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A rubber ball, your family's minivan, and you.

by Bboy Physics
Tags: ball, family, minivan, rubber
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Bboy Physics
#1
Jan2-13, 10:38 PM
P: 9
Just to be clear, this is NOT a homework question. It's honestly a question I have pondered for as long as I can remember sitting on the backseat of my family's minivan with a rubber ball.

I'd also like to clear out that you bear with me and if I look unintelligent or questionably retarded clinically; I am a simple high school student and new to physics(yet very interested) and I am aware that many of the members are probably veterans themselves and would scoff at a question that might have a very simple answer. But bear with me

But let's say you were sitting in a minivan moving at the speed limit(for simplicity sake let's say 35mph but it is irrelevant) and say you were sitting in the back of the van. Now say you had a rubber ball, and you throw this ball into the air. What happens is that ball just falls back into your hand(assuming you shot it straight up). But why doesn't the ball just hit you in the face after you throw it into the air, since once you let go of ball, there is nothing to keep it moving at the same speed as you or the car?

For Einstein's box and person with a ball equivalency for his theory of relativity, Einstein said that if a box with a person in it holding a ball were to fall towards the Earth from its atmosphere at g, the person would be free falling. But if the ball were let go from the free falling person, the ball would also seem like it's floating and motionless. Now the reason why it does that is because of gravity: everything falls at the same speed(regarding air friction and other external variables) and so it's logical as to why the ball does not hit the inside of the top of the box, simply because it is falling at the same rate of the box and person due to gravity.

But what about the minivan, there is no force that moves the ball at 35mph forwards(anymore, since me holding the ball and I with my butt on the seat). So what causes it to make it so that it's going straight up and down?

To be clear I want to know the force that keeps the ball moving at the constant speed of the van despite nothing touching the ball after you throw it straight up.

The reason I put this in the GToR section is because of the box and person analogy; I didn't know where else to put it.
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Nugatory
#2
Jan2-13, 11:02 PM
Sci Advisor
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P: 3,428
Quote Quote by Bboy Physics View Post

But what about the minivan, there is no force that moves the ball at 35mph forwards(anymore, since me holding the ball and I with my butt on the seat). So what causes it to just look like it's going straight up and down?
The ball is moving at 35 mph while it's in your hands, right? You take your hands off the ball and it's still moving at 35 mph along with you and everything else in the minivan. There's no reason for it to slow down relative to you.

You could ask the same question about why the ball goes straight up and down again when you're just standing around outside. You aren't in a minivan moving at 35 mph... But you are standing on a planet that is rushing through space at many miles per second, and you don't even notice.

Your best bet would be to be to read up on Newton's three laws; that's the starting point for understanding all physics.
(and don't be surprised if the moderators move this thread into "general physics")
Nugatory
#3
Jan2-13, 11:06 PM
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Quote Quote by Bboy Physics View Post
To be clear I want to know the force that keeps the ball moving at the constant speed of the van despite nothing touching the ball after you throw it straight up.
No force is needed to keep something moving - force is only needed to change the speed

(or to cancel out some other force that is working to slow the object, such as friction.)

Bboy Physics
#4
Jan2-13, 11:07 PM
P: 9
A rubber ball, your family's minivan, and you.

Quote Quote by Nugatory View Post
No force is needed to keep something moving - force is only needed to change the speed

(or to cancel out some other force that is working to slow the object, such as friction.)
Ah! This helps deeply. Much appreciated! And the planet analogy helps as well!


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