Another energy question, sorry.

[Matt1234]

Homework Statement

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?

Homework 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

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. I am confused.

 

[Matt1234]

I found a link to the same problem on this forum. It doesn't make sense to me yet it yields the correct answer:
https://www.physicsforums.com/archive/index.php/t-176588.html


Where is Eg while all of this is happening? They miss Eg entirely??
They give you a vertical displacement yet no Eg? hmmm

 

[Matt1234]

Eg is at cos 90 = 0

I Suck.

 

[tiny-tim]

Hi Matt1234!

Where is Eg while all of this is happening? They miss Eg entirely??

Eg won't 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.

 

[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

 

[tiny-tim]

You no longer suck!

 

[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.