There are situations where you have two objects interacting with each other, which means that you’ll have two unknown variables you’ll solve for (usually acceleration). You can solve these types of problems in a couple different ways. First, you can look at the entire system and consider both objects as only one object. For example, the Earth and Moon might be combined into one object if we’re looking at objects that orbit the sun, so the mass of the Earth and Moon would be combined into a single mass, m, and would also have the same acceleration, a. This approach is used if you really don’t care about what’s going on between the two objects. Or you could treat each object as it’s own separate body and draw FBD for each one. This second approach is usually used if you need to know the forces acting between the two objects.
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Thanks so much for your eagle eye! I’ll get that corrected in the updated version of the video.
Similar to previous note, the F(rope) calculation shows 16,000kg while solving (rather than the given 1600kg), though the solution given is correct, based on the car’s given mass of 1600kg.
Quick editorial note: the combined mass of the two vehicles as given in the problem (1600kg + 4000kg) is 5600kg, rather than 56,000kg as written while solving the problem in the video. The solution given for acceleration in the video is correct, based on the original given masses (i.e., 4.46 m/s^2)