Two identical `5 kg` blocks are moving with same speed of `2 m//s` towards each other along a friction less 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:

Explanation

Here, `m_(1)=m_(2)=5kg,`
`v_(1)=2m//s, v_(2)=-2m//s`
Final velocity `v=0`
As no external forces are involves, therefore,
`vec(F)_(ext)=0, W_(ext)=vec(F)_(ext).vec(s)=Zero`
According to work energy theorem,
Total work done `=` Change is KE
`W_("ext")+W_("int")=` final KE - inital KE
`=0-((1)/(2)m_(1)v_(1)^(2)+(1)/(2)m_(2)v_(2)^(2))`
`0+W_(i nt)=-(1)/(2)[5(2)^(2)+5(-2)^(2)]=-20J`
`:. W_("int")=-20J`
Negative sign show that internal forces of action and reaction act on the two blocks in a direction opposite to their motion.