# A little help with an Isaac Physics Q?

Part D (B9.4)
A spring with natural length 75cm requires a force of 300N in order for it to stretch to 85cm. How much EPE would be stored in the spring if it were stretched to 90cm? Give your answer to 2 significant figures.
Original post by OneOfTheBillions
Part D (B9.4)
A spring with natural length 75cm requires a force of 300N in order for it to stretch to 85cm. How much EPE would be stored in the spring if it were stretched to 90cm? Give your answer to 2 significant figures.

E=1/2kx2

x is the extension, the difference between the natural length and the stretched length in meters
k is the spring constant... the force required to stretch the spring by 1 meter
E is the stored energy in Joules... expressed to 2 sig figs as requested by the question.
Original post by Joinedup
E=1/2kx2

x is the extension, the difference between the natural length and the stretched length in meters
k is the spring constant... the force required to stretch the spring by 1 meter
E is the stored energy in Joules... expressed to 2 sig figs as requested by the question.

Thanks but that is the equation i have been using but it doesn't give me the correct answer.
Original post by OneOfTheBillions
Thanks but that is the equation i have been using but it doesn't give me the correct answer.

energy stored in a spring: E=1/2 kx2
k =300/0.1 = 3000

so when x is 0.15

Original post by Joinedup
energy stored in a spring: E=1/2 kx2
k =300/0.1 = 3000
so when x is 0.15

If you don't mind, can you show me how you did it step by step. Please!
Original post by Parth_7
If you don't mind, can you show me how you did it step by step. Please!

There's not much to it.

The question is asking about energy stored so eventually you're going to want to use E=1/2kx^2

Find the spring constant k using the given information. Make sure you're calculating the extension properly, its the difference between unstretched and stretched lengths.

0.85-0.75=0.1

k=300/0.1

Then you keep the constant k and calculate 1/2kx^2 for the extension when the length is 0.9m