An object’s velocty will not change unless a force acts on the object. Therefore, if not net force acts upon an object then:

• an object at rest will remain at rest
• a moving object will maintan constant velocity.

This natural resistance to change is the inertia. The mass of an object measures its inertia. Mass and weight are not the same, however.

If F_net is the net force acting on an object of mass m, then the aceleration (a) follows the equation:

F_net = ma

• F_net is the sum of all forces acting on the object.
• If F_net = 0, then a = 0.
• A force of 1 kgºm/s² is 1 netwon (N).

If Object 1 exerts a force F₁ on Object 2, then Object 2 exerts a force F₂ on Object 1. These forces have the same magnitude but act in opposite directions (F₁ = -F₂) and act on different objects. These two forces form an action-reaction pair. For every action, there is an equal but opposite reaction. However, the effects of these equal-strength forces can be different. For example, imagine a collision between a bus and a pedestrian. Both the bus and the pedestrian experience equal force, but the pedestrian has less mass and so will undergo more acceleration. The bus will undergo very little acceleration.

When two surfaces are in contact, there is an electrical attraction between atoms of one surface and those of another. This is called friction. There are two kinds of friction:

• Kinetic (sliding) friction is when one surface is moving across another.
• Static friction is when the two surfaces are not moving across each other.

When you place a book on a table, there are two forces acting upon it: gravitational force and normal force. Normal force is perpindicular to the table, and cancels the weight of the book. This prevents the book from crashing through the table. Normal force is abbreviated F_N or N (not to be confused with the abbreviation for a newton).