# Homework Help: Magnitude of the Force of Friction

1. Jan 11, 2010

### Helpme7534

A wooden crate with a mass of 20.0 kg is dragged across a floor at a constant speed by a force of 85.0N. The magnitude of the force of friction here is ____ N.

I'm not even sure how to start.

2. Jan 11, 2010

### Staff: Mentor

What do Newton's laws say about constant speed motion?

3. Jan 11, 2010

### Helpme7534

F=ma

4. Jan 11, 2010

### Helpme7534

no acceleration?

5. Jan 11, 2010

### stanton

If it is dragged with constant speed, yes. Try to draw a free body diagram and find forces acting on the object. Now let's see. I can first find two forces.
Hint: 'apple and Newton', and [the block is being moved by what?]

6. Jan 11, 2010

### Helpme7534

k i appreciate the help but i need to no how to do it,
f=ma yes i no that, but i dont no how to get force of friction

7. Jan 11, 2010

### Staff: Mentor

If there is no acceleration, what is the net force?

8. Jan 11, 2010

### Helpme7534

85n?

9. Jan 11, 2010

### stanton

no... you have information that there is no acceleration. so a=0 plug that in f(net)=ma. What is net force now?

10. Jan 11, 2010

### Helpme7534

0, so 85 N is the force of friction?

11. Jan 11, 2010

### stanton

Yes, zero. But Sorry, force of friction is not 85n. We are pulling this with 85n. If force of friction equals the force pulling, the object won't move. Take a look at your another post in which I submitted by opinion.

12. Jan 12, 2010

### Lsos

Whoa I think you're wrong there stanton...and Helpme7534 is right. Don't confuse the guy.

If force of friction equals the force pulling, the object will move if it was already moving.

13. Sep 24, 2010

### mmludin08

A Painter of Mass 70kg stands on a platform of mass 10kg and pulls himself with an acceleration of 10 cm/sec^2. He pulls each rope with a force F as shown below.

a). Draw a free body diagram for the painter.
b). Compute the value of F.
c). Compute the contact force which the painter exerts on the platform.

(sorry i couldnt draw the image. which is man pulling down the two ropes from two directions.)