Air Resistance (Conceptual) - EK #125

[ilovemcat]

Question: 125 - Air resistance on a smooth ball launched over level ground:
(increases/decreases) the maximum height and (increases/decreases) the range.
























The solution to this question was that Air Resistance increases the maximum height and range. I assumed that it would decrease both because Air Resistance opposes an object motion. Since the object is being thrown upwards, Air Resistance would act downwards to shorten both the height and indirectly the range as well. Can anyone tell me if this is an error or am I misunderstanding the concept of Air Resistance.

Thank you

---

Members don't see this ad.

 

[RocketSurgeon]

This has to do with terminal velocity

Force due to air resistance is given by KV^2 (Where K is a constant for an object), Draw a FBD

This explains longer range because once the object reaches max height and starts coming down without friction it is accelerating downward with 10m/s^2 through out but with air resistance it accelerates only till when the terminal vel is reached and then at the same velocity so it travels lesser distance/unit time => more time for the horizontal component to increase range...

They making an assumption some where because you would have to integrate the force over the are to get the air resistance and theres no way a problem like that would be on the MCAT..

 

[ilovemcat]

This has to do with terminal velocity

Force due to air resistance is given by KV^2 (Where K is a constant for an object), Draw a FBD

This explains longer range because once the object reaches max height and starts coming down without friction it is accelerating downward with 10m/s^2 through out but with air resistance it accelerates only till when the terminal vel is reached and then at the same velocity so it travels lesser distance/unit time => more time for the horizontal component to increase range...

They making an assumption some where because you would have to integrate the force over the are to get the air resistance and theres no way a problem like that would be on the MCAT..

Thanks for the explanation, but I'm still really confused lol.

Here's another question: Two balls with exactly the same size and shape are launched with the same initial velocity from the surface of a perfectly flat plane. When air resistance is considered, the ball with the greater mass will have a:

(Longer/Shorter) flight time and a (greater/lower) maximum height.

I know that once the two balls begin falling after reaching peak velocity, the ball with the larger mass takes more time to reach terminal velocity (because it weights more) while the smaller ball would reach terminal velocity much sooner and begin falling at a constant speed.

The heavier ball would reach a faster speed then the smaller ball. I mean wouldn't that shorten the time? The answer was that Flight time would be longer and maximum height would be greater, and I do not understand why at all.

 

[IntelInside]

Thanks for the explanation, but I'm still really confused lol.

Here's another question: Two balls with exactly the same size and shape are launched with the same initial velocity from the surface of a perfectly flat plane. When air resistance is considered, the ball with the greater mass will have a:

(Longer/Shorter) flight time and a (greater/lower) maximum height.

I know that once the two balls begin falling after reaching peak velocity, the ball with the larger mass takes more time to reach terminal velocity (because it weights more) while the smaller ball would reach terminal velocity much sooner and begin falling at a constant speed.

The heavier ball would reach a faster speed then the smaller ball. I mean wouldn't that shorten the time? The answer was that Flight time would be longer and maximum height would be greater, and I do not understand why at all.

The heavier ball is less affected by air resistance therefore it can "power" its way through the air molecules more effectively achieving a greater height and because we achieve a greater height it will be in the air longer

 

[UIUCstudent]

That's strange. I also have the book and the answer says D. it decreases range and height. Explanation reads air resistance is a force against motion. I have the 2003 edition off amazon a less than a year ago

 

[Pamplemousse123]

that's a weird one.
forgive me if I'm not supposed to post links.

See this:
http://i33.photobucket.com/albums/d87/Aye_Way/PicforMiDeng.jpg

 

[LaurenMCAT]

Question: 125 - Air resistance on a smooth ball launched over level ground:
(increases/decreases) the maximum height and (increases/decreases) the range.

I'm with Pample...IF we compared the trajectory of a thrown object with vs. without air resistance, the object WITH air resistance would reach a lower max height and would travel a shorter range. That's weird that the answer was wrong. Or are we reading the question incorrectly?

 

[ilovemcat]

I'm with Pample...IF we compared the trajectory of a thrown object with vs. without air resistance, the object WITH air resistance would reach a lower max height and would travel a shorter range. That's weird that the answer was wrong. Or are we reading the question incorrectly?

Yeah, that's what I thought, but no it's typed exactly like the book. Weird question.

Even with the second question (above), I'm not totally understanding the concept. Can anyone explain the logic behind that?

This whole "air resistance" thing is really frustrating me to no end. I'm just gonna skip over it lol.

 

[ilovemcat]

that's a weird one.
forgive me if I'm not supposed to post links.

See this:
http://i33.photobucket.com/albums/d87/Aye_Way/PicforMiDeng.jpg

I think I understand now. The first problem listed above - the answer is just wrong. For the second problem, the picture you showed is for an object with the same mass and size. It compares the effect of air resistance on the same object.

However, in the second question above it asks what is the effect of (increasing) the mass of one of the balls (both same shape and size). Both balls would have the same force of air resistance at the same velocity (because air resistance is independent of mass. it only depends on velocity, shape, and size). What changes though is the effect of that force of air resistance on slowing down the ball to terminal velocity. The heavier ball would reach a much higher height than the ball with lower mass and because range distance is dependent on height, an object with a larger height would experience a longer range (because of increased air time).

I hope this makes since, lol.

 

[RocketSurgeon]

I think I understand now. The first problem listed above - the answer is just wrong. For the second problem, the picture you showed is for an object with the same mass and size. It compares the effect of air resistance on the same object.

However, in the second question above it asks what is the effect of (increasing) the mass of one of the balls (both same shape and size). Both balls would have the same force of air resistance at the same velocity (because air resistance is independent of mass. it only depends on velocity, shape, and size). What changes though is the effect of that force of air resistance on slowing down the ball to terminal velocity. The heavier ball would reach a much higher height than the ball with lower mass and because range distance is dependent on height, an object with a larger height would experience a longer range (because of increased air time).

I hope this makes since, lol.

Makes sense !!

also http://mythbustersresults.com/episode47 lol

--->A regulation NFL football will fly farther when filled with helium as opposed to compressed air at regulation pressure (13 psi).

 

[ilovemcat]

Makes sense !!

also http://mythbustersresults.com/episode47 lol

--->A regulation NFL football will fly farther when filled with helium as opposed to compressed air at regulation pressure (13 psi).

Thanks interesting, haha. Maybe when the Saints play the Seahawks this Saturday, a ref could sneak in a helium inflated ball >