# Hookean systems

1. Sep 15, 2007

### phillyj

hi everyone,
I'm stuck on a problem that uses Hook's law but it's not straight forward. What must i do when there is an object pulled by an another object and connected with a spring. do i have to combine the force due to gravity on each of the object? can someone tell me a reference site for this. thanks

Last edited: Sep 15, 2007
2. Sep 16, 2007

Could you provide a sketch? Start with a free body diagram. Gravity is acting on every body with non-neglible mass, right? That's a start point.

3. Sep 16, 2007

### phillyj

heres the diagram. the bigger mass is connected to a small one by a spring with (k) given. A force pulls on the larger mass and the spring compresses a certain length from equilibrium.

http://img529.imageshack.us/img529/8886/physicio4.th.jpg [Broken]

The problem is this: 2 crates, 640 kg and 490kg, are connected by a spring with k=8.1 kN/m and it's propelled along a level frictionless plane by a horizontal force of 15 N. The spring compresses 5.1 cm, what is the applied force?

I have no clue where to start.

Last edited by a moderator: May 3, 2017
4. Sep 16, 2007

### phillyj

anyone even understand how we would go about doing this?

5. Sep 16, 2007

### phillyj

would a combination of F1m1=F2m2 and hook's law help?

6. Sep 17, 2007

### dynamicsolo

You'll want to start by considering the horizontal forces acting on each crate. (Since there is no friction between the crates and the surface they slide on, we don't really need to know about the vertical forces, as we'll have no use for that information.)

I might note that if the spring is being compressed, the crates are presumably being pushed from behind; were they being towed from the front, the spring should be stretched. I don't think this changes the result, however.

Last edited by a moderator: May 3, 2017