# Homework Help: General Physics 1

1. Mar 1, 2005

### Kp0684

General Physics 1....

A crate of mass 30.0kg rests on level surface. The coefficient of kinetic friction between the crate and surface is .400 . What force applied at an angle of 30.0 degrees below the horizontal (i.e. pushing down) is required to keep the mass moving at constant speed?......................on this problem i came up with the equation Fn= Uk[mgcos(30)]......with Uk=.400......Fn=102N......as the answer......iam i doing this right.....need help......please.

2. Mar 1, 2005

### scholzie

You made some erroneous assumtions. The normal force of the crate is not just $mgcos\theta$, if the angle is that of an applied force. This method assumes that the box is on an incline (in which case that would be true). This is a level surface, so the normal force is caused by the box's full weight (mg) and the force you apply ($F_y=F\cos\theta$) by pushing or pulling.

Force of friction: $f=\mu N$ with $N=mg+F\sin30$ and so $f=\mu\left(mg + F\sin30\right)$

To move at a constant speed, $F\cos30$ must equal the force of the friction trying to keep it from moving:
$$F_{x net} = F\cos30 - F_{friction} = ma =0$$

That should help.

PS, as an answer, I got close to $180N$ - I'll hold off on the exact answer I got until you've done the problem. :)

Last edited: Mar 1, 2005