# I'm stumped

A 2.0 kg wood box slides down a vertical wood wall while you push on it at a 45 degree angle. What magnitude of force should you apply to cause the box to slide down at a constant speed? (Problem 51, Chapter 5, Knight)

I can't figure out this problem and do not have the text to refer to (alibris.com sent me the wrong book). I'm not having problems with FBD's. I tried to set the acceleration to zero so it travels at a constant speed but with zero acceleration there is zero force. I'm confused.

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Physics Monkey
Homework Helper
godplayer said:
I tried to set the acceleration to zero so it travels at a constant speed but with zero acceleration there is zero force. I'm confused.
This is exactly right, you want zero net force. How else can the box move with constant velocity?

There are four forces in the problem, the force of gravity, the normal force, the friction force, and your applied force. Resolve these four into components and find the magnitude of the applied force necessary to have zero net force vertically.

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Pengwuino
Gold Member
Well remember, with constant speed, there is zero NET force.

Oh. . . now I'm really confused.

That seems to easy!

I thought that if there was a zero NET force that the box would be at rest.

Physics Monkey
Homework Helper
If there is zero net force, the acceleration is zero. But if the body is already moving then it keeps moving by Newton's 1st Law.

Pengwuino
Gold Member
godplayer said:
I thought that if there was a zero NET force that the box would be at rest.
Nope, it would just be moving at the same speed its been moving. Remember newton's first law. What you must imply however is that the box is already moving down that 45 degree plane when you start actually pushing back

Thanks! That helps a lot!

I'm not sure how to find the normal force. I knoe its horizontal from the wall but I am unsure of how to find it. I need it to find the force of friction.

Physics Monkey
Homework Helper
You can find the normal by balancing the forces in the horizontal direction (remember the applied force has a component in that direction). The box certainly isn't accelerating away from the wall.

Yeah I know but there were know horizontal forces given in the problem.

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Physics Monkey