EK 1001 Physics #260

[Se Parker]

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Here's the question:

Which of the following is true about a system that is NOT in equilibrium?

A. The motion of the system cannot be linear
B. The sum of the forces is proportional to the acceleration of the system
C. The velocity of the system is changing at a constant rate
D. The acceleration of the system is proportional to its mass

The book says the answer is B, although I feel like it could be either B or D

F=MA

Answer B is right because according to the formula, Force is directly proportional to Acceleration; however, D is also right because Acceleration is inversely proportional to mass.

I wanted to just move on and mark this as a typo but I started wondering if maybe whenever I see "proportional" on an MCAT question I'm supposed to assume it means directly proportional unless it says inversely proportional...but personally I think that's stupid. What do you guys think? Am I missing something?

 

[MedPR]

The acceleration of a system is independent of a mass. Think about gravity. Every mass accelerates at -10m/s^2.

 

[Se Parker]

The acceleration of a system is independent of a mass. Think about gravity. Every mass accelerates at -10m/s^2.

Makes perfect sense. I'm thinking of the Force being constant for some reason...

Thanks

 

[milski]

The acceleration of a system is independent of a mass. Think about gravity. Every mass accelerates at -10m/s^2.

That is only true because the gravity force is also proportional to the mass. Acceleration does depend on the mass and is inversely proportional to it. (a=1/m * F)

If D was correct, higher mass would have higher acceleration for the same force.

 

[ljc]

Acceleration is inversely proportional to mass. This is the opposite of what D says.

 

[milski]

Makes perfect sense. I'm thinking of the Force being constant for some reason...

Thanks

That's the right way to think about it. Only for a constant force increased mass will give you decreased acceleration, making it inversely proportional, not proportional.