What is the spring constant for this spring?

[jannx3]

Homework Statement

A mass of 50.0 g mass is hung from a spring, spring is elongated to 40 cm, an additional mass of 20g is added, with a new reading of 45cm

a) what is the spring constant for this spring?
b) another mass of 30g is added to the spring, what is the new reading?

Homework Equations

k= mg/x

The Attempt at a Solution

not sure if this is the right approach
a)
k= change in mass x g / change in height
0.020x9.8/0.05 =3.92n/m
b)mg=kx
0.03x9.8=3.92x
x=0.075 so it extend another 7.5cm?

 

[NascentOxygen]

not sure if this is the right approach
a)
k= change in mass x g / change in height
0.020x9.8/0.05 =3.92n/m
b)mg=kx
0.03x9.8=3.92x
x=0.075 so it extend another 7.5cm? ✔

Use a capital "N" for Newtons.

 

[Chestermiller]

The numbers in your solution don't seem to match the numbers in the problem statement.

Chet

 

[BiGyElLoWhAt]

why not just set up a N2L equation for this? If the spring stretches an amount x with a mass m attached to it and comes to rest again, then that means that net force is zero, and mg = kx, you only need 1 weight and 1 distance.

 

[Doc Al]

The numbers in your solution don't seem to match the numbers in the problem statement.

They look fine to me.

A mass of 50.0 g mass is hung from a spring, spring is elongated to 40 cm,

I interpret the 40 cm as the total length of the spring, including its unstretched length.

 

[BiGyElLoWhAt]

Ahhh, I see what you're saying, so you do actually need two masses. Good call Doc.
elongated to 40 cm