# Homework Help: What is force if acceleration is .75? (inclined plane)

1. Dec 10, 2011

### skysunsand

1. The problem statement, all variables and given/known data
A horizontal force F is exerted on a 20kg box to slide it up a 30 degree incline. The friction force retarding the motion is 80N. How large must F be if the acceleration of the moving box is to be .75 m/s^2?

2. Relevant equations
All the forces on the box in the x and y direction have to equal zero

F=ma
F-Ff-Fn = ma

3. The attempt at a solution

Using F-Ff-Fn,

F-80-20*9.8*sin(30) = 20*.75
F-178= 15
F= 193 N

Which isn't even close to what the answer is supposed to be, which is F=220 N

2. Dec 10, 2011

### Staff: Mentor

If the box is accelerating then there must be a net force. Newton's first law.

3. Dec 10, 2011

### skysunsand

How does that change my equations?

4. Dec 10, 2011

### Staff: Mentor

Draw a Free Body Diagram for the box. There must be a net force that produces an acceleration of 0.75m/s2 (acting up-slope). So what forces act along the slope?