A force is an influence that causes physical change. There are ten basic kinds of forces: applied, gravitational, normal, friction, air resistance, tension, spring, electrical, magnetic and upthrust. Each of these forces does something different: some pull and some push, while others cause an object to change form.
Applied force is transferred from a person or object to another person or object. If a man is pushing his chair across a room, he is using applied force on the chair.
Large objects have a gravitational force, which attracts smaller objects. The best example of a gravitational force is the Earth's interaction with people, animals and objects. Even the moon is pulled toward the Earth by a gravitational force.
Normal force is exerted on an object when it is in contact with another object. When a coffee cup is resting on a table, the table is exerting a normal upward force on the cup to support its weight.
Friction occurs when an object moves -- or tries to move -- across a surface and the surface opposes the object's movement. The amount of friction that is generated depends on how strongly the two surfaces are being pushed together and the nature of the surfaces. For example, pushing a book across a glassy surface will not create much friction, while sliding your feet across carpet will produce more friction.
Air Resistance Force
Air resistance acts on objects as they travel through the air. This force will oppose motion, but only factors in when objects travel at high speeds or have a large surface area. For example, air resistance is smaller on a notebook falling from the desk than a kite falling from a tree.
Tension passes through strings, cables, ropes or wires when they are being pulled in opposite directions. The tension force is directed along the length of the wire and pulls equally on objects at opposite ends of the string, cable, rope or wire.
Spring force is exerted when a compressed or stretched spring is trying to return to an inert state. The string always wants to return to equilibrium and will do what it can to do so.
Electrical force is an attraction between positively and negatively charged objects. The closer the objects are to each other, the higher the electrical force.
Magnetic force is an attraction force usually associated with electrical currents and magnets. Magnetic force attracts opposite forces. Each magnet has a north and a south end, each of which attracts the opposite ends of another magnet. For example, a north magnetic end will attract a south magnetic end, and vice versa. North ends of magnets repel each other, and vice versa. Magnets also create an attractive force with certain metals.
Upthrust is more commonly known as buoyancy. This is the upward thrust that is caused by fluid pressure on objects, such as the pressure that allows boats to float.
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