Jim needs to send a parcel, but wants to save money on postage. Maybe there's a solution. After all, he's sending a helicopter in a box.

The explanation

If you've seen the Chopper in a box video, you'll be wondering why the box doesn't weigh less when the helicopter's in the air. Many people, quite reasonably, think that if the helicopter is flying - then it shouldn’t be registering on the weighing scales. It seems only natural.

But something needs to keep the helicopter up – in this case the downward air pressure from its blades. The force of this air pressure must (at least) match the helicopter’s weight (otherwise the helicopter wouldn’t stay up).

Imagine if there was a large blast of wind down onto the top of the package – you’d probably expect the package to ‘weigh’ more when the blast came. This is a result of the air pressure from the blast – and the gravity acting on the mass of the package. When the helicopter takes off that blast of air from its blades is also pressing down on the scales with a force equal to, if not more than, the weight of the helicopter. So the package as a whole weighs the same (if not a little more).

What could you do next?

• Do you think it would make a difference if the box was bigger?
• Do you think the box has to be sealed?
• You can probably see we had to tether the helicopter (because we’re not very good at steering!). Do you think this made any difference?

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