The tendency of objects to resist change in their state of motion is called inertia. Inertia is measured quantitatively by the object's mass. Objects will undergo changes in motion only in the presence of a net (unbalanced) force.

 

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. This is because a force is defined as the interaction between two objects. In the metric system, forces are measured in a unit called a newton. Forces occur only in pairs, one action and the other reaction, both of which constitute the interaction between one thing and the other. Neither force exists without the other. Since action and reaction forces act on different objects, action and reaction forces can never cancel each other.

 

Whenever you speak of net force, you are speaking about ALL of the forces acting on one, unique object. These forces are often summarized in a freebody diagram. If the forces cancel each other, then the net force acting on the body is equal to zero. When this happens, the object is said to be in a state of equilibrium:

• static equilibrium occurs when the object is at rest;
• dynamic equilibrium occurs when the object is moving at a constant velocity.

When a net unbalanced force is impressed upon an object, the object will accelerate. The acceleration is directly proportional to the unbalanced force and is inversely proportional to the object's mass. Symbolically this is written as a ~ F/m. Acceleration is always in the direction of the net unbalanced force. An net unbalanced force of 1 N will result in a 1 kg mass experiencing an acceleration of 1 m/sec².

 

When objects fall in a vacuum, the net force is simply the object's weight and the object is said to be in a state of freefall. We state that the acceleration is a = -g where the variable g denotes that acceleration is due to gravity, 9.8 m/sec². When objects fall through air, the net force is equal to the weight minus the force of air resistance, and the acceleration has a magnitude less than g. If and when the force of air resistance equals the weight of a falling object, acceleration terminates, and the object falls at constant speed called its terminal velocity.

 

 

An object will maintain a constant velocity until an unbalanced, outside force acts upon it.

 

        OR

 

An object at rest will remain at rest, while an object moving at a constant velocity will continue to move in that fashion until it is acted upon by an unbalanced, outside force.

 

 

The acceleration an object experiences is directly proportional to, and in the same direction as, the net force acting upon it and is inversely proportional to the object's mass.

 

net F = ma

 

 

If object A exerts a force on object B then object B exerts an equal but opposite force on object A.

 

F_AB = - F_BA