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Originally posted by Lunar Phase 7 I dont't wanna fall out about this, I respect the fact you have took time to think this through, and you do seem to know your stuff, but I still say you are wrong. Or at least, the least right.

FOr sure. This is a scientific debate, I have no intention of making it personal.

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http://www.physlink.com/Education/AskExperts/ae6.cfm Might be interesting to read.

In both of the answers given air resistance is discounted. The first answer discusses it briefly in the second paragraph only to say that the shape plays a role, which still does not discount the role of weight.

The second answer excludes air resistance all together which means it is irrelevant to terminal velocity.

In a vacuum neither object ever reaches terminal velocity and both, regardless of mass or shape accelerate at the same rate til they hit the ground simultaneously (including an elephant and a feather).

Weight is not a deciding factor for air resistance.
The drag force is dependant only on the object's velocity,area,shape and the density of the gas through which it is falling.
The drag force on 2 objects similar in all aspects, except for weight, will still be the same.

But what will not be same is the terminal velocity of each object.
The terminal velocity is reached when drag force balances gravity.
A heavier body requires higher drag force to balance gravity,
and since drag force is proportional to velocity,
it => heavier bodies have higher terminal velocities.

Since drag force is independent of mass, but heavier objects have higher terminal velocities,
it => that heavier objects take more time to reach terminal velocity than lighter objects.

And hence, 2 objects with different masses (but identical in all other aspects) will not reach terminal velocity at the same time.

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Originally posted by Omega_M Weight is not a deciding factor for air resistance. The drag force is dependant only on the object's velocity,area,shape and the density of the gas through which it is falling. The drag force on 2 objects similar in all aspects, except for weight, will still be the same. But what will not be same is the terminal velocity of each object. The terminal velocity is reached when drag force balances gravity. A heavier body requires higher drag force to balance gravity, and since drag force is proportional to velocity, it => heavier bodies have higher terminal velocities. Since drag force is independent of mass, but heavier objects have higher terminal velocities, it => that heavier objects take more time to reach that terminal velocity than lighter objects. And hence, 2 objects with different masses (but identical in all other aspects) will not reach terminal velocity at the same time.

GET ON MSN I NEED YOUR CALCULUS HELP!

edit: plz

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Originally posted by Omega_M I don't quite understand how my statement is misleading. All I am doing is stating Newton's second law. Infact, your statement that acceleration due to gravity is constant, is misleading for objects falling in air. As the object starts falling down the force of gravity is dominant and initially, there is no drag. As the object speeds up, drag acts upwards and starts to nullify the effect of gravity. It implies that acceleration starts with a finite positive value and ends up being zero at the instant the body reaches terminal velocity. After that, the object continues to fall with a constant velocity and zero acceleration. If acceleration due to gravity is constant, it implies that the object always has a constant net downward force acting on it, and that it always speed up. But that is just not the case in air or any medium except for vacuum.

I'm not sure what I posted implied any different than what you said here. The rate of acceleration due to gravity IS a constant, although this question urges us to consider the effect of air resistance on an object's acceleration due to gravity, which we can all agree, causes acceleration to slow and eventually stop.

I never said that acceleration of any given object is ALWAYS constant because in the case of question two, that would require ignoring the OTHER forces acting upon the falling object, which is an essential part of answering this problem.

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Originally posted by Omega_M 1)Speed of light is a fundamental constant

This is true to an extent, but I won't delve into that right now

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Originally posted by tubularbills 1) B - visible light, in the form of the full specturm [i.e. white light] spreds out more than any color does individually [i.e., the sun]. the laser light is infrared, and does not scatter as much, since it's of a different wavelength. 2) B - since the objects are not in a vaccum, the heavier object [object B] will reach terminal velocity faster. 3) C - there is more pressure underneath the ball because pressure decreases with heigh in the atmosphere. the second part of the question is worded awfully awkward. i don't know what they're getting at. but, it has to be either A/C so pick one. its been awhile since i've had physics like this, but that was the quick and dirtay rationale i had on those questions.

your right on evey one except number 3, the pressure above the ball is higher than the pressure below the ball because of its upwards movement, the ball is pushing air ahead of it which causes a pressure differential.

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Originally posted by colonelcrisp your right on evey one except number 3, the pressure above the ball is higher than the pressure below the ball because of its upwards movement, the ball is pushing air ahead of it which causes a pressure differential.

He is also wrong on q2 - the lighter object will reach terminal velocity first - it doesn't have to accelerate for as long a time for the drag to be equal to the downward force.

Also q1.. the sun will emit a wider spectrum of light than a torch bulb.

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Originally posted by D-res This is true to an extent, but I won't delve into that right now

did I forget to mention it is in vacuum Not in water or hair gel.

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Originally posted by Omega_M Weight is not a deciding factor for air resistance. The drag force is dependant only on the object's velocity,area,shape and the density of the gas through which it is falling. The drag force on 2 objects similar in all aspects, except for weight, will still be the same. But what will not be same is the terminal velocity of each object. The terminal velocity is reached when drag force balances gravity. A heavier body requires higher drag force to balance gravity, and since drag force is proportional to velocity, it => heavier bodies have higher terminal velocities. Since drag force is independent of mass, but heavier objects have higher terminal velocities, it => that heavier objects take more time to reach terminal velocity than lighter objects. And hence, 2 objects with different masses (but identical in all other aspects) will not reach terminal velocity at the same time.

exactly.

ALRIGHT guys thanks for all ur helps, but now here it is...its time for the REAL answers as i found out today...

1) Which light spreads out most (or fastest)(not sure which makes more sense)?

that question was omitted from the test, as the instructor himself said it is nonsense and too vague..

2) Two objects, object A and B, free fall together from a high altitude. Assume Object A weighs less and both object A and object B have the same shape. Which object will reach terminal velocity quicker?

Answer: a) Object A

Since altitudes and shapes are about the same, object A and object B initially start out falling together at same speed, and each second both are accelerating at a velocity of 32 m/s^2, or 9.8 m/s^2. Once each object hits terminal velocity, it stops accelerating and continues falling until it hits the ground (continuing a constant fall at the speed at which it last accelerated at before it reached terminal velocity). Therefore, the lighter object reaches terminal velocity faster, so as a result, stops accelerating during its fall pretty quickly (much quicker than its heavier counterpart). So it keeps falling at the new constant speed, while Object B, the heavier one, continues accelerating for a bit longer. Therefore, Object B, ends up hitting the ground first, while Object A gave up acceleration and is still slowly but surely coming down at a constant speed.

3) If you throw a baseball up to the sky, there is ________ air pressure at the bottom of the ball than at the top of it, which means there is ______ velocity under the ball than there is at the bottom.

Answer: a) more, less

When ball is going up, there is more pressure below it and slower air particles moving under it, while above it, there is less pressure and faster air particles moving. This concept is called Bernoulli's Principle, or Bernoulli's Lift.

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2) Two objects, object A and B, free fall together from a high altitude. Assume Object A weighs less and both object A and object B have the same shape. Which object will reach terminal velocity quicker? Answer: a) Object A Since altitudes and shapes are about the same, object A and object B initially start out falling together at same speed, and each second both are accelerating at a velocity of 32 m/s^2, or 9.8 m/s^2. Once each object hits terminal velocity, it stops accelerating and continues falling until it hits the ground (continuing a constant fall at the speed at which it last accelerated at before it reached terminal velocity). Therefore, the lighter object reaches terminal velocity faster, so as a result, stops accelerating during its fall pretty quickly (much quicker than its heavier counterpart). So it keeps falling at the new constant speed, while Object B, the heavier one, continues accelerating for a bit longer. Therefore, Object B, ends up hitting the ground first, while Object A gave up acceleration and is still slowly but surely coming down at a constant speed.

Pass

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3) If you throw a baseball up to the sky, there is ________ air pressure at the bottom of the ball than at the top of it, which means there is ______ velocity under the ball than there is at the bottom. Answer: a) more, less When ball is going up, there is more pressure below it and slower air particles moving under it, while above it, there is less pressure and faster air particles moving. This concept is called Bernoulli's Principle, or Bernoulli's Lift.

Fail

But looks kinda odd to me. A ball thrown upwards, gets an upward lift ? Bernoulli's lift relates to the forces on a wing that causes an airplane to rise up. But, the motion of plane and the direction of lift are perpendicular to each other. In this case, the both are in the same direction. Need to check up on this one.

yup, but thats the answer...thats what the professor said, and i checked it in the text as well, same principle as the airplane wing, that's the one

Here's my take on 2):

Please consider the following:

NOTE:
Both bodies are in a state of free-fall.
Thus, a condition of weightlessness will exist i.e microgravity.

Take a look at the terminal velocity equation.
It's inversely proportional to cross-sectional area and directly proportional to weight. The rest of the variables are constants.

Since, the weightless condition applies, the numerator can be neglected.

The only unknown quantity is cross-sectional area.

Take note here:

Larger objects have lower terminal velocity as compared with smaller objects.

So,

This is the trick in this question. Weight is a misleading quantity in this case.

However, they have mentioned both objects have the SAME SHAPE.

I believe, this signifies identical cross-section.

Thus, both objects will have the SAME terminal velocity.

If we look at the fundemental velocity equation,

v1 = u1 + gt1 (v1 = terminal velocity of A, u1 = init. velocity of A)
v2 = u2 + gt2 (v2 = terminal velocity of B, u2 = init. velocity of B)

Since v1 = v2,

u1 + gt1 = u2 + gt2

Since, the question says both bodies START falling TOGETHER, this would signify they are both initially at rest and start falling at the same instant.

Thus, u1 , u2 = 0

Therefore, t1 = t2.

Thus, they should reach terminal velocity at the same time.

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Originally posted by varun Here's my take on 2): Please consider the following: NOTE: Both bodies are in a state of free-fall. Thus, a condition of weightlessness will exist i.e microgravity. Take a look at the terminal velocity equation. It's inversely proportional to cross-sectional area and directly proportional to weight. The rest of the variables are constants. Since, the weightless condition applies, the numerator can be neglected. The only unknown quantity is cross-sectional area. Take note here: Larger objects have lower terminal velocity as compared with smaller objects. So, This is the trick in this question. Weight is a misleading quantity in this case. However, they have mentioned both objects have the SAME SHAPE. I believe, this signifies identical cross-section. Thus, both objects will have the SAME terminal velocity. If we look at the fundemental velocity equation, v1 = u1 + gt1 (v1 = terminal velocity of A, u1 = init. velocity of A) v2 = u2 + gt2 (v2 = terminal velocity of B, u2 = init. velocity of B) Since v1 = v2, u1 + gt1 = u2 + gt2 Since, the question says both bodies START falling TOGETHER, this would signify they are both initially at rest and start falling at the same instant. Thus, u1 , u2 = 0 Therefore, t1 = t2. Thus, they should reach terminal velocity at the same time.

The terminal velocity equation by definition applies to objects in freefall, hence it needs no tweaking or ignoring of any part of it, numerator or otherwise.

An object in freefall is not weightless. Terminal velocity is the velocity at which upward air resistance is equal to downward force (weight).

Again if ONLY the cross sectional area is relevant, and the weight is completely irrelevant for objects in freefall, as you suggest, then the elephant has a lower terminal velocity than the feather, which it will reach first meaning the feather must hit the ground first.


Isn't it within the ambit of everyone's common experience that when you drop a piece of polystyrene it drops much slower than a piece of metal of the same shape?

I'm still wondering how he can talk about an upward lift for an upward rising ball. I could be wrong ofcourse, but that looks really strange.
If the baseball in question is thrown horizontally, like how a pitcher would throw, then it might rise up due to the high pressure on the lower side.

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Originally posted by Omega_M I'm still wondering how he can talk about an upward lift for an upward rising ball. I could be wrong ofcourse, but that looks really strange. If the baseball in question is thrown horizontally, like how a pitcher would throw, then it might rise up due to the high pressure on the lower side.

Yeah I've been looking up stuff about bernoulli's principle and that answer doesn't make much sense either. If it's a high school physics teacher then I'd be inclined to say s/he is wrong.

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