Drag Force

[G1SG2]

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Suppose you have a question that tells you that two objects, A and B, in free fall are acted upon by the force of air resistance. Since the force is directly proportional to the velocity (I think the equation goes something like Fd=1/2*c*d*a*v^2 where c is the drag coefficient, d is the density, a is the area and v is the velocity). Now, if we're told that object A is acted upon by a greater force of air resistance, then it must be due to a greater velocity, right? Or does this also tell us that object A necessarily has a greater mass than object B?

 

[sv3]

Suppose you have a question that tells you that two objects, A and B, in free fall are acted upon by the force of air resistance. Since the force is directly proportional to the velocity (I think the equation goes something like Fd=1/2*c*d*a*v^2 where c is the drag coefficient, d is the density, a is the area and v is the velocity). Now, if we're told that object A is acted upon by a greater force of air resistance, then it must be due to a greater velocity, right? Or does this also tell us that object A necessarily has a greater mass than object B?

I would think the object with the greater mass has a greater inertia and is less likely to be affected by air resistance. I don't think TPR covers this and Ek briefly mentioned it so thats where im getting my info. So if i had to choose id say A has a higher velocity and less mass.

 

[G1SG2]

I would think the object with the greater mass has a greater inertia and is less likely to be affected by air resistance. I don't think TPR covers this and Ek briefly mentioned it so thats where im getting my info. So if i had to choose id say A has a higher velocity and less mass.

Yeah, that sounds about right. But wouldn't A have the higher mass, as per that reasoning (more mass, more inertia)? Anyone else want to chime in as well?

 

[sv3]

Yeah, that sounds about right. But wouldn't A have the higher mass, as per that reasoning (more mass, more inertia)? Anyone else want to chime in as well?

Not from what I know and thats very very little from 2 lines in Ek. Greater the mass, the less air resistance will bother it. So if its A that feels more air resistance, i would guess its lighter.

 

[minutemen11]

i think mass isnt as big a factor as surface area. So if object A has greater air resistance it follows that it has greater surface area. this is why cats spread themselves flat if they are falling, increasing surface area= greater air resistance= lower falling speed= cat might survive than if it fell like a ball.

 

[minutemen11]

on second thought i think mass does not play a role at all in air resistance: -0.5pv2ACdV
p is the density of the FLUID (air)
v= velocity
A= surface area (this is why greater surface area= greater air resistance)
C= is drag const.
V= nothing important just a unit vector not related to this question.

 

[Dr Gerrard]

Yeah, that is what I was leaning towards.

The effect of air resistance is lower when mass is greater, because net force, ma = mg - Fd.

If mg is greater, Fd is less significant. For example, if mg = infinity, air resistance would not play a role at all, although it would still be there.

This is assuming what is posted above is true.

 

[minutemen11]

Yeah, that is what I was leaning towards.

The effect of air resistance is lower when mass is greater, because net force, ma = mg - Fd.

If mg is greater, Fd is less significant. For example, if mg = infinity, air resistance would not play a role at all, although it would still be there.

This is assuming what is posted above is true.

Umm you can never ignore air resistance, its kinda like gravity (is air resistance a conservative force?). Mass does have have any effect on the air resistance, only the surface area.. look at my equation above there is no factor implicating mass. so regardless if you dropped a feather or a bowling ball, air resistance is always present.. I THINK..

EDIT: ok say you two have things of equivalent mass, like 2,000 kg being transported on a jumbo jet. One object is just a huge round ball, the other a flat piece. The flat piece wieghing the same amount as the round ball will fall at a lower speed due to the its greater surface area.

 

[G1SG2]

Umm you can never ignore air resistance, its kinda like gravity (is air resistance a conservative force?). Mass does have have any effect on the air resistance, only the surface area.. look at my equation above there is no factor implicating mass. so regardless if you dropped a feather or a bowling ball, air resistance is always present.. I THINK..

Hey, no, it's not a conservative force.

Okay, so according to the equation, the object that is acted upon with the greatest force of drag must be the one with the greater velocity, yes? Since Fd is directly proportional to V^2. It definitely has to do with velocity-the explanation for the question pertaining to this was:

"Greater air resistance on a larger object must be due to larger velocity (F is directly proportional to velocity); therefore, the larger object is falling with a greater speed (both experiencing no net force)"


In that question, I am told that the object was larger. I just wanted to see if that was something we could've predicted without being told that it was larger, just based upon the drag equation and good old inertia stuff.

 

[minutemen11]

OOOO sorry haha guess i was waay off then..

but why isnt air resistance a conservative force? i know that a conservative force is not dependent on the path taken and non conservative forces do depend on the path taken... sorry for being a *****, I never really learned these things in physics maybe i dont remember because i didnt have to take the last exams- i already had an A..

 

[Pulsar]

OOOO sorry haha guess i was waay off then..

but why isnt air resistance a conservative force? i know that a conservative force is not dependent on the path taken and non conservative forces do depend on the path taken... sorry for being a *****, I never really learned these things in physics maybe i dont remember because i didnt have to take the last exams- i already had an A..

Air resistance is a form of friction, and friction is not a conservative force. Energy is lost to the air in the form of heat and sound.

 

[G1SG2]

OOOO sorry haha guess i was waay off then..

but why isnt air resistance a conservative force? i know that a conservative force is not dependent on the path taken and non conservative forces do depend on the path taken... sorry for being a *****, I never really learned these things in physics maybe i dont remember because i didnt have to take the last exams- i already had an A..

Think of air resistance as a type of friction. Conservative forces CONSERVE mechanical energy. Friction takes away mechanical energy. If you launched a projectile with a certain KE, we usually have KEf=KEi by conservation of energy. However, this is when we ignore the effects of air resistance. If we didn't then KEf would be less than KEi. From the conservation equation, we have:

Ei=Ef or KEi+PEi=KEf+PEf. However, if we consider friction or air resistance, the equation will become Ei-Work done by friction=Ef, as friction slows our projectile down and takes away mechanical energy.

 

[minutemen11]

thanks! both are really good explanations

 

[amine2086]

Mass does not change the force of air resistance, but it changes how the object behaves in a given resistance. For example, you can throw a lead ball much farther than a plastic ball since the lead ball has much higher inertia, it can combat the force of air resistance a lot better.

 

[sv3]

Mass does not change the force of air resistance, but it changes how the object behaves in a given resistance. For example, you can throw a lead ball much farther than a plastic ball since the lead ball has much higher inertia, it can combat the force of air resistance a lot better.

so then we're down to velocity and surface area and shape of an object.....as factors that affect the FORCE of air resistance?

whereas mass doesn't change force, just changes behaviour?

is that right?

 

[G1SG2]

so then we're down to velocity and surface area and shape of an object.....as factors that affect the FORCE of air resistance?

whereas mass doesn't change force, just changes behaviour?

is that right?

Unless you get hit with a question that says something about WHY the larger object needed a greater force of air resistance to bring it to the same speed or level as a smaller object...then it would be because it has more inertia and therefore needed a greater force to accelerate it. I guess it has to do with the wording and the type of problem being asked. Isn't the MCAT great? lol.

 

[sv3]

Unless you get hit with a question that says something about WHY the larger object needed a greater force of air resistance to bring it to the same speed or level as a smaller object...then it would be because it has more inertia and therefore needed a greater force to accelerate it. I guess it has to do with the wording and the type of problem being asked. Isn't the MCAT great? lol.

Do you mean slow it down? Since the smaller object is travelling slower due to the greater force of air resistance, the larger object need a greater force of air resistance to slow it down more?

And no, i hate the mcat. it's aging me beyond my years and im already old dammit.

 

[G1SG2]

Do you mean slow it down? Since the smaller object is travelling slower due to the greater force of air resistance, the larger object need a greater force of air resistance to slow it down more?

And no, i hate the mcat. it's aging me beyond my years and im already old dammit.

Yup, to slow it down.

 

[amine2086]

so then we're down to velocity and surface area and shape of an object.....as factors that affect the FORCE of air resistance?

whereas mass doesn't change force, just changes behaviour?

is that right?

You got it.