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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