# Positive and negative signs on Work

1. Oct 18, 2015

### bigsmile

1. The problem statement, all variables and given/known data
An amount of work equal to 1.5 J is required to compress the spring in a spring gun. ?

2. Relevant equations
Wtotal = change of KE

3. The attempt at a solution

W=ΔKE=(1/2)m(Vf² - Vi²)

It sounds like the Vf >0 and would be maximum after the spring is compressed hard and get ready to launch the marble. Hence, the marble will have the maximum launch speed. In that case, Vi of marble = Vf of the spring.

Vi of the spring should be zero. It starts from rest before compression.

But how is the Vi and Vf of the spring help me find the "launch Speed " of the marble?

I may not understand the process of energy transfer and force interaction between the marble and spring. Quite confusing!

I am lost and am unsure where my reasoning is wrong. Need help!

2. Oct 18, 2015

### Staff: Mentor

What is your understanding of the force interaction between the marble and the spring? Do you know the equation for the force a spring exerts in terms of the spring constant and the displacement of the spring from its unextended length? Can you draw a free body diagram of the marble, showing the forces acting on it while it is in contact with the spring? Based on your free body diagram, can you write a force balance equation for the marble?

Chet