| How high do you think you can you jump? Get some of your friends together to find out. Tape a large piece of paper to the wall. (Brown wrapping paper or sheets of newspaper will do.) The shortest person in your group should be able to reach the bottom of the paper without standing on tiptoes. Dip one finger in dirt or ink. While standing with your feet flat on the floor, stretch your arm as far as you can and mark the highest point you can reach on the paper. Now jump and mark the paper by touching it at the top of your jump. Try it a few times and challenge your friends to jump higher. (Label each person's marks with initials or have each person use a different color ink or dirt.) Measure the difference in height between your standing and jumping marks. This is how high you can jump. Surprised? |
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How long can you stay in the air when
you jump? Ask someone to time you with a
stopwatch or a watch with a second hand.
(It won't be easy!) You can also time
jumping athletes who are playing
basketball or volleyball.
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You probably noticed that taller kids
didn't necessarily have higher jumps.
Remember, you measured the jump height
from your reach and not the ground. The
best jumpers in the world can clear
heights up to 2.4 meters (8 feet), but
they only lift their center of mass
considerably less than that. Did you have a hard time measuring your time in the air? You're not alone. Even the best jumper remains airborne for less than one second. Still, you can compare your airborne time with those of your friends by using the table below. Look down the first column for your jump height and read across to find your time.
Jump Height Time in Air
(centimeters)/(inches) (seconds)
25 10 0.4
50 20 0.6
75 30 0.8
100 39 0.9
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But is jumping really flying? Yes and
no. Some things like rockets,
cannonballs, and baseballs fly like
jumping kids_they are pushed into the
air by engines or muscles. But what
about airplanes? It's true they are
pushed forward by engines, but air is
what pushes and holds them up. Thrust, drag, weight, and lift are the four forces that usually work together to make things fly. You already know something about each of them, although you might not have called them by their names. If you thought that having stronger muscles or springy shoes or a rocket booster might help your jump, then you were thinking about thrust (the "muscle" that pushes you during flight). If you thought that a slick suit or helmet would make you jump higher or that a parachute would keep you in the air longer, then you were thinking about drag (the way air tends to slow things that fly). Obviously weight is important - if you could lose weight by changing clothes, dieting, or visiting the Moon, you would probably jump higher. |
Most people have experienced how thrust,
drag, and weight can help them jump
higher or "fly," but few people are
familiar with lift. Lift is a push that
comes from the air. You were thinking
about this force if you decided that
wearing wings or holding helium balloons
would help you jump higher. Planes and
birds have to be moving to get enough of
this push to fly; hot-air balloons are
light enough for their size that the air
will lift them up whether or not the
balloon is moving. You may want to find
out more about this key force to better
understand how things fly. Does everything that flies use all four forces? Nope. Only two forces - weight and thrust - help spacecraft fly. Lift and drag won't help spacecraft flying in space, where no air exists. This was a quick overview of the forces of flight. Don't stop here, though - look for more information about how high you can fly. |
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