Homework Help: Ideal Spring compression

1. Dec 10, 2004

king_naeem

An ideal spring is used to fire a 15.0-g block horizontally across a frictionless table top. The spring has a spring constant of 20 N/m and is initially compressed by 7.0 cm. The speed of the block as it leaves the spring is:

i do the following:

energy stored by spring: (kv^2)/2= 0.049

when the spring is at equilibrium position all the energy wwould be in kinetic thus i solve for the velocity,

but i get the wrong answer...what am i doing wrong?

2. Dec 10, 2004

ZapperZ

Staff Emeritus
Er... if you meant by "v" above as "velocity", then you need to double check the expression for the potential energy of a compressed spring. Hint: it involves the compressed LENGTH, and not dependent on any velocity.

Zz.

3. Dec 10, 2004

HallsofIvy

It's hard to tell what you are doing wrong since you don't show us HOW you solve for velocity!

The potential energy stored in the spring, with spring constant 20 N/m, by compressing it 0.07 meters is (1/2)(20)(0.07)2= 0.049 Joules.

Once the spring has "uncompressed", all of its potential energy goes into the kinetic energy of the block so (1/2)(0.015kg)v2= 0.049 and v= 2.56 m/s approximately.

4. Dec 10, 2004

king_naeem

aah...my problem was that i forgot to convert 15 grams into kilograms...ahh!!