Acceleration used should be -2.4.
If you used acceleration as +2.4, the tension should be negative according to its free diagram
mg-t=ma , which yields to 12.02 kg
A lamp hangs vertically from a cord in a descending elevator that declerates at 2.4m/s^2 a) if the tension in the cord is 89N what is the lamp's mass? b) what is the cord's tension when the elevator is ascends with an upward acceleration of 2.4m/s^2?
I know T=w where w=mg. Would I use T-mg=ma? I should be getting 31.3 kN for a) and 24.3kN for part b but I can't figure out my problem.
Yes, use
T - mg = ma.
T does NOT equal mg when the elevator is accelerating or decelerating.
In the first case,
89 N = m (g + a)
m = 89/(9.8 + 2.4) = 7.3 kg
I used a plus sign for "a" since it is decelerating and descending
The (a) problem asks for a mass, so I don't see how the answer can be in Newtons.
I don't agree with your part (b) "should" answer either.
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