Homework Help: Crates and pulleys

1. Sep 10, 2006

mikefitz

A 50-kg crate is suspended between the floor and the ceiling using two spring scales, one attached to the ceiling and one to the floor. If the lower scale reads 121 N, what is the reading of the upper scale? Ignore the weight of the scales.

So, I convert 50kg to N and get 490
Then I subtract 121 from 490 and get 369 N

Why isn't the reading on the ceiling 369; wouldn't that make sense? Thanks

2. Sep 10, 2006

Chi Meson

Why did you subtract 121N? The lower scale is not supporting the crate, it's pulling down on it.

3. Sep 10, 2006

mikefitz

I thought the two readings would have to equal the total mass of the crate? My understanding of pulleys is really lacking, and these notes given to me from my teacher make little sense (he is not the best instructor and has failed to give me a good explanation for them)

4. Sep 10, 2006

Chi Meson

Those notes don't match the problem you describe. Where the heck did the pulleys come from? According to the question in the OP, there are two spring scales, one aboved and one below. Take away the word "scale" and you have two springs. ONe pulls up, one pulls down. The one above will have to pull up with the force equal to the weight of the crate plus the amount by which the lower spring pulls down.

The notes are for "Atwood's" machine, that's a different problem.