# The Force of Gravity

Starring

Script By

Vicki Cobb, Education World Science Editor

Synopsis

Discover gravity -- the force that’s always with us.

Genre

Physical Science

Props Required

• lots of small objects (for example, chalk, pencils, paper clips, erasers)
• rubber bands
• shoes

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,

• Earth's gravity attracts the moon.
• The moon's gravity attracts the water on the surface of the Earth, causing tides.
• The sun's gravity attracts the Earth.

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.

Stage Direction

These activities provide lots of opportunity for creative experimenting, so you might let students work with a partner.

The Plot

Act I
You can elicit the following observations about gravity from a class discussion and from some simple experimenting:

• All objects fall to Earth, even if they are thrown up in the air first. You can illustrate this concept by dropping a ball from any level, or by throwing it up into the air and letting it drop back to the ground on its own. That illustrates that (1) every force has a direction and (2) the direction of the force of gravity is down.
• All objects pick up speed as they fall. Starting from zero, they pick up velocity until they strike the ground. The higher the object when it starts the fall, the faster it will be going when it strikes the earth.

Act II
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.

Act III
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.

The End