### Question

A stationary van is filled with 1 tone of bees sitting down. Is it true that the overall weight of the van is the same if bees are flying instead of sitting?

Of course this is true! How else could the bees push up if there were not an equal, but opposite, force pushing down? This is an example of Newton's Third Law. This Law of the universe says that for every force there is an equal and opposite force.

I hope this helps you to better understand Newton's Third Law. Most people get confused when they try to understand this Law. this is easy to do because people often forget that the two forces that are equal and opposite act on different objects. There is a famous story about a farmer and his horse. The horse is a lazy beast who spends his time eating hay and lazily reading science books. One day the farmer attaches a heavy cart to the horse and demands that the horse pull the cart. 'Well,' says the horse, 'I cannot pull the cart because, according to Newton's third law, if I apply a force to the cart the cart will apply an equal and opposite force on me. The net result will be that I cannot pull the cart since all the forces will be balanced. And, as you know, Newton's first law says that if there are equal forces acting on an object the net result is no change in motion. Therefore, it is impossible for me to pull this cart.' The farmer was very up set! Do you know what he could say to convince the horse to move?
Answered by: Tom Young, M.S., Science Teacher, Whitehouse High School, Texas

Yes. For an object to be supported by air, its weight needs to be countered with an upward force of equal magnitude. Newton's third law of motion also says that if the air provides an upward force on the bees, it feels an opposite force of the same magnitude pushing down on it. In your example, the downward force on the air results in a downward force on the floor of the van equal to the weight of the bees.

If you replace the bees with grasshoppers which don't fly, but get into the air by pushing off from the van's floor, however, the situation is different. The air is not supporting them as it does the flying bees (jumping can still occur in a vacuum). So if all the grasshoppers jump at the same time, it results in the van temporarily 'weighing' MORE, then less, until they all returned to the floor. You can do this experiment yourself by getting on a bathroom scale and jumping up. While in the air, the scale registers zero. Just before, and upon landing, it registers MORE than your weight.
Answered by: Paul Walorski, B.A., Part-time Physics Instructor

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### Science Quote

'I have deep faith that the principle of the universe will be beautiful and simple.'

Albert Einstein
(1879-1955)