# Magnitude of the Force of Friction

1. Jan 11, 2010

### Helpme7534

1. The problem statement, all variables and given/known data
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.

2. Relevant equations

3. The attempt at a solution

2. Jan 11, 2010

### rl.bhat

According to Newton's first law, when a body moves with constant velocity?

3. Jan 11, 2010

### Helpme7534

? no acceleration?

4. Jan 11, 2010

### rl.bhat

Can you state the Newton's first law?

5. Jan 11, 2010

### jegues

Correct, now expand on this using Newtons 2nd law.

6. Jan 11, 2010

### jegues

I'm confused, constant velocity should mean that there is 0 acceleration?

7. Jan 11, 2010

### Helpme7534

newtons first law is pretty much intertia, an object in motion tends to stay in motion, its 11:00pm my time and ive been struggling with it for a while, kinda frustrated so i no its rude but if u dont mind i need a lil fast help

8. Jan 11, 2010

### jegues

In the absence of a net force, a body either is at rest or moves in a straight line with constant speed.

Apply this knowledge with Newton's 2nd law to draw your conclusion.

$$\sum F = ma$$

9. Jan 11, 2010

### Helpme7534

k heres what i got

F=ma
85N=20kg(a)
a=4.25 m/s2

I dont no how to get force of friction

10. Jan 11, 2010

### jegues

You told me before that with constant velocity, acceleration was 0. You should be able to conclude that,

$$\sum F = 0$$

Now simply choose a coordinate system for your x axis (which way is positive which way is negative) and assign each horizontal force the proper sign and solve for the force of friction.

You need to review your understanding of Newtons laws.

11. Jan 11, 2010

### stanton

And mind you that friction force acts the opposite direction from the force acting on the object. i.e., if you move a block to the right, friction force act to the left. So, friction force would have minus sign.

12. Jan 11, 2010

### Helpme7534

well could the answer be a negative number then? I need a 3 digit answer

the second part of the question is

The coefficient of friction here is a.bc x 10 ^-d
The values of a, b, c, and d respectivly are ____

13. Jan 11, 2010

### stanton

Sorry, I might have confused you. What I want to say is that find all force act on the object, use net force equation(f=ma=0, which I told you in another post). net force will be
total force-friction force because friction acts the opposite. That is why I mentioned minus sign. I hope I am going right. Could someone check this for me?
Net force=total force-friction force = 0

14. Jan 11, 2010

### Helpme7534

wow so confused so my final answer for the first part will be zero? and for the seconds part it will be zero??

15. Jan 11, 2010

### stanton

No. Now take a closer and careful look at the eqation I gave it to you.
total force-force of friction=0.
This does not imply force of friction is zero.
Equate this eqation we get
force of friction=total force.
now find WHat force acts on the object as I told you before?
we find gravity and pulling force.
so, mg+pulling force=total force
(weight of the object)(9.8)+85n=total force.

And I am so sorry. Here I am staying right now is 12:30 in the evening. And visit this website. It has specific information that will help you to find answer. There it tells you how to find the total force too.
http://www.physics247.com/physics-homework-help/friction-and-weight.php

Last edited: Jan 11, 2010
16. Jan 11, 2010

### Helpme7534

so 281.2 is the total force but where do i go from there?

17. Jan 11, 2010

### jegues

This is incorrect. We're applying Newton's 2nd law in a 1 dimensional motion.

18. Jan 11, 2010

### Helpme7534

lets clarify i only have physics 20-1, and i really need help with this questions, can someone please show me the formulas to use and how to fill them in? it would be greatly appreciated