# Matchbox Coin Challenge

Push a coin into a matchbox on the base side of the inner box as shown

The challenge is to get the coin to go upwards through the box to come out of the top, but you can’t touch the coin. I was shown this by the wonderful Syailendra Harahap

What are you going to do?

Dont watch the video below until you have tried to solve the problem (really don’t watch it yet!)

When you do watch it pause the video between Act 1 and Act 3 whilst you think.

Act 1: You are going to tap the top of the box. What questions does this make you think of?

Act 2: What might happen if…. ?

• You tapped the box harder?
• You used a lighter/heavier/bigger coin?
• you used a full matchbox/heavier one?
• you used a bigger matchbox

Act 3: The reveal

So what is happening?

The coin cannot be moving upwards as the only force that is exerted is downwards.

The coin starts in a state of grip with friction holding it in place in the box.

As the box is tapped it moves downwards, however the coin has inertia and so does not move with the box, it stays fairly still whilst the box moves past it. The frictional force is in a state of slip at this point and the box moves relative to the coin. As the box slows down friction returns to a state of grip. The coin appears to be moving upwards inside the box and will continue to do so until it reaches the top.