# Work, energy and power

1. Nov 29, 2009

### eureka_beyond

1. The problem statement, all variables and given/known data
the question and answers are in the attachment, I don't understand question 8b) (ii), why is the work done against friction =f x d and not 10N x d?

2. Relevant equations

3. The attempt at a solution

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• ###### 掃描0007.pdf
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2. Nov 29, 2009

### Staff: Mentor

For some reason, I cannot view those attachments. Can you describe the problem?
The obvious question is: What does the friction force equal?

3. Nov 29, 2009

### eureka_beyond

okay, the question goes like this:
on a rough horizontal Ground, a 3 kg block at rest is moved be a Force of 10N. The acceleration of the block is 2ms-2.
if the force ceases to act after 4 seconds,
what 's the further distance traveled by the block before it comes to rest?

4. Nov 29, 2009

### Staff: Mentor

That means the 10 N force is no longer applied.
What's the only force acting to slow down the block?

5. Nov 29, 2009

### eureka_beyond

that's got to be the friction, right?

6. Nov 29, 2009

### Staff: Mentor

Yes. Now use the given information to figure out the friction force. (Hint: Newton's 2nd law.)

7. Nov 29, 2009

### eureka_beyond

by Newton's 2nd law
F=ma
Let the friction be f
10-f=3(2)
f=4N

8. Nov 29, 2009

### Staff: Mentor

Good!

9. Nov 29, 2009

### eureka_beyond

yay! thx a million. but there's another thing, how do I get the further distance traveled by the block before it comes to rest?

10. Nov 29, 2009

### Staff: Mentor

Several ways:
(1) You can find the acceleration and use kinematics. (First figure out how fast the block is moving at the end of 4 seconds.)
(2) You can set the work done by friction equal to the change in kinetic energy.

11. Nov 29, 2009

### eureka_beyond

ah, I see. The book must have been using the second way you describe, I get it now, thx a lot.