search.noResults

search.searching

note.createNoteMessage

search.noResults

search.searching

#### orderForm.title

orderForm.productCode
orderForm.description
orderForm.quantity
orderForm.itemPrice
orderForm.price
orderForm.totalPrice
orderForm.noItems
I

was walking along, minding my own

business. 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. Cool! Cool? Yeah. It’s okay. I didn’t get hurt. And my fall was a reminder that gravity exists! Gravity is a force. A force is something

that pushes or pulls. Gravity makes one object attract another. When I went fl ying, Earth’s gravity pulled me down to the ground. 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 powerful stuff , right? Yes and no. Gravity can be weak. To see how weak, just liſt this magazine. T e gravity of Earth is trying to pull the magazine down. Yet you are holding it up. Now let the magazine fall. Earth’s gravity pulls it down. Without your liſt ing the magazine, gravity takes over. Gravity can also work over a distance.

T row a ball into the air. Earth’s gravity pulls it down even though Earth isn’t touching it. To throw the ball, you have to touch it. T e farther you get from Earth, the more

weakly its gravity pulls on you. If you move far enough away, it won’t pull on you at all.

12 NATIONAL GEOGRAPHIC EXPLORER

Putting Gravity to the Test Gravity has 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, if you drop a bowling ball and a marble, they will hit the ground at the same time. Hold on! T e bowling ball has more

mass than the marble. So there’s a stronger force of gravity between it and Earth than Earth and the marble. T at’s true. But because of its greater

mass, it’s also harder to get the bowling ball to move. T e greater force of gravity is off set by the greater diffi culty in getting the 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. T at’s because Earth has air that pushes on the feather and slows it down.

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24