The net force on an object is the vector sum of all the forces acting on the object. Newton's first law says that if this sum is zero, the state of motion of the object does not change. Essentially, it makes the following two points:

• An object that is not moving will not move until a net force acts upon it.

• An object that is in motion will not change its velocity (accelerate) until a net force acts upon it.

FREE BODY DIAGRAMS

To identify the various forces that act on the body, resulting in any one of the states of rest, constant velocity, change in direction, change in shape, acceleration, change in rotational energy etc , can be determined only when we are able to identify all the forces that act on the body. For serving this purpose, we use FREE BODY DIAGRAMS.

These are simplified representations of an object (the body) in a problem, and includes force vectors acting on the object. This body is free because the diagram will show it without its surroundings; i.e. the body is 'free' of its environment. This eliminates unnecessary information which might be given in a problem.

The main forces that act on the body are:

Gravity
Normal Force
Friction
Push or Pull
Tension

Type of Force and its Symbol	Description of Force
Applied Force Fapp	An applied force is a force which is applied to an object by another object or by a person. If a person is pushing a desk across the room, then there is an applied force acting upon the desk. The applied force is the force exerted on the desk by the person.
Gravity Force (also known as Weight) Fgrav	The force of gravity is the force with which the earth, moon, or other massive body attracts an object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity which is directed "downward" towards the center of the earth. The force of gravity on an object on earth is always equal to the weight of the object as given by the equation: Fgrav = m * g where: g = acceleration of gravity = 9.8 m/s2 (on Earth) m = mass (in kg)
Normal Force Fnorm	The normal force is the support force exerted upon an object which is in contact with another stable object. 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. On occasion, a normal force is exerted horizontally between two objects which are in contact with each other.
Friction Force Ffrict	The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. The friction force opposes the motion of the object. For example, if a book moves across the surface of a desk, the desk exerts a friction force in the direction opposite to the motion of the book. Friction results when two surfaces are pressed together closely, causing attractive intermolecular forces between the molecules of the two different surfaces. As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together. The friction force can be calculated using the equation:
Air Resistance Force Fair	Air resistance is a special type of frictional force which acts upon objects as they travel through the air. Like all frictional forces, the force of air resistance always opposes the motion of the object. This force will frequently be ignored due to its negligible magnitude. It is most noticeable for objects which travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.
Tensional Force Ftens	Tension is the force which is transmitted through a string, rope, or wire when it is pulled tight by forces acting at each end. The tensional force is directed along the wire and pulls equally on the objects on either end of the wire.
Spring Force Fspring	The spring force is the force exerted by a compressed or stretched spring upon any object which is attached to it. This force acts to restores the object, which compresses or stretches a spring, to its rest or equilibrium position. For most springs (specifically, for those said to obey "Hooke's Law"), the magnitude of the force is directly proportional to the amount of stretch or compression.

Examples:

• A book is at rest on a table top. Diagram the forces acting on the book.



• A girl is suspended motionless from the ceiling by two ropes. A free-body diagram for this situation looks like this:

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