# General Physics- Frictional Forces on Objects

1. Aug 7, 2012

### Physics-Pure

1. The problem statement, all variables and given/known data
A block of mass 3 kg, which has an initial
speed of 9 m/s at time t = 0, slides on a
horizontal surface.
Find the magnitude of the work that must
be done on the block to bring it to rest.
I already got 121.5 for this problem

If a constant friction force of magnitude
4 Newtons is exerted on the block by the surface, ﬁnd the magnitude of the acceleration of
the block.

How far does the block slide before it comes
to rest?

A team of dogs drags a 94.8 kg sled 2.63 km
over a horizontal surface at a constant speed.
The coeﬃcient of friction between the sled
and the snow is 0.228.
The acceleration of gravity is 9.8 m/s^2
Find the work done by the dogs.

Find the energy lost due to friction.

2. Relevant equations

F = μmg

W = Fd

W = μmgd

3. The attempt at a solution

I got the first third of problem one, but I don't know how to account for the frictional force.

For the second problem, I don't know how to account for the force of gravity.

Thanks anyone,
Physics-Pure
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Aug 7, 2012

### tiny-tim

Welcome to PF!

Hi Physics-Pure! Welcome to PF!

(try using the X2 button just above the Reply box )
yup!
you're told what the frictional force is, so just use F = ma
it comes in your own equation F = µmg

3. Aug 7, 2012

### Physics-Pure

Thank you very much :D

Now, what about part three of question one, and part two of question two?

4. Aug 7, 2012

### tiny-tim

come on, you know the rules

show us how far you've got!

5. Aug 7, 2012

### Physics-Pure

I don't know how to begin them. D:

6. Aug 7, 2012

### Staff: Mentor

Please summarize what you have so far, and what you do not understand. We certainly will not do your work for you, but perhaps if you summarize what is confusing you, we can offer a hint...

7. Aug 7, 2012

### Physics-Pure

Well the problem is, is that I'm skipping honors physics at my high school and taking AP Physics as a Freshman. However, I am required to complete honors physics review work. Therefore, I have no physics knowledge whatsoever, and do not know how to begin the third part of question 1 and the second part of question two.

8. Aug 7, 2012

### Staff: Mentor

That doesn't help. Please summarize the questions that you are having trouble with. It's not clear where the question numbers are...

9. Aug 7, 2012

### Physics-Pure

Those are the two sections to which I am referring.

10. Aug 7, 2012

### Staff: Mentor

If I understand where you are, you have the energy required to bring it to rest, yes? What is the equation relating work (energy), force and distance?

11. Aug 7, 2012

### Physics-Pure

W = Fd. I have the answers to parts one and two of the first question, and part one of the second question.

12. Aug 7, 2012

### Staff: Mentor

That's the right equation. You already calculated the work, and you know the force...

13. Aug 7, 2012

### Physics-Pure

Alright, thank you. Now moving onto the second part of question two?

14. Aug 7, 2012

### Staff: Mentor

Yes, please do. Re-read the full question, and see if you can figure it out for yourself. Think about where energies are coming from and going...

15. Aug 7, 2012

### Physics-Pure

I know that the work done by the dogs was 557.089 KJ, but I don't know how to find the energy lost due to the friction coefficient.

16. Aug 7, 2012

### Staff: Mentor

Was the sled accerating or moving at a constant speed? If there is no acceleration, what did all that dog work go into doing?