Condition of Sliding of a Block over another Block

The concept of friction is very important in variety of of problems involving concept of Newton’s Laws of Motion. Whenever a block is placed over the rough surface of another block then it is also important to analyze the condition on external effects under which the upper block starts sliding.

Several problems

are framed on such condition.

When a body is subjected to external forces and if it is at rest relative to the surface on which it is placed

then the friction acting at the contact would be static friction. If external force is increased the

magnitude of static friction between bodies will increase and as it approaches limiting value, bodies will

start sliding.

This concept can be easily applied when the lower block is at rest but when both blocks are

in motion, we need to be very careful while solving the equations of Laws of Motion as the analysis is

now done relative to lower block.

See the video below on how to apply the concept in finding the

condition of sliding when a block is moving over another block –

To apply the logic in various situations, see the examples explained in below videos –

Misconception: While solving problem of finding condition of sliding of a block over another the most

common mistake students do by simply calculating static friction and equate it with limiting friction in

the reference frame of lower block and ignore the motion of lower block. Due to acceleration of lower

block the static friction on upper block changes. If you analyze the problem in the reference frame of

lower block which is accelerating always consider the Pseudo Force on the upper block relative to lower


There can be variety of problems which can be framed on this concept of sliding of a body over another


See the below advance illustrations to develop a thorough understanding on application of this