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AS Physics question, please help!!!

Calculate how much extra work must be done in order to stretch a spring from 17cm to 20cm if its spring constant is 300Nm^−1 and natural length 15cm.

I am really struggling with this physics question :frown:
So this what I came up with so far.

I calculated force,
F=300 X 5X10^2 N= 15N

Then I did,
EPE=1/2 X 15 X (3X10^2) J= 0.225J

But this website keeps telling me it is wrong :s-smilie:
Reply 2
I might be wrong but this is my attempt:

There are two extensions taking place here - firstly from natural length 15x10^-2m to 17x10^-2m - ΔL=2x10^-2
- second from the first extension 17x10^-2m to the new extension 20x10^-2m - ΔL=3x10^-2, however we want the extension from natural length which is 5x10^-2
The question asks for the extra work done so I think you need to work out the difference between the two stretches from natural length, which would be as follows:
Stretch 1: F=kΔL=300x2x10^-2=6N
Stretch 2: F=kΔL=300x5x10^-2=15N

Then you know elastic energy stored is given by E = 1/2xFxΔL
So ΔE = (1/2x15x5x10^-2)-(1/2x6x2x10^-2)
= 0.315J

Like I said, this is probably wrong so don't take my word for it especially as I haven't yet covered this topic in lessons (and exams are in 5 weeks - not great!)
(edited 5 years ago)
Original post by BDE
I might be wrong but this is my attempt:

There are two extensions taking place here - firstly from natural length 15x10^-2m to 17x10^-2m - ΔL=2x10^-2
- second from the first extension 17x10^-2m to the new extension 10x10^-2m - ΔL=3x10^-2, however we want the extension from natural length which is 5x10^-2
The question asks for the extra work done so I think you need to work out the distance between the two stretches from natural length, which would be as follows:
Stretch 1: F=kΔL=300x2x10^-2=6N
Stretch 2: F=kΔL=300x5x10^-2=15N

Then you know elastic energy stored is given by E = 1/2xFxΔL
So ΔE = (1/2x15x5x10^-2)-(1/2x6x2x10^-2)
= 0.315J

Like I said, this is probably wrong so don't take my word for it especially as I haven't yet covered this topic in lessons (and exams are in 5 weeks - not great!)


Omgg you are a lifesaver!!! Thank you so much!!!!
It might help to think of the problem like this:

Work done to stretch to 20cm = Work done to stretch to 17cm + Work done to stretch an extra 3cm.

Use this equation to try and get the answer :smile:
Original post by darkforest
It might help to think of the problem like this:

Work done to stretch to 20cm = Work done to stretch to 17cm + Work done to stretch an extra 3cm.

Use this equation to try and get the answer :smile:


Thanksss !! I think I understand how to do it now :smile:
Reply 6
Original post by username3684524
It might help to think of the problem like this:

Work done to stretch to 20cm = Work done to stretch to 17cm + Work done to stretch an extra 3cm.

Use this equation to try and get the answer :smile:

Thanks BDE still useful after 3 years!!!

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