3kg-----\"A\"------4kg------\"B\"------> \"F\" Two masses are attached via ropes

Question

3kg-----"A"------4kg------"B"------> "F"

Two masses are attached via ropes and are free to slide on a frictionless table. Harry pulls with force F such that the meter "A" reads 84N. What is the force "F"?

Explanation / Answer

The whole system accelerates with some acceleration. So, if the force on the 3kg mass is 84N, then the acceleration is F = ma -> a = F/m = (84N)/(3kg) = 28 m/s^2. This is the same acceleration on the 4kg mass. So, what's the force pulling on the 4kg mass? Well, the total mass it's pulling on is 7kg, and F = ma = (7kg)(28m/s^2) = 196N. Note: Sorry about the previous answer, I forgot to consider the two masses as a whole system. The whole system has a mass of 7kg, which is why you use that.

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