Newton’s Laws of Motion

Newton’s First Law• If the external net force on an object is zero,

the object will remain at rest or continue to move at a constant velocity (it is in equilibrium)

• Examples:• A car on cruise control• Sky diver who has reached terminal velocity• A boulder on the ground at rest

• This law is often called the Law of Inertia, as inertia is a property of matter which causes an object to resist changes to its motion.

Newton’s Second Law• The acceleration of an object is directly

proportional to the net force acting on an object, and is inversely proportional to the mass of the object

a = Fnet / m• From this equation, one notices that the net

force and acceleration of an object are always in the same direction. As well, the equation above is often rearranged in the form

Fnet = ma

Example 1Sandy pedals her bicycle with a force of 12.0N [S]. A headwind exerts a force of 2.00N against her motion. If the total mass of Sandra and the bicycle is 65.0kg, what is her acceleration?

Newton’s Third Law• For every action force, there is a

simultaneous reaction force equal in magnitude but opposite in direction.

• This means that for every action force, there is an equal and opposite reaction force (i.e. FA = -FR). However, it is important to remember that the action-reaction fore pairs act on a different object, thus different FBDs.

Action-Reaction Pair

Newton’s Third Law Examples

Example 2Identify at least 6 pairs of action-reaction force pairs in the following image