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Physics question from TPR

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goodYvening

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Hi everyone, I answered incorrectly and still don't understand how to figure out the answer, despite TPR's provided reasoning. Please help.

Consider a quadruped of roughly the same mass and proportion as the human above (say, a large cat), with roughly the same time of contact of the foot with the ground and the same force exerted in the running limbs. Which of the following is true of bipeds and quadrupeds maintaining a steady run?

A)
The biped expends more energy in producing upward motion.

B)
The quadruped expends more energy in producing upward motion.

C)
The biped must use either more force or more time to bring limbs into position to break a fall.

D)
The biped exerts a greater force during the thrusting portion of the run.
 

depasqualemi

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If I were to answer this I'd put C. I don't know if I'm correct though. My reasoning is that the question addresses the fact that both the biped and quadruped are of equal mass, and because Kinetic energy is a function of mass and velocity (which both are held constant) A and B must be false. Force is also a function of mass, as well as acceleration (both again, held constant) so D must be false. What is the correct answer?
 

goodYvening

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You are correct, the answer is C. I know the question says they maintain a stead run, but it does not say they maintain the same velocity. How did you know/were able to assume that?
 

depasqualemi

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They're asking about force and kinetic energy, so from there I think, "ok what is the formula for KE and F". I do this because these variables are dependent on multiple factors, including velocity for both. However, the question never addresses velocity, so I know that isn't an important factor to determining my answer, so I assumed both move at the same speed. KE doesn't determine how fast you move, it's how fast you move that determines KE.
 

ThePrincetonReview

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Hi everyone, I answered incorrectly and still don't understand how to figure out the answer, despite TPR's provided reasoning. Please help.

Consider a quadruped of roughly the same mass and proportion as the human above (say, a large cat), with roughly the same time of contact of the foot with the ground and the same force exerted in the running limbs. Which of the following is true of bipeds and quadrupeds maintaining a steady run?

A)
The biped expends more energy in producing upward motion.

B)
The quadruped expends more energy in producing upward motion.

C)
The biped must use either more force or more time to bring limbs into position to break a fall.

D)
The biped exerts a greater force during the thrusting portion of the run.

When you read questions, make sure to try to catch all the important details! The question stem mentions that the quadruped has roughly the same mass as humans (the biped). As a result, the energy expended in producing upward motion is the same (remember PE=mgh) for bipeds and quadrupeds (choices A and B are wrong). Between choices C and D, things get a little trickier. Choice D states that the biped exerts greater force during the thrusting portion. If you think about it, that could be true or it could be false. You don't know because there is nothing in the passage or question stem to support or contradict this answer choice. Choice C states that bipeds must use more force or time to break a fall. If you think about it, quadrupeds have two limbs to break a fall while bipeds have only one. In that case, it is reasonable to conclude that bipeds would need more force or time to break their fall with their one limb (you may need to review your understanding of the impulse momentum theory to fully understand why!). Between choices C and D, the answer has to be C. On the MCAT, make sure to always go with the answer choice that can be supported by your knowledge of science or the passage! Answer choices like D can often be misleading but are common traps that you should learn to avoid!

If you have any more questions regarding the physics concepts behind this question or how to effectively eliminate wrong answer choices on the MCAT, feel free to PM me!
 
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