I have here a problem asking me to find the tension force on the lower end of a rod with mass of 0.6 kg. The rod connects two blocks, the first / upper one has a mass of 5.0 kg while other/lower one has a mass of 4.0 kg. There is an upward force of 150 N applied on the whole system.
F = 150 N
Mass of whole system : 9.6 kg (mass of upper block+mass of rod + mass of lower)
a = 150 N/9.6 kg
a= 15.625 m/s^2
The Attempt at a Solution
Knowing that the system has acceleration:
Fnet = Tb (tension force on point b/ lower end of rod) - (Weight of rod + Weight of lower block)
Tb - (Wrod + Wlowrblck) = ma
HERE IS MY QUESTION (only capitalised for emphasis)
Tb - (5.88 N + 39.2 N) = ma
Tb - 45.08 N = m (15.625 m/s^2)
At this juncture I was having 2nd thoughts on what mass to use but since this problem is in multiple choice, I had to try out some things:
Tb - 45.08 = (4.6 kg)(15.625 m/s^2) -------> the mass I used is the sum of the masses of rod and lower block
Tb = 116.96 N -------> This answer was too large and was not part of the choices.
I had an alternative:
Tb - 45.08 = (0.6 kg)(15.625 m/s^2) ---------> this time I plugged in only the mass of the rod
Tb = 54.455 or approximately 54 N -------> This answer of mine was part of the choices so decided to stick with it.
If my 2nd solution is correct, why should I only consider the mass of the rod and not include the mass of the lower block?
Thank You very much
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