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kjundvr

A physics question

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The answer is inertia: Inertia is the tendency of things to resist changes in motion. An object's inertia is related to it's mass or how much matter something has. The greater the mass of an object, the greater it's inertia. Therefore, the more resistant the object is in a change of it's motion.

Adding weight to a skydiver will increase his/her mass and create a greater inertia or resistance to a decrease in speed exerted on the jumper's body by air molecules.

Fall rate can also be increased by decreasing surface area presented to the wind in the form of drag i.e. wearing a tighter jumpsuit made of materials like nylon and spandex vs. a baggy free-fly suit made of polycotton.
Roy Bacon: "Elvises, light your fires."

Sting: "Be yourself no matter what they say."

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If I whirl an object around on a string, there is most definitely a measurable outwardly directed force on the anchor point of the string, as well as a measurable tension in the string. Any force measuring device, such as a load cell, is capable of recording it. Newton's 3rd law requires that it exist as a reaction to the centripetal force acting on the object moving in a circle. Calling it "centrifugal" is as good a name as any.



I don't disagree with you, but as you correctly point out, that outward force is on the anchor point in the center, and most people who use the term "centrifugal" apply it to the object that is rotating.



Ay! There's the rub! :)
How did your 10-way go?
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The only sure way to survive a canopy collision is not to have one.

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An object which is falling through the atmosphere is subjected to two external forces. One force is the gravitational force, expressed as the weight of the object. The other force is the air resistance, or drag of the object...



You forgot one more force - Archimedean force -lifting force which allow us to make baloon jump :)
Why drink and drive, if you can smoke and fly?

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If terminal velocity was just related to inertia, more massive objects would fall slower, since they'd resist the acceleration due to the force of gravity more, right? :)
Inertia is, like you said, related to mass. Terminal velocity is related to both mass (weight really) AND drag coeficient. Two objects of equal mass (equal inertia) can have very different fall rates. Just like the example of the packed parachute vs the open parachute. Same mass, same inertia, different terminal velocities.

Don't make it more complicated than it is... it's already rocket science. :P

Dave

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An object which is falling through the atmosphere is subjected to two external forces. One force is the gravitational force, expressed as the weight of the object. The other force is the air resistance, or drag of the object...



You forgot one more force - Archimedean force -lifting force which allow us to make baloon jump :)



That and Force of Habit.

"How come you jump out of airplanes?"

"It's what I do."

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An object which is falling through the atmosphere is subjected to two external forces. One force is the gravitational force, expressed as the weight of the object. The other force is the air resistance, or drag of the object...



You forgot one more force - Archimedean force -lifting force which allow us to make baloon jump :)



That and Force of Habit.

"How come you jump out of airplanes?"

"It's what I do."



If Ivan is around there's also "The Force".
...

The only sure way to survive a canopy collision is not to have one.

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I don't know about that....I still have graduate level engineering courses that have us use spherical cows!



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Freshmen physics still started out with a sphirical horse in a vacum...hell, there was even a question pertaining to a 2 diminsional box to make it slightly easy to do the equations.:o



Don't you guys bitch about the oversimplified problems!!:D I sure as hell don't want to have to start factoring air resistance into everything, that shit's complicated. Not to mention all the other things you have to consider for objects that aren't a point mass, or aren't symetrical.. whew!:P

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If terminal velocity was just related to inertia, more massive objects would fall slower, since they'd resist the acceleration due to the force of gravity more, right? :)



Hey PilotDave: For one, this statement makes absolutely no sense logically at all to me, second, I never said anything about terminal velocity only being related to inertia. If you read what I said I said that by adding weight to the same surface area of the jumper you are increasing the jumper's mass and thereby decreasing his/her resistance to an opposing force while in motion. The motion in this case is created by the acceleration due to gravity and the resistance in this case would be the air molecules of the relative wind bouncing off your body. The balance between those, the jumper's mass and the surface area presented, is what determines an object's terminal veloctity or descent rate.

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Inertia is, like you said, related to mass. Terminal velocity is related to both mass (weight really) AND drag coeficient.

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Yes, and again that is why I mentioned how a jumpsuit can increase or decrease the drag by increasing and/or decreasing surface area presented to the wind. You gave the example of an open parachute. Same principle.

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Don't make it more complicated than it is... it's already rocket science.

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I don't think I made it any more complicated than it is. Personally I would just like you to explain what you meant by the very first statment of your reply to my post. :S I think I get what you are trying to convey but it still doesn't make sense.

Roy Bacon: "Elvises, light your fires."

Sting: "Be yourself no matter what they say."

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The answer is inertia: Inertia is the tendency of things to resist changes in motion. An object's inertia is related to it's mass or how much matter something has. The greater the mass of an object, the greater it's inertia. Therefore, the more resistant the object is in a change of it's motion.



The object in question is being accelerated from 0 to terminal velocity!

Inertia gets eliminated from the equation immediately. The more massive an object, the more resistant it is to acceleration. But the more massive the object, the greater the force applied by gravity. End result is no difference, as proven in vacuums all the time.

It's all about the countering force of drag, based on surface area, which is vaguely the 2D representation of an object. But we're 3d objects, and so mass increases faster than that surface area does.

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The answer is inertia: Inertia is the tendency of things to resist changes in motion. An object's inertia is related to it's mass or how much matter something has. The greater the mass of an object, the greater it's inertia. Therefore, the more resistant the object is in a change of it's motion.



The object in question is being accelerated from 0 to terminal velocity!

Inertia gets eliminated from the equation immediately. The more massive an object, the more resistant it is to acceleration. But the more massive the object, the greater the force applied by gravity. End result is no difference, as proven in vacuums all the time.

It's all about the countering force of drag, based on surface area, which is vaguely the 2D representation of an object. But we're 3d objects, and so mass increases faster than that surface area does.



How do you know you're three dimensional, and not just a 3-d projection of a poly-dimensional entity? Evidence?

What we need is a parameter for people that serves the same function as the ballistic coefficient serves for bullets.
...

The only sure way to survive a canopy collision is not to have one.

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>We'd be more comfortable on a full otter load if we weren't.

Only if you could do a fourth dimensional slide to the left, so people see only, say, your liver in the 3D projection. Think how many livers we could fit in an otter!

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>We'd be more comfortable on a full otter load if we weren't.

Only if you could do a fourth dimensional slide to the left, so people see only, say, your liver in the 3D projection. Think how many livers we could fit in an otter!



I don't know about an Otter full of livers, but I've seen a bunch of pricks on one.
...

The only sure way to survive a canopy collision is not to have one.

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The magnitude of terminal velocity depends on the weight of the falling body. For a heavy object, the terminal velocity is generally greater than a light object. This is because air resistance is proportional to the falling body's velocity squared. For an object to experience terminal velocity, air resistance must balance weight. An example that shows this phenomenon was the classic illustration of a rock and a feather being dropped simultaneously. In a vacuum with zero air resistance, these two objects will experience the same acceleration. But on the earth this is not true. Air resistance will equal weight more quickly for the feather than it would for the rock. Thus the rock would accelerate longer and experience a terminal velocity greater than the feather.



Let me aquaint you with the 'Weak Equivalency Principle.'

Basically, it states that when one object is very large compared to others, like the earth compared to the people and things on it surface, the gravitational force between the earth and any of the smaller objects on its surface appears to be equivalent regardless of apparently large differences between the smaller objects.

That means your rock and your feather will fall at equivalent rates near the earth's surface if they are protected from experiencing air resistance durning the fall.

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The object in question is being accelerated from 0 to terminal velocity!

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Maybe I misunderstood the question but what I gathered from it was the person was asking about weight vests and the physical principle behind why they can be used to help two jumpers attain similar terminal velocities.

Because he did not specify someone using these weight vests to B.A.S.E. jump off of bridges or hovering helicopters I just assumed he was talking about jumping out of airplanes :S. If it is airplanes he is talking about jumping out of, you are not accelerating from 0-120.

We don't skydive in vacuums therefore it is necessary to take into consideration the resistance of the air molecules bouncing off of us which we call the relative wind. And if we are presenting something to the relative wind, it then becomes important as to what surface area we are presenting and how much mass is behind it which when combined with the acceleration of gravity is equivalent to weight.

Even in a wind tunnel the wind resistance will need to be turned up for different people doing different things. For instance, a free-flyer on his head in a tunnel requires stronger wind power to keep him in the same "floating spot" as a very lightweight belly flyer due to his increased mass and decreased surface area presented to the wind.

Roy Bacon: "Elvises, light your fires."

Sting: "Be yourself no matter what they say."

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your answer now is conceptually correct, but your first answer about inertia was plain wrong. Inertia is the inherent resistance of an object to change in motion, as you said. If gravity were a force, this would mean that two objects then two objects of different masses would accelerate at different rates even in a vacuum due to inertia. But gravity is an acceleration. Weight is a force that is determined by multiplying an objects mass with the accleration of the gravity close to the earth. Therefore, as an objects mass increases, the force acting "down" on it due to gravity also increases. Hence, objects fall at the same rate in a vacuum. Terminal velocity is related strictly to an objects mass (since gravity is usually constant it cancels out, and you just need mass and not weight), surface area, and coefficient of drag. Nowhere in there does inertia enter into anything. Again, dont try to make it more complicated than it is. You could put inertias into calculations for terminal velocity, but they would go away and have no effect.

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I wrote a whole long reply, then decided not to bother posting it. Now I changed my mind...don't ya hate when you do that?

I'm not disagreeing with you that an object with higher inertia will have a higher terminal velocity, everything else being equal. But that just isn't explaining what's really going on. You don't NEED inertia to explain why a heavier object falls faster. Inertia is just a concept....you've explained it just fine... it's an object's resistance to a change in motion. It will take more force (wind resistance) to get a more massive object in freefall to stop accelerating, just like you said.

But I think you can both simplify and be more precise by talking about the equilibrium of weight and drag. Drag is a function of velocity squared. At terminal velocity, drag must equal weight... no net force, otherwise there'll be acceleration (where newton's inertia law, F=ma comes in). If you increase weight, you must also increase V^2 for the equilibrium to exist.

You've said the same thing... I just think inertia is a more complicated way of saying it, and doesn't really explain exactly what's going on. It's a force balance. Mass (inertia) is not a force.

Dave

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Terminal velocity is related strictly to an objects mass (since gravity is usually constant it cancels out, and you just need mass and not weight)



Maybe I'm just too tired or dumb to see this, but why?

The equilibrium for terminal velocity is W=D, right? Gravity is part of W, but it is not part of D, so how does it cancel out?

Dave

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It is impoper to say that something has more inertia than something else. Momentum and Kinetic energy can be thought of that way, but not inertia.
People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am

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