# Spring constant and conservation of energy

1. Mar 17, 2009

### Knfoster

1. The problem statement, all variables and given/known data
The ball launcher in a pinbgall machine has a spring that has a force constant of 36 N/cm. THe surface on which the ball moves is inclined theta=10.1 degrees with respect to the horizontal. If the spring is initially conressed 4.25 cm, find the launching speed of a .120 kg bawhen the plunger is released. Friction and the mass of the plunger e negligible.

2. Relevant equations

PE=.5k(xf-xi)^2
KE=.5mv^2

3. The attempt at a solution
I'not sure what launching speed is supposed to be, it supposed to be an answer that doesn't require units. Does that mean I need to be finding the spring constant? If so, do I just set the KE=PE?

2. Mar 17, 2009

### LowlyPion

They give you the spring constant. 36 N/cm.

So think conservation of energy.

Spring potential depressed = kinetic energy at release point plus additional increase in gravitational potential energy at release point .

3. Mar 17, 2009

### Feldoh

The force constant is the spring constant. You're looking for a velocity.

Think about it if we compress a spring at some angle we are storing potential energy in the spring, but we also have to take into account gravity, does this help?

4. Mar 17, 2009

### Knfoster

It can't be simply velocity that I'm looking for because an answer not requiring units is needed. Any other ideas?

5. Mar 17, 2009

### Feldoh

I'm pretty sure it's got units -- it asks for the launching speed. Speed's have units of distance per time.

6. Mar 17, 2009

### Knfoster

I know... it should but, I'm doing homework through lon-capa and it's telling me that it doens't require units, so it can't be asking for velocity.

7. Mar 17, 2009