PhysLink.com Logo
Black Friday Sale 2017 - Educational Gifts and Toys
Black Friday Sale 2017 - Educational Gifts and Toys

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?
Asked by: Rob Mclachlan

Answer

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.

You can try an experiment that is close to the bee example for this law. You will need someone to help you. Get a bathroom scale and have your helper watch the scale as you stand on it. Have your helper record your weight. Now, bend your knees and jump up and off of the scale. (You might ruin your scales if you land back down on them, but, you can explain to your mother that it was done in the name of science!) At the exact same instant that you push down on the scales to jump up have your helper read the scale. You will find that you have exceeded your weight in order to jump. This is because in order to push up an equal and opposite force had to be pushed down. Now, if you were a bee with wings you would exceed your weight only for an instant at take-off and then while you hovered the force you would have to push down with would equal your weight.

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


Science Quote

'As long as men are free to ask what they must; free to say what they think; free to think what they will; freedom can never be lost and science can never regress. '

J. Robert Oppenheimer
(1904-1966)


All rights reserved. © Copyright '1995-'2017 PhysLink.com