# Plunger on compressed spring

• Thread starter awiest82
• Start date
• #1
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0

## Homework Statement

A plunger of mass m rests on a spring with constant k. The spring is pressed down a distance x and released from rest. How large does the the mass need to be in order to keep the plunger in contact with the spring?

PEf+KEf=PEi

kx-mg+N=0

## The Attempt at a Solution

Using the Conservation of energy equation and solving for m, I get:

kx2/(2gh+vf2) = m

I have a suspicion that I somehow have to incorporate the second equation into the answer, but am stumped as to how I proceed.

## Answers and Replies

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• #2
Filip Larsen
Gold Member
1,256
183
Perhaps you can find a way through by analyzing the acceleration of the spring-mass and consider what limits this must be within.

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