# Conservation of energy broken?



## audhulma (Feb 17, 2009)

So for a physics experiment my friends and I were to say something that effects maximum velocity of a sled. We measured the velocity as displacement over time, then plotted the data. We noticed something-the more massive people had a faster velocity on the same hill, same spot, same sled. According to conservation of energy this shouldn't happen...any ideas?


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## shelley (Feb 17, 2009)

Why shouldn't it happen?


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## Robert-Y (Feb 17, 2009)

The law of conservation of energy isn't right, at least that's what I think I learnt from physics.


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## audhulma (Feb 17, 2009)

Well as I calculate it, gravitational potential energy at the top is equal to the kinetic energy and loss of energy to friction at the bottom, therefore:
mgh=1/2mv^2+friction. As friction depends on normal force which depends on mass, the masses in each term should cancel.


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## fanwuq (Feb 17, 2009)

I need to think more about this problem. All I can thing of now is that Greater mass = greater force and force along the plane of the hill = cos (angle of hill)*Fg - Ff. Ff= sin (same angle) * Fg * coefficient of friction of the surface.

W=Fd
So heavier people have more potential energy proportional to their mass.
mgh=1/2mv^2, so m would cancel out.
I guess Sir E Brum is right.


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## Sir E Brum (Feb 17, 2009)

You are assuming you live in a perfect world and that the coefficient of friction was constant and no curvature in the hill.

Oh I would also like to see your raw data and your procedure for ascertaining this data.


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## AvGalen (Feb 17, 2009)

First guess: http://en.wikipedia.org/wiki/Potential_energy

How much energy does it cost to bring "more massive people" up the hill?


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## 4Chan (Feb 17, 2009)

But conservation of energy must be right!
People have been trying to resolve the issue of entropy and laws of thermodynamics for decades.


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## audhulma (Feb 17, 2009)

Ok, so my idea is that several factors effected the results:
1) The heavier people compressed the snow, lowering the coefficient of friction
2) The curvature of the hill leads to interference
3) The snow may have been rougher in patches, causing the coefficient of friction to vary
Does this sound reasonable?


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## abr71310 (Feb 17, 2009)

Cubes=Life said:


> But conservation of energy must be right!
> People have been trying to resolve the issue of entropy and laws of thermodynamics for decades.



It's called a "Law" for a reason.
_A law generalizes a body of observations. At the time it is made, no exceptions have been found to a law. Scientific laws explain things, but they do not describe them. One way to tell a law and a theory apart is to ask if the description gives you a means to explain 'why'.

There is no 'proof' or absolute 'truth' in science. The closest we get are facts, which are indisputable observations. Note, however, if you define proof as arriving at a logical conclusion, based on the evidence, then there is 'proof' in science._

The laws of thermodynamics are most certainly correct for most cases, but it doesn't say "all", or else it wouldn't BE a law.

Just like how the *LAW OF GRAVITY* doesn't apply to Apparent Weight (or is it the other weight...??) cases.



audhulma said:


> So for a physics experiment my friends and I were to say something that effects maximum velocity of a sled. We measured the velocity as displacement over time, then plotted the data. We noticed something-the more massive people had a faster velocity on the same hill, same spot, same sled. According to conservation of energy this shouldn't happen...any ideas?



We did this experiment in our classroom with weights and carts on a slanted plane.
That shouldn't happen, you're right; it means that something is obviously wrong with the way you measured the data...

According to "inclined plane problems" (Go look it up, I'm too darn lazy to figure it out myself), when you extend gravity to the cartesian plane (x, y coordinates), mass will cancel (when you look at the forces at work).

Was there an acceleration or were the forces in all directions balanced?


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## shelley (Feb 17, 2009)

audhulma said:


> Well as I calculate it, gravitational potential energy at the top is equal to the kinetic energy and loss of energy to friction at the bottom, therefore:
> mgh=1/2mv^2+friction. As friction depends on normal force which depends on mass, the masses in each term should cancel.



That would be true if we lived in an ideal world of physics where all relevant factors are stated in the problem and we can all be approximated as point masses.

How did each sled start? Did you push each sled with the same amount of force or sit on an incline and wait for gravity to do its thing? If not, that must also be taken into account.


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## suhas2112 (Feb 17, 2009)

Doesn't heavier people = more momentum?!?


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## badmephisto (Feb 17, 2009)

Don't be silly -- of course energy is conserved. Problem here is that your model that includes only gravity etc. is too trivial to model the system and hence the discrepancy. Remember that friction / air drag are non-conservative forces. In other words if they are not negligible, then the total energy of your system will indeed change.


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## JBCM627 (Feb 17, 2009)

shelley said:


> we can all be approximated as point masses.



Heeee, I'm a point mass.


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## Ewks (Feb 17, 2009)

My solution would be that the heavier person created more pressure to the snow under him which made it melt and decreased the friction. And we're speaking of just a model which is there just to give us a hint of how it might turn out to be. Plus it works perfectly only in an ideal world in our world there are many things which have the potential to make the calculation wrong.


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## Ellis (Feb 17, 2009)

I can't help but smile every time I see this thread title.


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## nitrocan (Feb 17, 2009)

audhulma said:


> Ok, so my idea is that several factors effected the results:
> *1) The heavier people compressed the snow, lowering the coefficient of friction*
> 2) The curvature of the hill leads to interference
> 3) The snow may have been rougher in patches, causing the coefficient of friction to vary
> Does this sound reasonable?



Kind of, yes.

And you always have the friction of the air that slows you down.

Think of a heavy car, and a light car. The faster you go with the light car, the more it will jump and skid. But the heavy car will hold on to the surface because it has weight, so you will need more power to move it and the weight supplies it with more friction so it doesn't skid.


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## suhas2112 (Feb 18, 2009)

Okay, sorry about my previous post, i wasn't thinking... You guys are taking this WAAAAAY out of hand... The first thing that we were taught in mechanics was that "The law of conservation of energy can be applied when there are *ONLY* conservative forces involved "
http://en.wikipedia.org/wiki/Conservative_Forces

So, due to friction, air resistance and other non-conservative forces, the law of conservation of energy cannot be applied.

EDIT: sorry, i didn't read badmephisto's post... I'm just adding to that...


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## nitrocan (Feb 18, 2009)

suhas2112 said:


> Okay, sorry about my previous post, i wasn't thinking... You guys are taking this WAAAAAY out of hand... The first thing that we were taught in mechanics was that "The law of conservation of energy can be applied when there are *ONLY* conservative forces involved "
> http://en.wikipedia.org/wiki/Conservative_Forces
> 
> So, due to friction, air resistance and other non-conservative forces, the law of conservation of energy cannot be applied.
> ...



Friction converts some of the energy to heat so it's not actually lost. It has left the system if you think of the system only consisting of the sled and the snow.


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## suhas2112 (Feb 18, 2009)

nitrocan said:


> suhas2112 said:
> 
> 
> > Okay, sorry about my previous post, i wasn't thinking... You guys are taking this WAAAAAY out of hand... The first thing that we were taught in mechanics was that "The law of conservation of energy can be applied when there are *ONLY* conservative forces involved "
> ...



Yes. That is precisely why it is a non-conservative force...


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## blah (Feb 18, 2009)

audhulma said:


> mgh=1/2mv^2+friction



You just placed energy and force in the same linear equation.


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## badmephisto (Feb 18, 2009)

blah said:


> audhulma said:
> 
> 
> > mgh=1/2mv^2+friction
> ...



This thread is so much lulz.


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