The force of gravity is a downward pull. So the force of gravity must be overcome whenever anything is lifted. Gravity does not always oppose our efforts. We often use its force to good advantage. Besides holding us to the earth, gravity is useful in many other ways. Every time we weigh something, we measure the amount of force with which gravity pulls down on it. Gravity helps us whenever we want to slide or roll something down a slope. Gravity also makes rain and snow fall to the earth. Because of gravity, water from the rain and melted snow sinks into the ground or runs downhill into streams and lakes. Most of the water we use comes either from streams and lakes or from the ground. Wherever water comes from, gravity plays an important part in keeping us supplied with it.
            Some things are pushed up because of gravity. When you suck lemonade through a straw or pump water from a well, gravity makes the liquid move upward. Gravity also drives dirt from a rug up into a vacuum cleaner. Due to gravity, boats float in water and balloons float in air.

What is Gravity?

            People have known since the earliest times that objects fall toward the earth unless they are held up in some way. But no one understood why this happens until about 300 years ago. Then Sir Isaac Newton, a great British scientist, stated an important scientific principle that explains this and many other facts. From Newton's studies and from many experiments, scientists have learned that all matter has the property of attracting all other matter. This property of matter is called gravitation, or gravitational attraction. The principle that describes how matter attracts other matter is called the Principle of Universal Gravitation. The first part of the principle is stated in this way: Every particle of matter in the universe attracts every other particle. The second part of the principle tells how strong the force of attraction will be under various conditions.
            What we call gravity is the gravitational attraction between the earth and all the materials on and around it. Objects fall to earth because of gravity. And because of gravity, objects also have weight. The harder gravity pulls downward on an object, the heavier the object is, or the more it weighs.
            According to the second part of Newton's Principle, two kinds of things determine the amount of gravitational attraction between any two objects. One thing is the distance between the centers of the two objects. The closer the centers are to each other, the more strongly the objects attract each other. As the distance between the centers increases, the attraction decreases rapidly. At a great enough distance, the attraction is so small that it has no practical effect at all.
            The attraction between two objects also depends on the amount of matter in each object. The amount of matter in any object is called its mass. The greater the masses of the objects, the greater is the force of attraction between them. Even objects like pins attract each other. But because their masses are so small, the gravitational attraction between such objects is usually too weak to notice.
            The earth has a huge mass. It attracts objects that are close to it with enough force to make them fall to earth unless they are held up in some way. The force of gravitational attraction that makes an object fall to earth is a pull exerted by both the earth and the object. The object that falls to earth pulls just as hard on the earth as the earth pulls on the object. So the earth falls too. It falls toward the object. But the mass of the earth is so great compared with the mass of the object that the earth's motion is much too small to notice.
            The weight of an object is the amount of force with which gravity pulls down on it. The amount of this force depends on three things: (1) the mass of the object, (2) the mass of the earth, and (3) the distance between the centers of the two. The mass of the earth is always just about the same. And on the earth's surface, the distance between the center of an object and the center of the earth is always much the same. So under these conditions, the weight of an object depends almost entirely on its mass.
            However, weight and mass are not the same thing. Weight is the amount of force with which gravity pulls down on any object. Mass is the amount of matter in it. The difference can be shown by an experiment using a very sensitive spring balance. This kind of balance measures the force with which gravity pulls down on an object. A spring in the balance is stretched when the object is placed on the balance. The greater the force with which gravity pulls on the object, the more the spring is stretched. So this balance measures weight, not mass. If this balance is used to weigh an object first at sea level and then on a mountain top, there is a difference in the weight. The object weighs slightly less on the mountain top. However, the mass of the object has not changed.
            The Principle of Universal Gravitation explains why this is so. The weight of an object depends not only on its mass but also on the earth's mass and the distance between the center of the object and the center of the earth. On a mountain top this distance is slightly greater than it is when the object is at sea level. So the force of gravitational attraction between the two is less than at sea level. The weight of the object is less, although its mass remains the same.
            The fact that the weight of anything depends on its position is very important to space scientists. For example, if a space ship that weights 20,000 pounds at sea level gets 80,000 miles away from the center of the earth, its weight is only about 50 pounds. If the space ship should go to the moon and land on it, its weight due to the gravitational attraction of the moon would be about 3300 pounds.
            For many purposes, it is not necessary to distinguish between an object's weight and its mass. But for other purposes the distinction is very important. The weight of a certain volume of a material is called its density. Physicists have a more exact definition of density. They say that density is the mass of a certain volume of a material. That is, they make the distinction between the ideas of weight and mass, because it is important in their studies.