NEWTON’S LAW

An imbalance of forces (F) means that the sum of all the forces in any one direction is greater than the sum of all the forces in the opposite direction.

 

Newton’s Law says:

 

·         that if there is an imbalance of forces (F) acting on a particle, then this must cause an acceleration. Another way of stating this is to say, that if a particle is accelerating, then there must be an imbalance of forces.

·         If there is an imbalance of forces (F), then F= ma where: 

 

F   = total force in direction of motion caused by an imbalance of forces

m   = mass

a    = acceleration

 

HINTS:

• Always draw an accurate diagram with ALL the forces and with an arrow for acceleration
• Look at the acceleration arrow to see which way it is pointing
• Find all the forces which are pointing in exactly the same direction as the acceleration arrow and all the forces which are pointing in the exact opposite direction (including components of forces which are not horizontal or vertical).
• The F part of F = ma will be all the forces in the same direction as the acceleration arrow
  
   all the forces in the opposite directon.

 

Simple Case

 

The only force in the direction of a is F so,

 

F          = ma

 

 

2 Force Case

 

   

Acceleration a is pointing to the right. Force A is in the same direction, force R is in the opposite direction so,

 

F          = ma

A

 R  = ma

 

3 Force Case

 

             

 

Acceleration a is pointing to the right. Force A has a horizontal component in the same direction, force R has a horizontal component in the opposite direction so,

 

F                                  = ma

Acosa  Bcosb  R    = ma