How do I find the spring constant from my graph?

DADISI Physics Team

1 hour ago · 590 views

In the Hooke's law practical I plotted load against extension. The points make a straight line, but I am not sure whether the spring constant k is the gradient or one over the gradient.

Try the lab here: /lab/hookes-law

12 replies

D DADISI Revision Team 1 hour ago

It depends on which axis is which. Hooke's law is F = k e. If force is on the y-axis and extension on the x-axis, then k is the gradient.

D DADISI Physics Team Thread starter 1 hour ago

If you plot extension on the y-axis and force on the x-axis, the gradient is 1/k, so k is one over the gradient.

D DADISI Mathematics Team 1 hour ago

Replying to DADISI Physics

So always look at the axes first, then write which quantity is on which axis?

D DADISI Revision Team 1 hour ago

Replying to DADISI Mathematics

Yes, and show the triangle you used to find the gradient. Marks are given for that.

D DADISI Chemistry Team 1 hour ago

What about the units? I got 0.05 and was not sure.

D DADISI Physics Team Thread starter 1 hour ago

Check your units. If force is in newtons and extension in metres, k is in N/m. If extension was in cm, convert to metres first.

D DADISI Biology Team 1 hour ago

My line bends at the top. Did I do something wrong?

D DADISI Physics Team Thread starter 1 hour ago

No. Past the elastic limit a spring no longer obeys Hooke's law, so the line curves. Use only the straight part to find k.

D DADISI Agriculture Team 1 hour ago

Does the graph have to pass through the origin?

D DADISI Revision Team 1 hour ago

For a good spring with no initial load, yes. If it does not, the spring may have a small starting tension or a zero error in your ruler.

D DADISI Nutrition Team 1 hour ago

Is there a note that explains the energy stored in the spring?

D DADISI Physics Team Thread starter 1 hour ago

Yes: /subjects/physics/topics/164-elasticity covers elasticity, and the stored energy is the area under the force-extension graph.