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Physcis

An object X of weight 2w is on top of another object Y of weight w. They are both falling in vacuum. They don't move apart. They are in contact , as if a single object, and are falling. What is the force exerted by X on Y. The answer is 0. I get it its 0 because they are accelerating downwards with g. Any resultant force acting will cause the acceleration to increase. But they are in contact. Shouldn't there be a contact force?
A contact force is any type of force that requires contact to occur, but two objects being in contact does not itself bring about a pair of action/reaction force - there has to be relative motion or a tendency for it, absent in this case, given that the two objects have identical acceleration (g) and the same velocity at any given instant.

In a way it's comparable to e.g. two adjacent objects both sitting at rest on a planar surface. They could be touching but not exerting any force on each other.
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ryanhassan
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Original post by randombiochemist
A contact force is any type of force that requires contact to occur, but two objects being in contact does not itself bring about a pair of action/reaction force - there has to be relative motion or a tendency for it, absent in this case, given that the two objects have identical acceleration (g) and the same velocity at any given instant.

In a way it's comparable to e.g. two adjacent objects both sitting at rest on a planar surface. They could be touching but not exerting any force on each other.

But for example, a book is resting on a table. The book exerts a contact force on the table and vice versa. This occurs because they are in contact. In my example they are in contact. So why no contact force?
Original post by ryanhassan
But for example, a book is resting on a table. The book exerts a contact force on the table and vice versa. This occurs because they are in contact. In my example they are in contact. So why no contact force?


With the book-on-a-table scenario, gravity acting on the book would've caused it to accelerate towards the ground had there not been an upwards force from the table to balance it. The book stays at rest because the two forces are equal in magnitude.

In the free-falling objects example, it's a vacuum and each object experiences no force other than that due to gravity. As you've said, otherwise their acceleration wouldn't both be g.

I suppose another way to get one's head around this would be to imagine if one of the objects disappeared - would that affect the state of the other one? The table disappearing would cause the book to fall. But if either of X and Y disappeared, the other would still be free-falling at g.
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ryanhassan
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Original post by randombiochemist
With the book-on-a-table scenario, gravity acting on the book would've caused it to accelerate towards the ground had there not been an upwards force from the table to balance it. The book stays at rest because the two forces are equal in magnitude.

In the free-falling objects example, it's a vacuum and each object experiences no force other than that due to gravity. As you've said, otherwise their acceleration wouldn't both be g.

I suppose another way to get one's head around this would be to imagine if one of the objects disappeared - would that affect the state of the other one? The table disappearing would cause the book to fall. But if either of X and Y disappeared, the other would still be free-falling at g.

Thank you so much. I get it now! Do you mind chatting?
Original post by ryanhassan
Thank you so much. I get it now! Do you mind chatting?


No problem :smile: And not at all - hello - what did you want to talk about?
Reply 6
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ryanhassan
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Original post by randombiochemist
No problem :smile: And not at all - hello - what did you want to talk about?

How can we chat? Is there an inbox system in here like Facebook?

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