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## ksaimouli 2 years ago newton plane

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1. ksaimouli

|dw:1358006505942:dw|

2. ksaimouli

can u draw components indicate +and -

3. ksaimouli

@mathmate

4. mathmate

Have you learned about FBD?

5. ksaimouli

i am always getting T-Mg cos 60 =Ma

6. ksaimouli

what is FBD

7. mathmate

Free body diagram.

8. ksaimouli

i know

9. mathmate

So the next step is to isolate the object and put all forces that are applied TO the body.

10. mathmate

Memorizing formulas is dangerous, because sometimes theta can refer to something else.

11. ksaimouli

|dw:1358006756106:dw|

12. ksaimouli

i did not memorize i drew FBD

13. ksaimouli

something wrong with vectors

14. mathmate

|dw:1358006741723:dw| Do you have something like this?

15. ksaimouli

^ what i have

16. mathmate

According your your FBD, "T-Mg cos 60 =Ma" is not correct.

17. ksaimouli

i mean T-Mg sin 60

18. ksaimouli

=Ma

19. mathmate

That's good. Can you continue?

20. ksaimouli

yes i can but i am getting acceleration as -

21. ksaimouli

|dw:1358007157522:dw|

22. ksaimouli

this is other half

23. ksaimouli

-T+mg sin 30 =Ma

24. ksaimouli

M is 5kg for both find the acceleration

25. mathmate

|dw:1358007244777:dw| So you have two unknowns (T and a) and two equations. :)

26. ksaimouli

ok so i got mgsin 30-mgsin 60=ma+ma

27. ksaimouli

m=5kg for both i got a=-1.830 which is wrong

28. ksaimouli

i must get a=1.830

29. mathmate

Your equation assumed that the masses accelerate DOWN the 30 deg. plane, so a<0 is correct! :)

30. mathmate

It is logical that the masses slide down the 60 deg. plane. So put mg sin(60)-T = ma , and T-mg sin(30) = ma This assumes mgsin(60)>T (=> a>0) and T>mg sin(30) , again => a>0

31. ksaimouli

so what ur doing is making accelration + right

32. ksaimouli

thanks

33. mathmate

You don't always have to guess the direction right the first time. Make a reasonable guess, using logic, and if it comes out negative, reexamine to see if it is logical, or if you made a mistake. This is how scientiests work: examine the work when there is a surprise.

34. mathmate

yw! :)

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