Confused about wind resistance

[neelyboy]

In TBR, wind resistance is stated as being a drag force that "opposes the direction of an object's velocity", and that the magnitude of it depends on the "contact area, viscosity and speed." Also, it is implied that mass plays a role in determining which of two objects will experience a greater wind resistance. "Our empirical conclusion is that objects with a large contact area-to-mass ratio experience a more significant impact of wind resistance than their denser counterparts." Hence, they explain this is why a solid ball is not as impacted by wind resistance as a hollowed-out ball.

HOWEVER,
EK1001 Physics problem # 129 asks, "Which of the following would most likely experience the greatest force of air resistance if dropped from an airplane?"
A. a feather
B. an elephant
C. a bullet
D. a toaster

The explanation says "The surface area, not the mass, changes air resistance."

Therefore I am completely confused! TBR states that denser objects do not experience as much wind resistance, and hence I thought a feather would be the answer to this question (The answer is B, an elephant).

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[justhanging]

The magnitude of wind resistance is not determined by the mass but the impact that wind resistance has is. Things that have more mass have greater inertia and will resist wind resistance and any other force more. Your quotes answer the question, magnitude is determined by "contact area, viscosity and speed" and "objects with a large contact area-to-mass ratio experience a more significant IMPACT of wind resistance."

Hope that helps.

 

[neelyboy]

The magnitude of wind resistance is not determined by the mass but the impact that wind resistance has is. Things that have more mass have greater inertia and will resist wind resistance and any other force more. Your quotes answer the question, magnitude is determined by "contact area, viscosity and speed" and "objects with a large contact area-to-mass ratio experience a more significant IMPACT of wind resistance."

Hope that helps.

I just thought that the mass of an elephant would it make it so much more dense than a feather. Right?

 

[justhanging]

Sure but mass doesn't determine magnitude of wind resistance. All density will tell you is how much impact wind resistance will have.

 

[BerkReviewTeach]

Sure but mass doesn't determine magnitude of wind resistance. All density will tell you is how much impact wind resistance will have.

This is the perfect answer, but if it's okay, I might pile on a little.

The feather will have its very small drag force roughly equal to its very small weight, so its flight will be significantly slowed by wind resistance. Wind resistance has a great impact because area:mass is so large.

The elephant will have its medium drag force as much less than its very huge weight, so its flight will be barely slowed by wind resistance. Wind resistance has a small impact because area:mass is so small.

So the elephant will strike the ground before the feather if both were dropped from the same height. (Note to PETA: this is purely hypothetical and no elephants were harmed in the making of this explanation). But the EK question is asking about magnitude of the drag force caused by wind resistance. Both are falling through the same medium, so viscosity can be ignored. But the elephant has greater contact area than the feather and is falling with a greater magnitude of velocity than the feather, so it will experience a greater magnitude of wind resistance than the feather.

Hope that helps.