# Another energy question, sorry.

1. Jul 11, 2009

### Matt1234

1. The problem statement, all variables and given/known data
A horizontal spring, of force constant 12 N/m is mounted at the edge of a lab benchto shoot marbles at targets on the floor 0.93 m below. A marble of mass 8.3 x 10 -3 Kg is shot from the spring, which is initally compressed a distance of 4.0 cm. How far does the marble travel horizontally before hitting the floor?

2. Relevant equations

Eg = mgh
Ee =0.5 k x^2
Ek= 0.5 m v^2

time to hit floor
d = 0.5 (g) t^2

and Vx = d / t

3. The attempt at a solution

Ek = Eg+ Ee

then find t then use t to find d, that gave me 1.97 m but the answer in the book was 0.66 m. This may be due to the fact that i do not know the launch angle and could not incorporate it into my calculation. Im confused.

2. Jul 11, 2009

### Matt1234

3. Jul 11, 2009

### Matt1234

Eg is at cos 90 = 0

I Suck.

4. Jul 11, 2009

### tiny-tim

Hi Matt1234!
Eg wont help you at all …

The energy equation will only give you the KE when it hits the floor, but that doesn't help since you don't know the angle at which it hits the floor.

Forget Eg (=mgh) …

you know the initial speed, and you know it's horizontal, so go from there, using the standard constant acceleration equations.

5. Jul 12, 2009

### Matt1234

Thank you for your help, i got it.

Et = Ee

then used Ee to find the velocity
then found time to hit the floor.

then found d

6. Jul 12, 2009

### tiny-tim

You no longer suck!

7. Jul 12, 2009

### Matt1234

Hehe, well this is my second time taking the course, hopefully i can get a good enough mark to get into the program i like.