How Do You Calculate Acceleration with Friction Involved?

[bship810]

Homework Statement

A person pushes horizontally with a force of 280 N on a 40 kg crate to move it across a level floor. The coefficient of kinetic friction is 0.30. the magnitude of the frictional force is 118.44. What is the magnitude of the crate's acceleration?

Homework Equations

Fn=ma
fk=uk*Fn

The Attempt at a Solution

I figured out the first problem which was finding the magnitude of frictional force. I think I have to take 280N-118.44N for some reason. Thats just a gut feeling though.

Im just not totally sure which equations to use and what order to use them

Help would be great

 

[D H]

Don't just guess. Draw a picture. In which directions are the two horizontal forces applied to the crate?

 

[bship810]

ok, so the frictional force is countering the person's force. that would mean there is a 161.56N force still remaining pushing the box forward.

How do get acceleration from this? a=Fn/m? so that would give me" a=(161.56)/(40). thus a=4.04. is this right?