You and Your Students!
Vicki Cobb, Education World Science Editor
Discover gravity -- the force that’s always with us.
Setting the Scene (Background)
Gravity is the force you can't escape as long as you are on Earth! A gravitational field is similar to electrical and magnet fields. All of them pull objects that are in their force fields. One big difference is that gravity only attracts. It doesn't repel. For example,
Unlike a gravitational field, electrical and magnetic fields attract and repel.
Another difference is that gravity is a very weak force, much weaker than electrical or magnetic forces.
These activities provide lots of opportunity for creative experimenting, so you might let students work with a partner.
You can elicit the following observations about gravity from a class discussion and from some simple experimenting:
One of the most common misconceptions about gravity is that heavier objects fall faster than lighter objects. Have your students invent a way -- a dropping contest -- to prove that heavier objects do not necessarily fall faster than lighter objects. Have students select small objects -- preferably objects small enough so they can hold them in their hands. (Some of the objects listed in the Props section above will suit this activity nicely.) Let students hold a different object in each hand and let go simultaneously while others watch and listen as the objects fall to the floor.
This requires focused attention, much like the attention a baseball umpire must give as the uses the sound of the ball hitting the glove to determine if a runner is safe or not.
There may be small variations, but usually the difference is too small to detect. Every dropping race is a tie -- except for objects that are so light (a leaf or a sheet of paper, for example) that air can fight the pull of gravity.
Physicists have determined that the acceleration of gravity is a constant. Absent other forces -- such as air resistance and buoyancy -- an object will fall 32 feet per second squared. That means that at the end of the first second the object will be traveling 32 feet per second, or about 21 miles per hour. At the end of the second second, the object will be traveling 42 miles per hour.
Since the acceleration of gravity is a constant, what is weight? Simply put: weight is a measure of the amount of matter in an object by the strength of its attraction to the earth. The more matter there is, the stronger the gravitational attraction. (The bigger you are, the harder you fall.) But you don't have to fall to measure weight. The stretch of a rubber band is an effective way to illustrate that.
Have kids tie different shoes to same size rubber bands and compare how much the bands stretch. In this case, rubber bands are a simple scale. The longer the stretch, the heavier the shoes. (Students' bathroom scales at home are more accurate. They measure weight in pounds and kilograms. )
The elastic bands illustrate that the shoes are definitely of different weight, but if students drop two shoes of different weight at the same time, from the same height, they will hit the ground at the same time.
Behind the Scenes
These experiments should give kids an appreciation for the problems of controlling experiments and getting accurate measurements. They show why experiments must be repeated.
There is a myth that Galileo dropped a cannon ball and a wooden ball off the top of the Leaning Tower of Pisa. (Actually, he thought of the experiment and in later years his students actually performed it.) The two balls arrived on the ground at approximately the same instant. (The wooden one was slowed slightly by air resistance, but Galileo had anticipated that result and factored it in.) The important thing was that that experiment proved that the acceleration of gravity is a constant and is independent of mass.
For more information and activities for teaching about fields of force, check out
Sources of Forces: Science Fun with Force Fields
by Vicki Cobb, illustrated by Steve Haefele (The Millbrook Press, 2002).
Article By Vicki Cobb
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