# Spring Constants/Work And Energy

1. Feb 17, 2008

### eann595

I'm having a bit of a problem with these homework questions, if anyone could help out that would be great!!

1. A 15 kg mass is attached to a massless spring by a light string that passes over a frictionless pulley. The spring has a force constant k= 500 N/m and is unstretched when the mass is relased. What is the velocity when it has fallen a distance of .30 m?

2. A 0.75 kg sphere is dropped through a tall column of liquid. When the sphere has fallen a distance of 2.0 m, it is observed to have a velocity of 2.5 m/s
a. how much work was done by the frictional force exerted on the sphere by the liquid?
b. what is the average force of friction during the displacement of 2.0 m?

2. Feb 17, 2008

### _Mayday_

PF usually asks that you show an attempt first.

3. Feb 17, 2008

### Staff: Mentor

In both problems, analyze what happens to the mechanical energy.

4. Feb 18, 2008

### sblinguas916

I'm struggling with the same questions as eann595, but I attempted to work them out so please help!:

1.) a 0.75 kg sphere is dropped through a tall column of liquid. When the sphere has fallen a distance of 2.0 m, it is observed to have a velocity of 2.5 m/s.
a.) How much work was done by the frictional force exerted on the sphere by the
liquid?

* I need the net work of friction, so I started with Wnet(Fr) = change in KE = ma.
* Fr x cos(180) x d = 1/2mv^2 = ma.
* Fr x -1 x 2.0m = 1/2(0.75kg)(2.5m/s)^2 = (0.75kg)(a)
* -2.0m x Fr = 2.34 kg x m squared/sec squared = (0.75)(a)

I have a feeling that b/c acceleration is not given, it equals g aka 9.8 m/s^2 (??) Am I on the right track with this problem?

b.) What is the average force of friction during the displacement of 2.0m?

This one I think I need to know the answer to letter a.) to solve.