#### What is gravity and effects of gravity? How to overcome gravity? The resistance of gravity.

Many kinds of work need to be done in our world. Cars, buses, trucks, trains, ships, airplanes, rockets, and their loads have to be moved. Soil must be plowed and cultivated so that crops will grow. Water must be pumped, grain must be ground into flour, and logs must be sawed for lumber. Materials have to be lifted into place for new buildings. Then people, furniture, machines, and supplies must be carried up when the buildings are in use. You can easily think of many other kinds of work that haye to be done.

Each kind of work is somewhat different from the others, but they are all alike in one way: When any kind of work is done, something is moved. To make it move, a force must overcome some kind of resistance, or opposing force. Energy is used to exert the force that overcomes the resistance. As scientists studied the various kinds of work, they discovered an important fact: Most work is done and most energy is used in overcoming four kinds of resistance. You have already learned something about two of these kinds, but the other two kinds you have probably not studied before.

Of course, a force must be exerted to lift an object. This force is needed to overcome a resistance. Only a little force is needed to lift a pencil, but a much bigger force must be used to lift a chair. So some force must always be exerted, because every object resists being lifted. This kind of resistance is caused by the earth’s pull on the object. The earth attracts, or pulls toward itself, all objects in our world. This attraction, or pull by the earth, is the force called gravity. It pulls downward all the time on matter everywhere. To overcome gravity, the upward force exerted on an object must be greater than the downward force of gravity. Even the small force used to lift a pencil is greater than the downward force of its weight.

Weight is one characteristic, or property, of matter. What we call weight is really the force with which gravity pulls down on an object. In fact, an old meaning of gravity is weight, or heaviness. When you pick up an object, you must use enough force to overcome its weight. Suppose a chair weighs 20 pounds. This means that gravity exerts a downward force of 20 pounds on the chair. To lift the chair, you must exert an upward force of more than 20 pounds. Much less force is needed to pick up a pencil that weighs less than an ounce.

Whenever any object is lifted or moved upward, the resistance of gravity must be overcome. In other words, the upward force exerted on the object must be greater than the downward force of its weight. Without other forces to overcome the resistance of gravity, nothing could be lifted. Energy is used to exert these forces and make things move upward.

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