you and an apple get on a bus, and the bus starts moving down the street, you and the apple will both have the same velocity as the bus. (That makes sense, right, because you're both moving along.) In order to stop something that's moving, you need to use a force to slow it down, just like you need to use a force to get things moving in the first place. (For example, see this previously asked question.) There's a physics law (Newton's first law of motion) which tells us that "objects in motion tend to stay in motion". So, when you throw the apple in the air while on the bus, it's already moving forward at the same speed as the bus, and there's essentially no force to slow down it's motion in this direction (assuming it doesn't bounce off the ceiling). Therefore, while it's in the air, the apple moves forward with you, the bus, and the other passengers, and it comes down in your hand.

It's the same deal with the Earth. We're all on the moving Earth, and we're travelling at the same speed as Earth. So when we jump up, we keep travelling around at the same speed we were moving at before because there's no force to stop us. Now, if a huge force was applied to the solid Earth (like a big impact) and caused it to stop spinning in a single instant, we'd be in trouble because the Earth would have stopped moving, but since no force was applied to us, we'd still be travelling at the same speed we were going before the impact (really fast). I guess if all the people were glued to the Earth, then the force of the impact would translate to us as well and we would slow down, but in reality we're free to fly forward.

I think a car accident is a good analogy for this. If you're travelling really fast down the road and the car stops very suddenly (like you hit something), then your body will fly forward because you had a forward velocity and will tend to stay in motion in that direction. If you're stuck to the car with a seatbelt, you'll stay in the car because the seatbelt exerts a force that holds you in place. But if you're not wearing a seatbelt you may well fly out of the car. Similarly, if Earth stopped really fast and we weren't held down, we would fly pretty fast. But as long as Earth is moving, we move around with it so that when we jump up, we're actually moving up and around at the same time such that we come down in the same place.

#### Lynn Carter

Lynn uses radar astronomy to study the planets, especially Venus. She got her PhD in Astronomy from Cornell in Summer 2004 and is now working at the Smithsonian in Washington D.C. on the Mars Express radar.