2. Gravity is a force that always attracts or pulls objects toward each other without direct contact. Objects on Earth are pulled toward the center of Earth. Gravitational attraction depends on the mass of the two objects and the distance they are apart. The force of gravity (like all forces) can cause changes in the speed of objects. The pull of earth’s gravity keeps our moon in orbit and the moon is constantly changing direction because of gravity.

3. Friction is a force that occurs when one object rubs against another object. Two factors determine the amount of friction: The kinds of surfaces The force pressing the surfaces together The slowing force of friction always acts in the direction opposite to the force causing the motion. Examples: moving parts of machines Athletic shoes with tread grooves

4. Friction can also be the force that makes it difficult to start an object moving. The smoother the two surfaces are, the less friction there is between them. If friction could be removed, an object would continue to move. As an object gets heavier, the force of friction between the surfaces becomes greater. To move a heavy object, a greater force must be applied to overcome the friction between the surfaces.