I


was walking along. I didn’t see the


curb. Next thing I know, I’m airborne. T en I’m on the pavement, along with the bananas I just bought. I didn’t get hurt. But my fall reminded me of the force of gravity! A force is something that pushes or


pulls. Gravity makes one object attract another. Earth’s gravity pulled me down to the ground. T e force of gravity begins with


mass. Mass is the amount of material an object has. Earth has more mass than I do. So its gravity pulls on me.


Weak or Strong? So, gravity is powerful stuff , right! Yes and no. Gravity can be weak. To see how weak, liſt this magazine. Earth’s gravity is trying to pull it down. Yet you are holding it up. Now let it fall. Earth’s gravity pulls it down again. Gravity can work over a distance,


too. T row a ball into the air. Earth’s gravity pulls it down. Yet, the farther you get from an object, the less its gravity pulls on you.


12 NATIONAL GEOGRAPHIC EXPLORER


Putting Gravity to the Test Gravity has some other qualities. T is one may surprise you. Drop a bowling ball and a marble. T ey hit the ground at the same time. T e bowling ball has more mass


than the marble. So there’s a stronger force of gravity between it and Earth than between Earth and the marble. But because of its mass, it’s also


harder to get the bowling ball to move. T e diffi culty in getting the ball to move counteracts the force of gravity. So, they hit the ground at the same time. What happens if you drop a


hammer and a feather! T e hammer hits the ground fi rst, right? On the moon, this changes. On the moon, there’s no air to


slow things down. So these objects hit the lunar surface at exactly the same time!


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