How to Calculate the Speed of a Block Released from a Spring

[robertmatthew]

Homework Statement

A block of mass 1.20 kg is placed in contact with a spring with a force constant of k = 2.20 N/m. When released, the spring pushes the block and expands a distance of 0.420 meters. If the floor is frictionless, the speed of the block as it leaves the spring is ? m/s.

Homework Equations

Elastic energy is Ue = .5kd²
Energy in a block-spring system is E = .5kx² + .5mv²

The Attempt at a Solution

Well the problem is I haven't been able to get anywhere, because in my obviously incorrect thinking, if the block has yet to be released, then all the energy is potential, so E = Ue, but if E = Ue, then .5kx² = 0, which can't be right. So, in short, I really don't know what I'm doing here. Am I using the wrong equations or something?

 

[PhanthomJay]

Homework Statement

A block of mass 1.20 kg is placed in contact with a spring with a force constant of k = 2.20 N/m. When released, the spring pushes the block and expands a distance of 0.420 meters. If the floor is frictionless, the speed of the block as it leaves the spring is ? m/s.

Homework Equations

Elastic energy is Ue = .5kd²
Energy in a block-spring system is E = .5kx² + .5mv²

The Attempt at a Solution

Well the problem is I haven't been able to get anywhere, because in my obviously incorrect thinking, if the block has yet to be released, then all the energy is potential, so E = Ue, but if E = Ue, then .5kx² = 0, which can't be right. So, in short, I really don't know what I'm doing here. Am I using the wrong equations or something?

Wecome to PF!
When the block is not yet released, the spring is compressed by 0.420 m. So its initial PE is not 0.

 

[robertmatthew]

I just solved it, but I think my problem was that I was calculating the initial potential energy while also calculating the final kinetic energy, but still using E = PE, so I ended up with zero on one side and .5mv^2 on the other. I'm not really sure why I did Kf and Ui at the same time, I guess I'm having an off day, haha. But thank you!