I


was walking along, minding my own


business. I didn’t see the curb. T e next thing I knew, I was airborne. But only for a second. T en I was fl at on the pavement, along with the bananas I just bought. Cool! Yeah. It’s okay. I didn’t get hurt. And my fall was a reminder to me that gravity exists! Gravity is a force. A force is something that


pushes or pulls. Gravity attracts objects to each other. In this case, when I went fl ying, Earth's gravity pulled me down to the ground. In fact, it was pulling me toward Earth's center. T e ground just got in the way! When you think about it, that is pretty cool. T e force of gravity arises from mass. Mass


is the amount of material an object has. I have mass. You have mass. Earth has a whole lot of mass. T e more massive an object, the more strongly its gravity pulls on other objects.


Weak or Strong? So gravity is pretty powerful stuff , right? Yes and no. Gravity is actually a weak force. To see how weak, just liſt this magazine. T e gravity of the entire planet is trying to pull the magazine down, yet you are stronger than Earth! To see gravity in action, let the magazine


fall. Earth’s gravity pulls it down, and it lands on your desk. Without your liſt ing the magazine, gravity takes over. Gravity is powerful, though, because it can


work over a distance. When you throw a ball into the air, Earth’s gravity pulls the ball down even though Earth isn’t touching it. In contrast, for you to throw the ball, you have to touch it. But the farther you get from an object,


the weaker its pull on you will be. So if you go far from Earth, you will be aff ected less by its gravity.


12 NATIONAL GEOGRAPHIC EXPLORER


Putting Gravity to the Test Gravity has some other unusual characteristics. T is one may surprise you. Two objects—one heavy, the other light—fall equally fast, if you ignore what air does. For example, drop a bowling ball and a marble, and they hit the ground at the same time. Hold on, you say. T at doesn’t sound right.


T e bowling ball has more mass than the marble. T at means there’s a stronger force of gravity between it and Earth compared to Earth and the marble. T at’s true. But because of its mass, it’s


also harder to get the bowling ball to move. So Earth’s gravity speeds them up exactly the same. T e greater gravitational force between Earth and the bowling bowl is off set by the greater diffi culty in getting the darn bowling bowl to move. Don’t believe me? In 1971, an astronaut on


the moon dropped a hammer and a feather at the same time. On the moon, there’s no air to slow things down. T e moon’s gravity pulled the hammer and the feather down at the same speed. T ey hit the lunar surface at exactly the same time! Of course, on Earth, the hammer hits the


ground before the feather. But that’s because Earth has air that pushes against the feather and slows it down. Likewise, if you jump out of an airplane, the air pushes against your parachute—you did take a parachute, right?— and slows you down so that you land safely.


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