TBR, Physics, Forces, #35

[Sammy1024]

---

Members do not see this ad.

I was wondering why the answer is A instead of D. If they're asking for the magnitude of acceleration, it shouldn't matter whether m2 is going up or down right? Maybe I have it wrong.

Choice A is the best answer. Choice C is the first to be eliminated, because the kg unit doesn't cancel out. If we consider the case where m1 is greater than m2, then we predict by observation that the system will accelerate downward on the left side. The magnitude of "a" should increase as m1 increases and m2 decreases. Choice D definitely doesn't fit this observation, so it is eliminated. If m2 is greater than mi, then we would expect the mass on the left side to accelerate upwards, so the sign of a has to be different when m2 >m1 than it was when m1 > m2. Only choice A fits this restriction. The best answer is choice A.

 

[orangetea]

to approach these problems I like you employ the "extreme cases" approach which makes it a ton easier and faster.

Since you are stuck between A and D I will go through those to show you why A is the answer 🙂

First Scenario: Assume that mass 1 is REALLY BIG like 100 tons or something like that and mass 2 is 0. What would the acceleration be? (gravity, since M1 would be pulled down) okay so let's put that in our equations

D: 0(g)/(100 + 0)= 0 (which is not equal to g)
A: (100-0 )*g / (0-100) *g= 1/1 *g = g (yay!)


Second Scenario: Assume the opposite for m1 and m2. Now you will get negative g. (I assumed that down was positive and up was negative. But either way it should work out)

D: 100(g)/(0 + 100)= Some really big number (which is not equal to g)
A: (0-100 )*g / (100-0) *g= -1/1 *g = -g (yay!)

Hope that made sense! 🙂