# Where is the force coming from to compress the spring

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1. Mar 13, 2017

### Lafith

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

Given a plank is moved with uniform velocity v. To which a mass is connected by a spring of force constant k. What is work done by the external agent on the maximum compression of the spring
2. Relevant equations
Work done by spring=0.5*k*x2
Work-energy theorem: total work done =change in kinetic energy

3. The attempt at a solution
in order to compress a spring there has to be a force.but since it is given plank is moving with uniform velocity.where is the force coming from?
Since I couldn't figure or understand that I couldn't proceed further.

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2. Mar 13, 2017

### John Park

The screenshot makes it a bit clearer. The "plank" is moved with constant velocity--but what about the mass, m? It seems we have to neglect friction.

3. Mar 13, 2017

### Lafith

Then also why the mass moves...?

4. Mar 13, 2017

### John Park

What would happen to the mass if there was no spring, and no friction, and the plank suddenly started moving at velocity v0?

5. Mar 13, 2017

### haruspex

This is the necessary assumption, not made clear in the question.

6. Mar 13, 2017

### John Park

It's not a terribly well written question--nothing about friction, for one thing--but I don't think anything else makes sense.

7. Mar 14, 2017

### Lafith

Ok then what is the work done by spring in this situation .is it negative or positive?

8. Mar 14, 2017

### CWatters

Is that the exact wording of the question?

There is no mention of it starting from rest?

9. Mar 14, 2017

### John Park

Try to visualise what will happen if the whole system is at rest and then someone starts pulling the "plank". As I said, it might help if you start by imagining there's no spring and the mass is on a very slippery surface.

Also note that the question talks about "compression" of the spring.