Two identical `5 kg` blocks are moving with same speed of `2 m//s` towards each other along a frictionless horizontal surface . The two blocks collide , stick together and come to rest. Consider the two blocks as a system . The work done by the external and ijnternal force are respectively:

Options

A. `0,0`
B. `0,20 J`
C. `0, -20 J`
D. `21 J, -20 J`

Answer

C

Explanation

`F_(ext) = 0, :. W_(ext) = vecF_(ext) . Vec s = 0`
By work energy theorem: `W = K_(F) - K_(I)`
`W_(ext) + W_("int") = 0 - ((1)/(2) mv^(2) + (1)/(2) mv^(2))`
`W_("int") = - mv^(2) = - 5 xx 2^(2) = - 20 J`