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ksaimouli
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|dw:1358006505942:dw|
ksaimouli
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can u draw components indicate +and -
ksaimouli
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@mathmate
mathmate
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Have you learned about FBD?
ksaimouli
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i am always getting T-Mg cos 60 =Ma
ksaimouli
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what is FBD
mathmate
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Free body diagram.
ksaimouli
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i know
mathmate
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So the next step is to isolate the object and put all forces that are applied TO the body.
mathmate
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Memorizing formulas is dangerous, because sometimes theta can refer to something else.
ksaimouli
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|dw:1358006756106:dw|
ksaimouli
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i did not memorize i drew FBD
ksaimouli
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something wrong with vectors
mathmate
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|dw:1358006741723:dw|
Do you have something like this?
ksaimouli
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^ what i have
mathmate
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According your your FBD,
"T-Mg cos 60 =Ma" is not correct.
ksaimouli
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i mean T-Mg sin 60
ksaimouli
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=Ma
mathmate
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That's good. Can you continue?
ksaimouli
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yes i can but i am getting acceleration as -
ksaimouli
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|dw:1358007157522:dw|
ksaimouli
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this is other half
ksaimouli
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-T+mg sin 30 =Ma
ksaimouli
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M is 5kg for both find the acceleration
mathmate
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|dw:1358007244777:dw|
So you have two unknowns (T and a) and two equations. :)
ksaimouli
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ok so i got mgsin 30-mgsin 60=ma+ma
ksaimouli
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m=5kg for both i got a=-1.830 which is wrong
ksaimouli
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i must get a=1.830
mathmate
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Your equation assumed that the masses accelerate DOWN the 30 deg. plane, so a<0 is correct! :)
mathmate
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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
ksaimouli
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so what ur doing is making accelration + right
ksaimouli
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thanks
mathmate
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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.
mathmate
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yw! :)