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Old 6th February 2010, 13:58   #46
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Originally Posted by zaks View Post
You guys are all digressing too much into automobile jargon here. !
The question was answered long back but there appears to be a misconception that the weight of the car in the OP's model is irrelevant. I'm trying to clear that up and explain that the weight is very important, even when resistive forces are removed. And its not jargon, its basic physics

EDIT: Zaks, read the last few posts by Shantnu. I too am saying that the OP's model is correct but it is important that everyone agrees since this is not opinion, but physics.

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Old 6th February 2010, 14:05   #47
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Originally Posted by McLaren Rulez View Post
The question was answered long back but there appears to be a misconception that the weight of the car in the OP's model is irrelevant. I'm trying to clear that up and explain that the weight is very important, even when resistive forces are removed. And its not jargon, its basic physics
Yes, but in his question he has already taken a measure for the weight so whats there to debate about. For his assumptions his calculations are correct.
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Old 6th February 2010, 14:19   #48
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The precise term in inertia. You need to apply a force to move an object and its acceleration will depend on its mass. A heavy object will accelerate slowly compared to a light object given the same force.
If mass in the absence of friction affects rate of acceleration, it should also work the other way around. So youre saying that an object with more mass will also come to a halt slower than a light object in the absence of friction?

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Since tarmac and ice don't offer any friction, the acceleration will be identical. Levitation introduces gravity, an extra force, so that will be different.

No this would not happen. Since there is no friction, tarmac and ice are indistinguishable. So the 1000 kg object would accelerate faster.
I was taking about real world. But even in a world with no friction, as long as the force acting on the object is the same the rate of acc should also be same, irrespective of its weight.


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Last edited by Shan2nu : 6th February 2010 at 14:22.
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Old 6th February 2010, 14:39   #49
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The question was answered long back but there appears to be a misconception that the weight of the car in the OP's model is irrelevant. I'm trying to clear that up and explain that the weight is very important, even when resistive forces are removed. And its not jargon, its basic physics
I think you're trying to say that even without friction, gravitaional pull creates some sort of resistance (in other words mass of the object). Is that right?

But dude, if the 1600kg car is, lets say levitating (since you mentioned its not using any wheels), It means that the upward force acting on it is the same as the gravitational pull acting on it.

So in such a situation, wont the 2 forces working against each other be like pushing an object in the absence of gravity (during its state of levitation)? Would the mass still affect acceleration?

Shan2nu

Last edited by Shan2nu : 6th February 2010 at 14:40.
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Old 6th February 2010, 14:54   #50
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Originally Posted by Shan2nu View Post

So in such a situation, wont the 2 forces working against each other be like pushing an object in the absence of gravity (during its state of levitation)? Would the mass still affect acceleration?

Shan2nu
Gravity absent, No friction, No drag, No resistance..

We are just arguing..

Lets get in the real world and learn something new today..
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Old 6th February 2010, 15:08   #51
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Originally Posted by Shan2nu View Post
If mass in the absence of friction affects rate of acceleration, it should also work the other way around. So youre saying that an object with more mass will also come to a halt slower than a light object in the absence of friction?
Without friction, an object will never slow down. Slowing down is an acceleration so it needs a force. If you did introduce friction to slow the body down, then it is slightly different.

The friction acting on a body is given by the product of a friction coefficient (depends on the surface) and the normal force exerted by the surface on the body. If we consider a box moving on a horizontal surface, then the normal force is equal to the weight. So the frictional force depends on the weight. Now, the force of friction may be higher but a heavier object means that the mass is also higher. If, therefore, you push a big 1000 kg box on a flat surface with friction till it reaches velocity v and let it go, it will have a magnitude of deceleration a m/s/s, say. Now if you push a 1 kg box to the same velocity on the same surface, it will also decelerate at a m/s/s after you let it go. They will both stop in exactly the same distance. (Note that air resistance is not considered here)

The math is simple.

f=uN (f is the force of friction, u is the friction coefficient and N is the normal force)
So, f=uW (In this case, W, the weight is equal to N.)
So, f=umg (W is mg)

The total force acting on the body is F, say. Since friction is the only force we have,
F=f
But we know by Newton's second law,
F=ma

Thus, we have
umg=ma
So,
a=ug which is clearly independent of m.


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Originally Posted by Shan2nu View Post
But even in a world with no friction, as long as the force acting on the object is the same the rate of acc should also be same, irrespective of its weight.
No, this is not so. The acceleration will be inversely proportional to the mass. So a heavier object will have lower acceleration for the same force.

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Originally Posted by Shan2nu View Post
I think you're trying to say that even without friction, gravitaional pull creates some sort of resistance (in other words mass of the object). Is that right?
Not quite. If it helps, here's the math

F=GMm/r^2, where F is the force on a falling body, M is the mass of the earth (constant), r is the distance of the body from the earth's centre (virtually constant at low heights) and m is the mass of the object that's falling.

F=ma, by Newton's second law

So ma=GMm/r^2

So a=GM/r^2 which is independent of m. GM/r^2 is a constant and is given the notation g. The same g that is 9.8m/s/s

So, a=g for all bodies. No m in the equation which simply means that irrespective of the mass, acceleration of the falling object is 9.8m/s/s

This explains the falling body idea.

If you find that math unhelpful, think of it this way. A strong powerful man can throw a heavy metal ball ten feet. A weak sick boy can throw a tennis ball ten feet. In the first case, the heavy object had a large force acting on it and the light object had a small force. Yet both had the same acceleration (and consquently, same distance travelled). This is what happens to a falling body. Heavy objects are pulled with great force but their mass is also high. So the acceleration ends up being the same.

Quote:
Originally Posted by Shan2nu View Post
But dude, if the 1600kg car is, lets say levitating (since you mentioned its not using any wheels), It means that the upward force acting on it is the same as the gravitational pull acting on it.

So in such a situation, wont the 2 forces working against each other be like pushing an object in the absence of gravity (during its state of levitation)? Would the mass still affect acceleration?

Shan2nu
In levitation, yes, the forces of gravity and the applied force balance each other. Hence the body stays where it is and has no acceleration. The correct way to think of it is that the total force on the body is zero. Thus, there is zero acceleration. If, after adding all the forces and cancelling the opposing forces, there remains a net force, then the body will accelerate in that direction and the magnitude of its acceleration will depend on the mass and the net force.

Last edited by McLaren Rulez : 6th February 2010 at 15:28.
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Old 6th February 2010, 15:15   #52
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Gravity absent, No friction, No drag, No resistance..

We are just arguing..
LOL, couldn't help it, but hey what the heck, anything goes in a hypothetical situation.

If a 1600kg car doesn't use wheels or any form of contact with the ground, it has to levitate. And if its levitating, how can it weight 1600kgs. Wierd stuff....

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Old 6th February 2010, 15:31   #53
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Without friction, an object will never slow down. Slowing down is an acceleration so it needs a force. If you did introduce friction to slow the body down, then it is slightly different.

The friction acting on a body is given by the product of a friction coefficient (depends on the surface) and the normal force exerted by the surface on the body. If we consider a car moving on a horizontal surface, then the normal force is equal to the weight. So the frictional force depends on the weight. That is why a heavy object takes longer to slow down as compared to a light object on the same surface.


No, this is not so. The acceleration will be inversely proportional to the mass. So a heavier object will have lower acceleration for the same force.


Not quite. If it helps, here's the math

F=GMm/r^2, where F is the force on a falling body, M is the mass of the earth (constant), r is the distance of the body from the earth's centre (virtually constant at low heights) and m is the mass of the object that's falling.

F=ma, by Newton's second law

So ma=GMm/r^2

So a=GM/r^2 which is independent of m. GM/r^2 is a constant and is given the notation g. The same g that is 9.8m/s/s

This explains the falling body idea.

If you find that math unhelpful, think of it this way. A strong powerful man can throw a heavy metal ball ten feet. A weak sick boy can throw a tennis ball ten feet. In the first case, the heavy object had a large force acting on it and the light object had a small force. Yet both had the same acceleration (and consquently, same distance travelled). This is what happens to a falling body. Heavy objects are pulled with great force but their mass is also high. So the acceleration ends up being the same.


In levitation, yes, the forces of gravity and the applied force balance each other. Hence the body stays where it is and has no acceleration. The correct way to think of it is that the total force on the body is zero. Thus, there is zero acceleration. If, after adding all the forces and cancelling the opposing forces, there remains a net force, then the body will accelerate in that direction and the magnitude of its acceleration will depend on the mass and the net force.
But if the hypothetical car is levitating, there is no question of it weighing 1600kgs. And if the car is not levitating, there has to be some sort of contact between the car and the ground. And the moment you create a point of contact, there will be friction as the car's speed increases.

This is the part thats confusing.

Shan2nu

Last edited by Shan2nu : 6th February 2010 at 15:34.
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Old 6th February 2010, 15:32   #54
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Originally Posted by Shan2nu View Post
If a 1600kg car doesn't use wheels or any form of contact with the ground, it has to levitate. And if its levitating, how can it weight 1600kgs. Wierd stuff....

Shan2nu
Why would you think its levitating if it doesn't use wheels? The reason I said that the model doesn't use wheels was because wheels have to rotate as the car accelerates and that needs friction. So assume simply that either the car's wheels don't rotate or to make things simple, consider the car to be a big box that weights 1600 kg. I didn't get your levitation points earlier but now I see why you thought the car was floating in air.

The car still has mass which never changes (even when it levitates). What happens when it levitates is that the normal force becomes zero. But let's not go there.

Let the car be very much on the ground. Assume the ground is a superior version of ice that is so slick that there is no friction. Better now?

Last edited by McLaren Rulez : 6th February 2010 at 15:47.
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Old 6th February 2010, 15:53   #55
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Why would you think its levitating? The reason I said that the model doesn't use wheels was because wheels have to rotate as the car accelerates and that needs friction. So assume simply that either the car's wheels don't rotate or to make things simple, consider the car to be a big box that weights 1600 kg.

The car still has mass which never changes (even when it levitates). What happens when it levitates is that the normal force becomes zero. But let's not go there.

Let the car be very much on the ground. Assume the ground is a superior version of ice that is so slick that there is no friction. Better now?
Man you guys are just coming up with one unreal condition after another. What exactly is the point of such a test if you're gonna create your own environment?

We've literally gone from an average car to a box weighing 1600kgs sliding on a surface that has no friction.

Has something like this even been done in a lab?

Shan2nu

Last edited by Shan2nu : 6th February 2010 at 15:57.
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Old 6th February 2010, 16:03   #56
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Man you guys are just coming up with one unreal condition after another. What exactly is the point of such a test if you're gonna create your own environment?
No I'm not. What I'm telling you is exactly what was present in the OP's initial model. No friction, no air resistance and uniform power from the engine.

You assumed incorrectly that under zero friction, the acceleration would not depend on weight. A lengthy discussion followed and you talked about levitation which was something I did not understand. Nonetheless, I told you what would happen if the car were floating (for whatever reason). Let me clarify that again: Wheels not rotating does not imply that the car is floating. You can achieve zero friction with the car firmly on the ground. You just need a highly slippery surface. In fact, any idealization that neglects friction implicitly means a perfectly slippery surface. If you want to levitate the car to achieve zero friction, that's fine too! But that will give you the same results.

And FYI, a 1600 kg car that has no friction acting on it is identical for all calculations to a 1600 kg box sliding on a smooth surface. We haven't gone anywhere. I'm trying to explain the same idea using a box hoping you'd understand it more easily.

Then you wanted to turn friction on and off so those two cases came in. And you also brought the falling body analogy which was, again, incorrect. And then some smaller things such as your idea that a levitating car has no mass which is also wrong.

End of the day, your original misconception can be answered very simply. The weight matters. That's it.

Last edited by McLaren Rulez : 6th February 2010 at 16:14.
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Old 6th February 2010, 16:04   #57
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Originally Posted by Shan2nu View Post

We've literally gone from an average car to a box weighing 1600kgs sliding on a surface that has no friction.

Shan2nu



That is exactly what I was talking about..
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Old 6th February 2010, 16:15   #58
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Somehow im not getting the relation between zero friction and the weight of the car.

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Old 6th February 2010, 16:26   #59
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Somehow im not getting the relation between zero friction and the weight of the car.

Shan2nu
There are two ways to get zero friction. The simple way is to make the surface slippery. Perfectly slippery. If you refer to the earlier post on the equation for friction this is the case where u=o.

Alternatively, you can make it zero by making the normal force zero. One way to achieve this is your idea of levitating the car. Normal force is the force that the earth has to exert on the car. If you just leave the car, normal force=weight. If you levitate the car, normal force=0. If you push the car down, normal force=weight+downward force.

If this sounds alien, its time to revisit Newtonian physics. I think you have a misconception about the difference between weight and mass as well as Newton's second law. Any physics textbook/google would clarify this and then you'll see exactly what the OP's model represents.

Last edited by McLaren Rulez : 6th February 2010 at 16:28.
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Old 6th February 2010, 16:40   #60
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If this sounds alien, its time to revisit Newtonian physics.
Yes, it is alien with no friction or aero drag involved. Need to find out what this is all about.

But when you say levitation has normal force = 0. There will only be mass but no weight involved since there is no force acting on the object.

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