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Physics help! :(

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I came across these two physics questions that I have no clue how to answer.

  1. A crate of mass 100 kg is at rest on a horizontal floor. The coefficient of static friction between the crate and the floor is .4, and the coefficient of kinetic friction is .3. A force F of magnitude 344 N is then applied to the crate, parallel to the floor. Will the crate move, and if so, WHY?

  1. Two crates are stacked on top of each other on a horizontal floor. Crate #1 is on the bottom, and Crate #2 is on the top. Both crates have the SAME mass. Compared to the strength of the force F1 necessary to push only Crate #1 at a constant speed across the floor, the strength of the force F2 necessary to push the stack at the same constant speed across the floor is greater than F1 because...?

Any help is greatly appreciated and thank you in advance!
Posted: January 2011 Permalink


1. You know that Force of friction = mu * normal force. If the friction force is greater than 344 N, then it won't move. If its less it will move.

2. To move just one box, its F = M*a and you need acceleration to move. a = F/M. For two boxes you have double the mass basically. so you have F = 2M*a. a = F/2M. Essentially the force that is needed to move two boxes is double the force needed to move one box.

@below. Same thing. If friction is greater it won't move, if its less it will move.
EDIT: I got what your saying now lol. I wrote it wrong.
Jan 06 2011
^ Don't you mean if the friction force is smaller?
Jan 06 2011
I never really cared about Physics other than the theory, everything else, like numbers and stuff, i didn't learn anything.
Jan 06 2011
1)A bit of a shot in the dark but the force of friction is equal to mhu(0.7)times the force or gravity(9.8 x 100) would be 686 N. Therefore 344 N is not enough force to move it. But as I said I'm guessing.
2)Probably twice the amount of force because you have 2 objects.
Jan 06 2011
Thank you for all your help guys! I get #1 now but I still don't understand #2...
Jan 06 2011
if im reading the question correctly... its harder to move box 1 compare to box 2, because box 1 has the force of gravity of box 2 pullin down on it as well so its 2 x mass x 9.8 m/s while box 2 only has to deal with its own pull towards gravity so its just mass x 9.8m/s?
Jan 06 2011

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