## ashna 3 years ago Two identical 5kg blocks are moving with same speed of 2m/s towards each other along a frictionless horizontal surface . The blocks collide , stick together and comes to rest . Considering the two blocks as a system , calculate the work done by (1) external forces (2) internal forces

1. ashna

@akash123

2. akash123

work done by all the forces= change in the kinetic energy

3. ashna

okay

4. akash123

so what are the external forces exerting on these two blocks?

5. ashna

i did'n get the question , what external and what internal ?

6. ashna

gravity ?

7. akash123

there is no friction...so only external forces are gravity and normal reaction

8. ashna

okay

9. akash123

internal forces..when they'll collide with each other then they will exert forces on each other..that's the internal force

10. ashna

okay

11. akash123

it's fine?

12. akash123

ok

13. akash123

now will the external forces( gravity and normal reaction) do work?

14. ashna

no ?

15. akash123

|dw:1349445206541:dw|

16. akash123

yes...they wont do any work because they are perpendicular to d displacement...so no work done

17. ashna

okay understood

18. akash123

so from work energy theorem work done= change in the kinetic energy --> work done by external forces+ work done by the internal forces= change in the kinetic energy of the system

19. akash123

so u can calculate the work done by the internal forces

20. akash123

u know the work done by the external forces(=0) and the change in the KE of the system

21. ashna

okay

22. akash123

u'll calculate the change in the KE of the system?

23. akash123

final KE=0 initial KE= 1/2 m1 v1^2 + 1/2 m2 v2^2

24. ashna

okay

25. akash123

m1= m2=5kg and v1=v2=2m/s

26. ashna

20 J ?

27. akash123

yes

28. akash123

so work done by internal forces= (final KE- initial KE)= -20 J

29. ashna

okay

30. akash123

u got the meaning of -ve sign? I mean...why work done is -ve?

31. ashna

yes 0-20 ?

32. akash123

-ve work done because they r pulling down the energy of the system to zero

33. ashna

okay .. i understand ! thank you :)

34. akash123

:)