# Homework Help: Hooke's Law- measurement of mass/weight

1. Sep 14, 2014

### thennathalie

A body of unknown mass hangs from a spring and stretches it to 50 cm. If the spring constant is 104 N/cm determine:

a. the weight of the body
b. the mass of the body

k = mg/∆L
W= Mg

mass : k = mg/∆L 50/104= .4807692308
weight : W= Mg .4807692308*9.8=4.711538462

2. Sep 14, 2014

### Staff: Mentor

In your attempt to find the mass you've calculated ΔL/k, which is not correct. Your Relevant Equation says k = Mg/ΔL. Rearrange that equation to solve for M. Note that there's a g in there!

Since Mg is the weight and Mg appears in your first Relevant Equation, you can find the weight first by rearranging the equation to place Mg on the left.

3. Sep 14, 2014

### Staff: Mentor

Hi, and welcome to the Physics Forums.

4. Sep 14, 2014

### thennathalie

Okay, so what you're telling me is that the equation will be Mg= k/∆L
and not k=Mg/∆L/.

using the new equation:

Mg= k/∆L
Mg= 104/50 which gives me 2.08
Mg= 2.08

so the weight would be 2.08?

5. Sep 14, 2014

### thennathalie

My unit for weight is N.
And for Mass is Kg.

I have not, didn't think about that. Thanks

6. Sep 14, 2014

### Staff: Mentor

Ahhh. Nope :shy:

Hooks law is usually presented as (ignoring the sign implying the direction of the force):

F = k ΔL

Where F is the force applied to the spring, ΔL the change in its length.

In your case the force is a weight due to gravity, Mg, so that Mg = k ΔL

k = Mg/ΔL

This is just an algebraic rearrangement of the same formula. You need to be able to perform these manipulations of the formulas in order to be able to solve for whatever unknown values are buried in the formula

7. Sep 14, 2014

### thennathalie

Thanks for keeping up with me.
I am new to physics.

That means i would have to multiply 104*50 = 5200?

8. Sep 14, 2014

### Staff: Mentor

Yes, that's what the formula says. What are the units?

9. Sep 14, 2014

### thennathalie

Would be 5200 N because is weight.

So for the mass,

I would use the W=Mg formula.

5200=m9.8
M=530.6kg

10. Sep 14, 2014

### Staff: Mentor

Yup. Looks good.

11. Sep 14, 2014

### thennathalie

☺️ thank you so much.