It is most noticeable for objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas. This force will frequently be neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value). The force of air resistance is often observed to oppose the motion of an object. The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The friction force is discussed in more detail later on this page. The maximum amount of friction force that a surface can exert upon an object can be calculated using the formula below: As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together. Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces. For example, if a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion. Though it is not always the case, the friction force often opposes the motion of an object. There are at least two types of friction force - sliding and static friction. The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. For instance, if a person leans against a wall, the wall pushes horizontally on the person. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. The normal force is the support force exerted upon an object that is in contact with another stable object. (Caution: do not confuse weight with mass.) The force of gravity on earth is always equal to the weight of the object as found by the equation: Fgrav = m * g where g = 9.8 N/kg (on Earth) All objects upon earth experience a force of gravity that is directed 'downward' towards the center of the earth. By definition, this is the weight of the object. The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. Gravity Force (also known as Weight) F grav The applied force is the force exerted on the desk by the person.

If a person is pushing a desk across the room, then there is an applied force acting upon the object. An applied force is a force that is applied to an object by a person or another object.

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