Amachine can never be 100% efficient because some work is always lost due to .

Efficiency of machine is defined by

here we know that

= output useful work

= input energy

Now here if we obtain output work same as input thermal energy then it is 100% efficient

but this is only possible when we have no energy loss while it is not possible due to many reasons

one of the main cause of energy loss is friction

So when machine working there must be friction between its working parts due to which out input energy is lost in different parts and hence we can never be 100% efficient machine.

So here main reason is FRICTIONAL LOSS

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s