# Homework Help: Work, Energy, Displacement

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1. Feb 16, 2015

### Joe Armas

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

In the pictures.
2. Relevant equations
KE = 1/2mv^2.
PE = 1/2kx^2
Work external = Delta KE + Delta PE
W = F* d

3. The attempt at a solution
In the pictures.

On Problem 3 ai, I am not sure whether I have identified the correct horizontal forces. The hint gave two, but I believe there could also be a force that the wall exerts on the spring.

I am also not sure which way the force points when a spring is stretched and extended. I believe it is in the direction of the equilibrium unextended position. Please let me know if my answers/reasoning is correct. Thank you.

Last edited: Feb 16, 2015
2. Feb 17, 2015

### haruspex

They want you to identify the forces acting on the SB system from outside that system. Is the force between spring and block such a force?

3. Feb 17, 2015

### Staff: Mentor

Bear in mind that forces always occur in pairs: an action and an equal and opposite reaction. You can't exert a force on anything if it doesn't push back.

4. Feb 17, 2015

### Joe Armas

I see, so the force on the spring by the block is equal and opposite to the force by the block on the spring?. So is what I have for 3 a i correct?

5. Feb 17, 2015

### Joe Armas

No, correct? The only force that I can see is the wall.

6. Feb 17, 2015

### haruspex

That's right, the external forces are from string and wall.

7. Feb 17, 2015

### Joe Armas

The wall does zero work while the String does positive work right? Since displacement and force is in the same direction?

8. Feb 17, 2015

### haruspex

Yes.

9. Feb 17, 2015

### Joe Armas

Ok thanks for the help. Do you see any discrepancies on the back side?

10. Feb 17, 2015

### haruspex

Have another think about a iv. This is the work done by external forces on SB.

11. Feb 17, 2015

### Joe Armas

It should be positive right? The string does positive work while work does negative work. But why is the work net external not equal to delta potential energy here?

12. Feb 17, 2015

### Joe Armas

What situations can we use conservation of energy for net external work?

13. Feb 17, 2015

### haruspex

No, I'm suggesting you need to distinguish between work done on a system and and work done by a system. They have opposite signs.

14. Feb 17, 2015

### Joe Armas

The system gains potential energy because the spring is stretched. Isn't Wnet external equal to - delta PE? So it would be indeed be negative? That was my reasoning and it makes sense.

15. Feb 17, 2015

### lightgrav

system gains PE because spring is stretched (spring PE is always positive, like KE is always positive).
but no, Wnet,external (positive) is where that Energy comes from that is stored in the spring as PE (positive).

16. Feb 17, 2015

### haruspex

Work done on the system equals gain of work by the system. Both positive in this case.