# Using the conservation of energy find speed

## Homework Statement

A spring, having a force constant of 6.0x102 N/m, is held in a vertical position and compressed 0.30m. A 5.0 kg mass is then placed on top of the spring. THe mass is then releases. Neglecting air resistance and the mass of the spring

## The Attempt at a Solution

Ek1 = Ek2
1/2kx^2 = 1/2kx^2 + 1/2mv^2
1/2(600N/m)(0.3m)^2 = 1/2(600N/m)(0.2m)^2 + 1/2(5 kg)v^2
27 J - 12 J = (2.5 kg)v^2
2.45 m/s=V

Im confused if we need to add any sort of gravitational potential energy to the ball?

PhanthomJay
Homework Helper
Gold Member

## Homework Statement

A spring, having a force constant of 6.0x102 N/m, is held in a vertical position and compressed 0.30m. A 5.0 kg mass is then placed on top of the spring. THe mass is then releases. Neglecting air resistance and the mass of the spring

## The Attempt at a Solution

Ek1 = Ek2
1/2kx^2 = 1/2kx^2 + 1/2mv^2
1/2(600N/m)(0.3m)^2 = 1/2(600N/m)(0.2m)^2 + 1/2(5 kg)v^2
27 J - 12 J = (2.5 kg)v^2
2.45 m/s=V

Im confused if we need to add any sort of gravitational potential energy to the ball?
You need to consider both gravitational and spring potential energies and kinetic energy. Are you trying to determine the speed when the mass leaves the spring?

You need to consider both gravitational and spring potential energies and kinetic energy. Are you trying to determine the speed when the mass leaves the spring?

yes, the velocity when it has moved up 0.20 m from the compressed position on the spring.

PhanthomJay
Homework Helper
Gold Member
When it moves up 0.2 m, the spring is now compressed by how much? Don't forget the gravitational PE.

When it moves up 0.2 m, the spring is now compressed by how much? Don't forget the gravitational PE.

umm.... 0.1?

PhanthomJay
Homework Helper
Gold Member
Yes.

Yes.

so
Ek1 = Ek2
1/2kx^2 = 1/2kx^2 + 1/2mv^2
1/2(600N/m)(0.3m)^2 = 1/2(600N/m)(0.1m)^2 + 1/2(5 kg)v^2
27 J - 3 J = (2.5 kg)v^2
3.098 m/s=V

PhanthomJay
Homework Helper
Gold Member
What happened to the gravitational potential energy term you were inquiring about??

What happened to the gravitational potential energy term you were inquiring about??

umm what do you mean?

What happened to the gravitational potential energy term you were inquiring about??

Ek1 = Ek2
1/2kx^2 = 1/2kx^2 + 1/2mv^2
1/2(600N/m)(0.3m)^2 = 1/2(600N/m)(0.2m)^2 + 1/2(5 kg)v^2 + (5 kg)(9.8 m/s^2)(0.2 m)
27 J - 3 J - 9.8 J= (2.5 kg)v^2
2.38 m/s=V

PhanthomJay
Homework Helper
Gold Member
Looks good now!

looks good now!

thanks :d

Tell me who u are, u go to my school. DONT IGNORE ME

Tell me who u are, u go to my school. DONT IGNORE ME

? who r u?