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4b. Meaning of Force
 | A body will move if the applied force is greater than the limiting friction.
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May the force be with you!
Newton's laws of motion are simple, direct statements about forces and their relationship to motion. But what exactly is a force?
In general, a FORCE is either a push or a pull. You pull a barbell in lifting it off the ground or push a car that has stalled on the road. In both cases, a force is applied.
| Force is a vector quantity. Roughly speaking, a force is a push or a pull. |
Notice that the term "force" can describe not only how hard you push or pull but also in which direction. In other words, force has both a magnitude and a direction, which means that force is a vector, just like velocity and acceleration.
Although it is possible to list hundreds of different forces, they all fall into one of two categories: contact forces or action-at-a-distance forces. CONTACT FORCES are probably a bit easier to visualize and arise only when one object is in physical contact with (i.e., touching) another. For example, when someone presses the doorbell or pushes on the door to open it, a contact force is exerted on the doorbell or door.
Examples of Contact Forces
| Contact Force | Description |
| tensional force | a pulling force by a rope, wire, or other object being stretched |
| frictional force | a pulling force resulting when one surface slides or attempts to slide against another; for example, a ball rolling along the ground |
| air drag | a pushing force by the air opposing the motion of an object through the air; for example, a feather falling though the atmosphere |
| normal force | a pushing force perpendicular to two surfaces that are in contact; for example, a book resting on a table |
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ACTION-AT-A-DISTANCE forces, as their name suggests, are felt without such physical contact. Examples of these forces include gravity, the electrical force between two charges, and the magnetic force between two magnets.
Examples of Action-at-a-Distance Forces
| Action-at-a-Distance Force | Description |
| gravity | pulling force by one mass on another |
| electrostatic force | pushing or pulling force between charged objects |
| magnetic force | pushing or pulling force between certain types of materials |
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 | The lines surrounding this magnet show the force that will be exerted on any magnet that is brought near it.
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Now that it's clear roughly what a force is, let's figure out what a force does. From their own experiences, most people know that forces can produce changes in motion. Kicking a stationary soccer ball causes the ball to fly off with a certain velocity. If the ball goes up, the force of gravity will eventually bring it down. If the ball just speeds along the ground, the force of friction will slow it down and eventually bring it to rest. These observations can be generalized by saying: A force is a vector quantity capable of changing an object's velocity.
Keep in mind that velocity, too, is a vector and has both a magnitude and a direction. Notice also the word "capable," which is very significant here. Whereas a change in velocity is evidence of a force, a force does not necessarily produce a change in velocity. (Try pushing on a brick wall!) Only when there is a NET FORCE — that is, the forces acting on an object do not completely cancel each other — can a change in motion be expected. But that's the subject of the next section.
  
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