"Nature, and Nature's laws lay hid in night.  God said 'Let Newton be!' and all was light"
Alexander Pope

•    An interaction that can cause acceleration of an object is called a force - a push or pull on the object.  Investigation of the effect of forces on the motion of object led Sir Isaac Newton (1642-1727)to formulate his three laws of motion.
  


•  First Law

"In the absence of a net force acting on it, an object at rest will remain at rest and an object in motion will continue in motion at constant speed in a straight line"

When the net force on an object is zero, it does not necessarily mean there are no forces acting on the object.  Usually when the net force is zero there are several forces acting on an object which add (vectorially) to zero.
 

• Second Law

"The net force acting on an abject is equal to the product of the mass and acceleration of the object; the direction of the force is the same as that of the object's acceleration" 

 
• F = ma
• F - net force.
Units : Newtons  1 N = 1 kg.m/s² (SI), Pounds 1 lb = 1 slug.ft/s ²  (British)


• m - mass of object.  Operationally defined in terms of inertia, through the second law.  (Inertia - the reluctance of an object to change its state of rest or constant velocity motion).  All masses are defined relative to the "standard" kilogram .
Units :  kg (SI), slug (British)


• a - acceleration.  Rate of change of speed (velocity)
Units: m/s²  (SI),  ft/s²  (British)


• Mathematically the first law is the zero acceleration special case of the second law.
 
• Third Law

"When an object exerts a force on another object, the second object exerts on the first a force equal in magnitude but opposite in direction"


• Often stated, somewhat less precisely, as "Action equals Reaction"
• Forces always appear in pairs. e.g. a book resting on a table exerts a force on the table (action), the table exerts an equal force in the opposite direction on the book (reaction).
• Important.  Action - reaction forces act on different objects.
  
  

   Supplementary Problems 

"In a packed programme tonight, I shall be having a word with a man who goes in for meditation, because he thinks it's better than sitting around doing nothing ..."

Ronnie Barker