# Sliding block- thermal energy - please

1. Nov 4, 2007

### mshah3

1. The problem statement, all variables and given/known data
You push a block of lead across the floor, applying a horizontal force of 9 N. The block moves at a constant speed. The mass of the block is 1.5 kg and the heat capacity of lead is 130 J/(kg K). After pushing the block a distance of 20 m, you measure its temperature and find that it has increased by 0.3 K.
(a) What is the change in thermal energy of the block?
(b) How much work is done by the frictional force of the floor acting on the block?
(c) What is the change in the thermal energy of the floor?

2. Relevant equations

Etherm = mC deltaT

Etherm = Force * delta r = W

3. The attempt at a solution

PART A : i was thinkin mCdeltaT

Part B: either -Fd or could be zero since the force is perpendicular to motion

Part C: work minus etherm maybe

****need help asap*****THANKS

2. Nov 4, 2007

### PhanthomJay

yes
explain why you think the force is perpendicular (or parallel?) to the motion

****need help asap*****THANKS[/QUOTE]

3. Nov 4, 2007

### mshah3

well the block is moving to the right
and the work by floor is moving upward
making it perpendicular to motion

(honestly, part b and c are the ones im very unsure about)

thank you

4. Nov 4, 2007

### PhanthomJay

Oh, OK, there are two forces exerted by the floor on the block; the upward normal force of the floor on the block, and the friction (leftward) force of the floor on the block (acting parallel to the floor). The motion of the block is to the right. Using the definition of work, frcostheta, note that the normal force does no work,(since costheta =0), and only the friction force does work. How much work does it do?

5. Nov 4, 2007

-Fd ?

6. Nov 4, 2007

### learningphysics

The total change in energy is mc*deltaT.

So the work done by friction + the work done by the 9N force = mc*deltaT

7. Nov 4, 2007

### mshah3

oh ok that makes sense

so Wfric = mct - mgh

then would the etherm for floor be zero?

8. Nov 4, 2007

### learningphysics

not mgh... the displacement is not vertical... what is the work done by the 9N force... what is the definition of work?

9. Nov 4, 2007

### mshah3

well i meant change in distance

work = mgdetlar

10. Nov 4, 2007

### mshah3

or wait no ... Fd

11. Nov 4, 2007

### learningphysics

yes, exactly.

so:

Wfric = mct - Fd

12. Nov 4, 2007

### mshah3

alright, so how would i get to the etherm of the floor?

13. Nov 4, 2007

### learningphysics

What did you get as the work done by friction?

Note that friction is the work done by the floor on the block.

The only object doing any work on the floor is the block...

if x is the work done by the floor on the block... what is the work done by the block on the floor?

14. Nov 4, 2007

### mshah3

negative x

15. Nov 4, 2007

### learningphysics

exactly. what numbers do you get?