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
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
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