Jumping Discs
A "jumping disc" is a metal disc that is slightly curved and made of two different metals. It normally looks like A .
If it is pressed, it bends the other way to state B . As soon as it is released, it reverts to its original form, state A .
Now, if it is rubbed between the fingers to heat up and then pressed, it bends to state B and stays that way even when released. If it is then placed on a flat surface, it jumps after some time.
How does the time taken to jump change if it is placed on a colder surface?
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1 solution
The last experiment given in the question works on the concept of thermal expansion of metals. The two metals expand differently when heated hence switch between the two states as shown.
They expand and switch to state B when heated, and when placed on a flat surface, as they cool down, after some time they quickly snap back into state A, and hence jump.
This process can thus happen quicker if the disc cools down faster, so it will jump sooner if placed on a cold surface.