# Where is the force coming from to compress the spring

## Homework Statement

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

## Homework Equations

Work done by spring=0.5*k*x2
Work-energy theorem: total work done =change in kinetic energy

## 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.

#### Attachments

• Screenshot_2017-03-13-12-02-17-1.png
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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.

Lafith
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.
Then also why the mass moves...?

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

Lafith
haruspex
Homework Helper
Gold Member
2020 Award
plank suddenly started moving
This is the necessary assumption, not made clear in the question.

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

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

Lafith
It's not a terribly well written question--nothing about friction, for one thing--but I don't think anything else makes sense.
Ok then what is the work done by spring in this situation .is it negative or positive?

CWatters
Homework Helper
Gold Member
Is that the exact wording of the question?

There is no mention of it starting from rest?

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

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.