A block of mass m is placed on triangular block of mass M, which in turn is placed on a horizontal surface as shown in figure (9-E21). Assuming frictionless surfaces find the velocity of the triangular block when the smaller block reaches the bottom end.

The block m will slide down the inclined plane of mass M with acceleration 
(relative to the inclined plane).
The horizontal component of a1 will be
for which the block M will accelerate towards left.
Let the acceleration be a2. According to the concept of centre of mass, (in the horizontal direction) external force is zero.

Thus,
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 _________ (1)
So, the absolute acceleration of m on the block M alone the direction of the incline will be
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 ________ (2)
Let the time taken by the block m to reach the bottom end be ‘t’
Now,
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So, the velocity of the bigger block after time ‘t’ will be vm
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Now, putting the value of ‘a’ to the above equation

