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This Week's Experiment - #248 Relative Speed
This week's experiment is on relative speed. By that, I don't mean timing
how fast your grandmother can run. Instead, we are going to see that how
fast things seem to be traveling can vary quite a bit, depending on where you
are standing. For this experiment, you will need:
a car
a good driver
a long stretch of road
a cotton ball or small piece of tissue paper
a friend
OK, here is the idea. You are going to challenge someone to a throwing
contest, and you are going to win. Trust me. If you do this correctly, you
will win every time. Be sure that your driver knows what is going on and that
everyone keeps safety in mind. You don't want to distract the driver from
paying attention to the road.
Have your friend sit in the front, passenger seat. You will sit in the back
seat. Wait until the car is traveling fairly fast. Now, you are going to
see who can throw the cotton ball faster. You start by very gently tossing
the cotton ball to your friend. Now to win, all your friend has to do is
throw the cotton ball back to you and have it travel faster than it did when
you threw it. Your friend will probably throw the cotton as hard as he can,
which means he will lose.
If your friend threw the cotton ball much harder, why did I say that he lost?
It has to do with relative speed. From the inside of the car, your friends
toss seemed faster than yours, but we are looking at things from a broader
perspective.
Say for example that your car is traveling at 55 miles per hour. That means
that you, your friend and the cotton ball are also traveling 55 miles per
hour. When you toss the cotton ball to your friend, in the same direction
that the car is traveling, you add its speed to that of the car. A toss that
seems to be 2 miles per hour actually adds 2 miles per hour to the speed that
the cotton ball is already traveling. Someone standing by the side of the
road with a radar gun would see the cotton ball with a speed of 57 miles per
hour!
Now it is your friend's turn. I chose a cotton ball so air resistance would
slow it down and prevent it from reaching high speeds. Lets pretend that
your friend throws the cotton ball back towards you at a speed that seems to
be 20 miles per hour. From your perspective, it would seem that his throw
was 18 miles per hour faster than yours, but from a perspective outside the
car, things are very different. Since he is throwing the cotton in the
opposite direction, this time you subtract that speed from the speed of the
car, not add it. That means that your friend succeeded in slowing the cotton
ball down by 20 miles per hour. The person standing on the side of the road
would see his throw traveling only 35 miles per hour compared to your toss of
57. An easy win. The harder he throws the cotton, the more he slows the
cotton down compared to the road outside, and the larger your margin of
victory. Even if he throws the cotton very gently, he must still throw it to
you, which means it will be traveling in the opposite direction from the car.
He still can't win.
Of course to convince your friend that you won, you will have to understand
all of this well enough to explain it and have him understand it. Relative
speeds can be very interesting. You can even win this challenge without a
car. You just have to be sure that you are standing in the right spot. The
Earth is rotating at about 1000 miles per hour. Think about where the sun
rises and sets and you should be able to figure out which direction it is
turning. That should let you figure out where to stand and where to have
your friend stand, in order to win the challenge. Think about the turning
Earth in the same way as the moving car. If you position things correctly,
your throw will be adding to the speed of the Earth's rotation (letting your
cotton ball reach a speed of over 1000 miles per hour!) Your friend's toss
back to you will be in the opposite direction, making it slower. Just don't
play this trick on an astronomer. She might add in the speed of the Earth
traveling around the sun, or the speed of the sun and Earth traveling through
space to work out a way to win. It all depends on your perspective.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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