Forces A force is a push or a pull. A force has both size and direction. Some forces act only when objects touch, such as when a baseball bat hits a baseball. Other forces act even if the objects do not touch. Forces are measured in units called Newton (N). If you have a 5N force pulling to the right, and a 3N force pulling to the left, the net force is 2N to the right. Force can be measured using a special scale called a spring scale.
Forces on Objects To find the overall effect of forces acting on an object, you add the forces together. This gives you the nets force. Most objects have more than one force acting on them. Sometimes several forces act all in the same direction, such as when a ball is rolled downhill. Other times forces act in different directions. This is like a tug-of-war with a rope. Some people pull the rope one way, and others pull the opposite way. If the people on both sides are equally strong, then the rope doesn´t move. This is an example of balanced forces. But if the people on one side are stronger than the people on the other side, more force will come from the stronger group and they will pull the rope in their direction. This is an example of unbalanced forces.
Friction Friction is the force that resists the movement of one surface past another. The friction created by your hand is called sliding friction. Rolling friction resists the motion of a rolling object. Static friction resists the motion of an object just as it begins to move.
Helpful and harmful Friction Friction is useful to people in many ways. When you walk, for example, the friction crated between the bottom of your shoes and the floor keeps you from slipping and sliding. Friction can also be harmful. For example, the heat caused by friction can damage car engines.