How to Calculate the Minimum Force Needed to Move a Box on a Ramp with Friction?

[darlafelicity]

A box with a mass of 3.00 kg is at rest on a ramp that is at an angle of 20 degrees with the horizontal. The coefficient of static friction between the box and the ramp is 0.700. Use g = 9.80 m/s2. You now want to make the box move by applying a force. To start the box moving, what is the minimum force you need to apply if your force is directed ...

(a) perpendicular to the slope?

(b) horizontally?

 

[darlafelicity]

i got part (a) but I'm not sure why it works. mgcos20(mgsin20/static coefficient)

i still don't know how to do part (b) though.