# Mass And Spring

1. Oct 21, 2004

### ElectricMile

A 7.00 kg mass hanging from a spring scale is slowly lowered onto a vertical spring, as shown in the figure.

I got the first part, What does the spring scale read just before the mass touches the lower spring? wich was, 68.6 N

Dont understand how to approch, The scale reads 20.0 N when the lower spring has been compressed by 1.50cm . What is the value of the spring constant for the lower spring?....i know im trying to find k

and also, At what compression length will the scale read zero?

wouldnt this be when the mass is all the way on the spring?

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2. Oct 21, 2004

### ElectricMile

anyone got any help?

3. Oct 21, 2004

### Pyrrhus

Do the forces analysis and Remember Hooke's Law $F = kx$ or in Vectorial form $\vec{F} = k \vec{x}$

4. Oct 21, 2004

### ElectricMile

still wondering, i know F=kx, F, being 68.6 and x = 1.5 cm, right? but that k isnt the right answer. i must be doing it wrong, but what?

5. Oct 21, 2004

### Pyrrhus

You know springs has a restorative force that will restore the spring back to its equilibrium position.

6. Oct 21, 2004

### Staff: Mentor

It doesn't seem that you've made use of the given information: that the scale reads 20 N. If the scale reading is 20 N, what must be the force the spring is exerting on the mass?

7. Oct 22, 2004

### ElectricMile

ALRIGHT, heres what i did, mupltipled, 68.6*48.6=3333.4 put that in for my homework online and it says i have a rounding error, i tried 5 diffrent things and it says the same thing, now i have 2 trys or i get a 0, anyone know whats wrong?

Last edited: Oct 22, 2004
8. Oct 22, 2004

### Staff: Mentor

I have no idea what you are up to with that calculation! Looks like you randomly multiplied two numbers.

But... if the scale reads 20 N, that means the spring must be providing 68.6-20 = 48.6 N of force. The amount of compression associated with that spring force is given as 1.5 cm. Use F = kx to find k. (Careful of units!)