Homework Help: Physics

Posted by Kate on Sunday, February 28, 2010 at 6:27pm.

h t t p : / / w w w . j i s k h a . c o m / d i s p l a y . c g i ? i d = 1 2 6 7 3 9 3 5 9 3

ok heres the problem in a picture

h t t p : / / i m g 1 9 2 . i m a g e s h a c k . u s / i m g 1 9 2 / 4 5 0 0 / c a p t u r e x x w . j p g

ok I called the 4 kg block m3 and the 2 kg block m2 and the 1 kg block m1

I don't see what is wrong with my work

a. net force on m3 = m3 a = Fg3 - Fg2 - Fg1

divide by m3 to get a

a = (Fg3 - Fg2 - Fg1)/m3

made it look nice

a = (m3 g - m2 g - m1 g)/m3
= g - (m2g)/m3 - (m1 g)/m3
= g(1 - m2/m3 - m1/m3)
=9.8(1 - 2/4 - 1/4) = 2.5 m/s^2

b. net force on m3 = m3 a = Fg3 - Ft3
solved for the force of tension
Ft3 = -m3(a+g) = -4.0(2.5+9.8)= - 49 N

c. Net force m1 = m1 a = -Fg1 + Ft1
solved for Ft1
Ft1 = m1 (a+g) = 1.0(2.5 + 9.8) = 12 N

Physics - bobpursley, Sunday, February 28, 2010 at 7:42pm
what is wrong with your work:

you wrote: a = (m3 g - m2 g - m1 g)/m3

but that means m3*a=net force which is NOT true.

NET FORCE= totalmass*acceleration
total mass is Mass1+Mass2+mass3
you cannot apply net force to just one object.

you should have written:
a = (m3 g - m2 g - m1 g)/(m3+m2+m1)
Physics - Kate, Sunday, February 28, 2010 at 8:30pm
hmmmmmm thanks

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