This is an experiment, medium-difficult.

You'll need a Van der Graaf generator for this one. 

Get a syringe, you don't need the needle. Separate two equal pieces of iron wool. Place one of them inside the syringe and press it. Was it hard to press, how far did you manage to go? Mark that place with a permanent marker. Remove the wool from the syringe.

Now take the other piece of wool. With an insulant tool, charge it in the Van der Graaf Generator. Place it inside the syringe, being careful not to touch it except with the insulant tool. Press the syringe. Did you get to the same place as before?

My intuition tells me that this time the charge repulsion would make it difficult for you to press the syringe.

Should this work, you might consider that a charged wool inside a piston, now charged in a variable and controled form, could make a spring with variable spring constant K.

[With a spring like that you could design a mechanical dampener that was variable. Naturally, it need to go more pro than here.]