Topic : Direction of the work done by force of friction Consider a case when a block is placed on a horizontal platform at rest. It then starts accelerating with a constant acceleration such that, Friction present between the block and the horizontal platform > mass times its acceleration so clearly sliding won't take place.. but can someone tell me how with reference to the fixed frame is the work done by force of friction +ve??? actually one more doubt of mine is ... we know that friction opposes motion of a body. It always acts in a direction opposite to the motion of the body. and as stated "the motion of a body with constant acceleration, the firction being greather than the mass times its acceleration"... how can this be possible? is it all about limiting friction..? i m muddled .... kindly help... and i m a beginner.....!!