text: where 
  
  a= acceleration, ft. per seca (negative) 
  
  Fr=net retarding force, lbs. 
  g= gravitational acceleration, ft. per sec.’ 
  W=weight, 
  lbs. 
  M= mass, slugs 
  
  = Wig 
  
  The net retarding force on the airplane, Fr, is 
  the net of drag, D, braking friction, F, and 
  thrust, T. Thus, the acceleration (negative) 
  at any instant during the landing roll is : 
  
  d=$ (Df F--T) 