Coefficient of Friction & F = ma (Edexcel A Level Maths): Revision Note

Coefficient of Friction - F = ma

How do we apply Newton’s Second Law (F = ma) in problems involving friction?

• The coefficient of friction combined with F = ma allows you to determine an object's motion where friction is involved in a problem

• For problems where the surface is horizontal:

  
  Step 1. If necessary, resolve any angled forces into vertical and horizontal components
  
  

  Step 2. Calculate the normal reaction force
  
  

  • Be careful – if there are vertical forces other than gravity these will affect the value of R

  • with a horizontal surface R will always be directed vertically upwards

  • the magnitude of R will be such as to make the total vertical force on the object zero

  Step 3. Calculate F_MAX= μR and find the resultant (total force) of all the horizontal forces on the object
  
  

  • Remember – if the resultant of the other horizontal forces is less than or equal to F_MAX then friction will exactly balance those forces out and the object will remain stationary

  Step 4. Use F = ma to determine the acceleration of the object
  
  

• For non-horizontal surfaces see the notes on inclined planes