Question

A demonstration of interference is done in class. A Slinky™ is stretched out from end to end. Three cups are placed on the ground, positioned just to the side of the Slinky™. Two pulses are introduced into each end at the same time. The pulses travel towards each other as shown in the diagram.What will be observed after the pulses pass by each other?a. All three cups will be knocked over.b. Cups 1 and 3 will be knocked over; cup 2 will be left standing.c. Only cup 2 will be knocked over; cups 1 and 3 will be left standing.d. Nothing will happen to any of the cups; they will all be left standing.

Answer 1

ANSWER

c. Only cup 2 will be knocked over; cups 2 and 3 will be left standing

EXPLANATION

The two crests are moving towards each other and they have the same amplitude. When the wave on the left moves past the first cup it will remain untouched since the Slinky won't touch it. The same happens for the wave on the right and cup 3,

Then, both pulses will reach cup 2 at the same time. This will create a constructive interference, making a pulse with an amplitude greater than the amplitude of each original pulse,

So, cup 2 will be knocked over.

Then, as the pulses continue their paths, they go back to their original amplitudes,

And cups 1 and 3 are always safe.

Hence, only cup 2 will be knocked over; cups 2 and 3 will be left standing.