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Energy Lost Due To Friction

Ratch

Mar 10, 2013
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@duke37 already explained that aspect. You should read his post above. Very informative.

Not true, he did not explain how to get the instantaneous speed at the bottom of the incline other than the difficult task of direct velocity measurement. There is a better way. Can you find it?

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Laplace

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Not true, he did not explain how to get the instantaneous speed at the bottom of the incline
In post #15 @duke37 stated "The speed at the end of the descent will be double the average speed if the acceleration is constant."
 

Ratch

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In post #15 @duke37 stated "The speed at the end of the descent will be double the average speed if the acceleration is constant."

OK, I see what you mean now. Yes, that will work. I used the increase in time caused by the friction to compute the reduction in acceleration. Then multiply by the mass of the trolley to get the friction directly without computing any potential energy or kinetic energies.

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duke37

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There are various ways of calculating however, the friction and hence acceleration will not be constant but will depend on speed. This means that the a sophisticated method of measuring the velocity at the end of the trip will be advantageous.
 

Ratch

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There are various ways of calculating however, the friction and hence acceleration will not be constant but will depend on speed. This means that the a sophisticated method of measuring the velocity at the end of the trip will be advantageous.

Then how will knowing the speed at the end of the descent help find the changing friction during the descent of the incline? Anyway, I believe the problem assumes a constant frictional force. In that case, the acceleration will be constant, but less than it would be if not friction were present. As this link implies, friction can be a complicated subject unless simplified. http://hyperphysics.phy-astr.gsu.edu/hbase/frict.html

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dorke

Jun 20, 2015
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I wonder, what does that friction issue have to do with Electronics?o_O
maybe something like this:
A capacitor C1 charged to an initial voltage e connects to another capacitor C2 through wire resistance R.

untitled.JPG
 
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duke37

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I know of no direct link between friction and electronics but I spent much of my working life persuading instrumentation to measure friction of braking materials. In some cases, the friction was negligible when wet and in other cases could be quite high when the friction material was on fire.:)
 

Martaine2005

May 12, 2015
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Sounds like bicycle brakes Trevor!! Negligible / non existent when wet..:p

Martin
 

marah

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Hello ,
I'm doing now exactly the same experiment and I need some help with it , do still have this lab report ? I know this is from 3 years ago but just a try.
Thank you
 

duke37

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The thread is getting a bit long with several side issues, it might be better to start another one.

Read through this thread and tell us what problem you have.

Starting energy is the potential energy.
Finish energy is the kinetic energy, estimated from the final speed.
Energy lost is the difference between these two.
 

Ratch

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To the Ineffable All,

I looked at the problem again, and this time I determined that it is a trivial exercise in kinematics. No need to know the starting potential energy, coefficient of friction, final velocity, height above the ground, etc. All that is superfluous information that gets in the way of the solution. Here is all you need to know and how to find the solution.

1) Angle of the incline
2) Length of the incline
3) Time for object to move to the bottom starting from rest on top
4) Mass of object
Integrator741.JPG


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Laplace

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There is a less complicated approach. All that is really needed are the three equations of motion in a single linear dimension for a body under constant acceleration starting at rest, relating the acceleration(a), velocity(v), distance(s), and time(t):

1) v = a⋅t
2) s = ½a⋅t²
3) v²= 2a⋅s

Recognize that this is a constant acceleration situation: the incline is straight & at the same angle, so gravity exerts the same force on the object (parallel to the incline) at all positions; air resistance is negligible; rolling resistance is assumed to be constant. So the same force acts on the object at all times; therefore the acceleration is constant.

The situation is that an object rolls down an incline, converting potential energy to kinetic energy, while losing some of the potential energy to friction. Measuring the height of the incline gives the total potential energy. Measuring the total distance traveled down the incline, and the total time of travel, allows for calculating the constant acceleration parallel to the incline and the final velocity (also parallel to the incline). With the final velocity, calculate the kinetic energy. Energy lost due to friction is the difference between the initial potential & final kinetic energy.

Note: If the final kinetic energy is greater than the initial potential energy, I want to know your secret!
 

Ratch

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There is a less complicated approach. All that is really needed are the three equations of motion in a single linear dimension for a body under constant acceleration starting at rest, relating the acceleration(a), velocity(v), distance(s), and time(t):

1) v = a⋅t
2) s = ½a⋅t²
3) v²= 2a⋅s

Recognize that this is a constant acceleration situation: the incline is straight & at the same angle, so gravity exerts the same force on the object (parallel to the incline) at all positions; air resistance is negligible; rolling resistance is assumed to be constant. So the same force acts on the object at all times; therefore the acceleration is constant.

The situation is that an object rolls down an incline, converting potential energy to kinetic energy, while losing some of the potential energy to friction. Measuring the height of the incline gives the total potential energy. Measuring the total distance traveled down the incline, and the total time of travel, allows for calculating the constant acceleration parallel to the incline and the final velocity (also parallel to the incline). With the final velocity, calculate the kinetic energy. Energy lost due to friction is the difference between the initial potential & final kinetic energy.

Note: If the final kinetic energy is greater than the initial potential energy, I want to know your secret!
Did I not aver in the post before yours, that knowing the potential energy, kinetic energy, and velocity are not necessary to calculate the energy lost due to friction? Before you say otherwise, you should read my post and prove me wrong first. Only the time of descent needs to be measured. Force of friction and energy loss can be calculated from the time and physical dimensions of the incline.

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Laplace

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Force of friction and energy loss can be calculated from the time and physical dimensions of the incline.
Yes, that is certainly true. But there is a conceptually simpler approach. It's somewhat like arguing whether Newtonian or Lagrangian mechanics is the better method.
 

Ratch

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Yes, that is certainly true. But there is a conceptually simpler approach. It's somewhat like arguing whether Newtonian or Lagrangian mechanics is the better method.
Show us the method and approach. Inquiring minds would like to know.

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Laplace

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Inquiring minds would like to know.
You would need to ask @marah (post #30) whether the kinetic energy method cited by @duke37 (post #31) or the retardation force method (post #32) is the conceptually simpler one. I know I've already made my choice.
 

Ratch

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You would need to ask @marah (post #30) whether the kinetic energy method cited by @duke37 (post #31) or the retardation force method (post #32) is the conceptually simpler one. I know I've already made my choice.

Both the Duke's method and my method use the same concept, specifically, that energy is lost by friction. The difference is in the way the loss is determined. I don't think there is any doubt that my method is easier to implement because only the time needs to be measured, instead of before and after potential and kinetic energies. It is easier to calculate the loss of acceleration than reading a speedometer at the end of a descent. Don't you agree? If not, why not?

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duke37

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My method was to answer the question directly. The friction or angle of dangle are not needed.
It is necessary to know the final velocity in any method to know the energy loss. I gave a way of estimating this from the time of flight.
Why complicate things?
How do you measure the loss of acceleration?

Starting energy A = mgh
Ending energy B = mv^2/2
Energy loss is A - B

The final velocity (v) is twice the average velocity if the acceleration is constant. You could prove this by calculus if so inclined. If the acceleration varies then time will not give the final velocity and the the final velocity will need to be measured directly.
 

Ratch

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My method was to answer the question directly.
As was mine also.

The friction or angle of dangle are not needed.
In my method, the angle is needed, the friction is not. The angle is readily available, the friction is not.

It is necessary to know the final velocity in any method to know the energy loss.
No need to measure the any velocity with my method.

I gave a way of estimating this from the time of flight.
Time of descent is all that needs to be measured by my method.

Why complicate things?
Good question. Why get involved with measuring velocities and energies. My method only measures the time of descent and easily calculates the energy loss due to friction.

How do you measure the loss of acceleration?
Already shown in post #32. The acceleration loss is (g sin(theta) -a) , where "a" is easily found by knowing the time of descent and the length of the incline.

Starting energy A = mgh
Ending energy B = mv^2/2
Energy loss is A - B
While the above equations are correct, it is a hell of a lot easier to measure the time of descent that to measure the final velocity.

The final velocity (v) is twice the average velocity if the acceleration is constant. You could prove this by calculus if so inclined. If the acceleration varies then time will not give the final velocity and the the final velocity will need to be measured directly.
My method gives the average acceleration loss. So even if the deceleration in not constant, the average friction retardation force can be easily calculated and the true energy loss found.

Ratch
 
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