Movement of a piston

[TonyMcC]

Yes, at time t, otherwise you have a non-linear discontinuous manifold and modifications would have to be made to its treatment.

 

Wherever I look for a definition of "stop" it always implies "cease completely or for a time". I still think that as the piston passes through speed=0 in an infinitely small time it cannot be said to have stopped. I think v=0 is not sufficient by itself to define stop. I feel a proper definition of stop should be v=0 for a time. If it only paused for something like a picosecond I would say it stopped for a picosecond. But as it "stops" for an infinitely smaller time than that I question the statement that it can be said to have stopped at all.

I do think we are really playing with words as I think we all know what happens. Its nearly bedtime now in UK so I wish you goodnight.

Edited January 7, 2012 by TonyMcC

[StringJunky]

Tony

 

Rather than say it stopped at v=0, wouldn't it be better to say it reached equilibrium at v=0? It's a semantic problem that requires a semantic solution maybe?

[Dekan]

You are right, your friend is wrong.

 

In an ideal case, the piston moves up and down, and the velocity will reach zero both at the top and bottom on every stroke.

 

In reality, every thing is vibrating, and there will be a very tiny sideways motion to - so nothing really ever stops.

 

You seem to be conceding, that the up-and-down motion of the piston, stops at the top and bottom of every stroke.

 

But you're saying that the piston hasn't really stopped, because it's still going sideways?

 

Forgive me, but that's the impression I was getting!

[swansont]

A body in continuous motion doesn't stop, but the body that you are refering to is not in continuous motion because it does. The only state that this body is continuously in is accelerating.

 

 

I think you can make the argument that continuous motion means a and v are never simultaneously zero.

[Xittenn]

The OP was asking about a tie between circular motion and whether or not the piston comes to a stop. I still argue that the two are in fact not related, and that whether or not the piston is connected to a system in circular motion is irrelevant to whether or not it stops at its peak of oscillation.

 

 

--edit--

 

made a comment not sure I actually understood what was being said, edited out

Edited January 8, 2012 by Xittenn

[Xittenn]

Just to be clear on my meaning

 

I am saying that this:

 

"He argued that since it pivots on a circular motion from the bottom that it didn't." -JustinW's friend

 

Isn't a direct negation of this:

 

"I had just had a discussion with a friend of mine about whether a piston's motion stops before it changes direction." -JustinW

 

 

Unless of course it was intended to imply that as a consequence there would result an unbalanced force at all times. And I now see, as a consequence of Swansonts statements, why one might say that the piston does in fact not stop at the top of its stroke.

 

well I'm contented : D

[TonyMcC]

 

 

I'm contented : D

 

Me also - I've enjoyed the discussion.

Edited January 8, 2012 by TonyMcC

[JustinW]

Okay...... Something would have to stop or reach v=0 before changing a trajectory that is either up or down with no curvature to its motion. If the trajectory were more circular when changing it's path from up to down someone could say that it didn't stop before changing direction. Since we are talking about a piston which can only move on a vertical path, up or down, then it would have to come to a v=0 state in its direction change.

[TonyMcC]

Okay...... Something would have to stop or reach v=0 before changing a trajectory that is either up or down with no curvature to its motion. If the trajectory were more circular when changing it's path from up to down someone could say that it didn't stop before changing direction. Since we are talking about a piston which can only move on a vertical path, up or down, then it would have to come to a v=0 state in its direction change.

 

The general consensus seems to be that although v=0 for an instant, at that time a<>0 and unless v and a are both zero at the same time the piston cannot be in a state fairly described as "stopped". (see #29 by Swansont)

Edited January 9, 2012 by TonyMcC

[JustinW]

Agreed.

[doG]

IMO, Just because the acceleration is continuous does not mean that it doesn't pass through zero. There is an instant when acceleration goes from positive to negative as the direction changes. At this point acceleration would be zero as would the velocity at that instant, i.e. a=v=0. At that instant in time the piston could be considered to be stopped.

[Xittenn]

IMO, Just because the acceleration is continuous does not mean that it doesn't pass through zero. There is an instant when acceleration goes from positive to negative as the direction changes. At this point acceleration would be zero as would the velocity at that instant, i.e. a=v=0. At that instant in time the piston could be considered to be stopped.

 

I think the argument has changed though, from this statement of "the velocity v never actually attains zero", to "well maybe we might define a body in motion as having velocity or having acceleration at any given moment, or both". I hadn't seen it like this before, until Swansont brought it up. I kind of like the idea because we would then define the body as being in motion at its first instant of a force being applied from rest. We could also then say that a body remains in motion as it is accelerated through zero. These statements seem perfectly logical to me, and I don't see why we couldn't use this as definition. I think however, that the ambiguity should be removed from the definition; although, that might be somewhat difficult as it would be a pretty massive undertaking.

 

Is there somewhere where this is defined properly, and without ambiguity?

Edited January 10, 2012 by Xittenn

[iNow]

Is there somewhere where this is defined properly, and without ambiguity?

Yes. It happened in the very first response to this thread. Post #2.

[doG]

I think the argument has changed though, from this statement of "the velocity v never actually attains zero", to "well maybe we might define a body in motion as having velocity or having acceleration at any given moment, or both". I hadn't seen it like this before, until Swansont brought it up. I kind of like the idea because we would then define the body as being in motion at its first instant of a force being applied from rest. We could also then say that a body remains in motion as it is accelerated through zero. These statements seem perfectly logical to me, and I don't see why we couldn't use this as definition. I think however, that the ambiguity should be removed from the definition; although, that might be somewhat difficult as it would be a pretty massive undertaking.

 

Is there somewhere where this is defined properly, and without ambiguity?

Consider 2 examples:

 

1. A ball is tossed vertically straight up into the air. At the apex of its travel it's velocity will hit zero as the acceleration changes from positive to negative. At the apex a and v with both equal 0.

 

2. A ball is launched in a parabolic arc like a cannonball fired from a cannon. At the apex of its travel its velocity will be continuously greater than zero but its acceleration will change from positive to negative. V will not equal 0 until the ball lands and comes to rest so a and v will never equal 0 simultaneously until the ball comes to rest.

 

IMO, the motion of the piston is similar to example 1 because of its axial travel in the piston bore. This becomes very evident if you place a dial indicator in the spark plug bore and bring the piston to top dead center in order to mechanically align the camshaft. You find there is a small amount of travel of the crankshaft, 1°-2°, where there is no change in the piston's position. This is caused by the change in angle of the connecting rod relative to the axis of the bore as the crankshaft moves across top dead center. During that period of travel you could consider the piston stopped.

[Xittenn]

Consider 2 examples:

 

1. A ball is tossed vertically straight up into the air. At the apex of its travel it's velocity will hit zero as the acceleration changes from positive to negative. At the apex a and v with both equal 0.

 

 

The acceleration due to gravity near earths surface is [math] 9.8 \frac{m}{s^2} [/math] down at all times on any body that is allowed to fall freely. This doesn't change even if the object has an upward velocity. I don't know where people (you and iNow) are saying that there is a change in acceleration at the apex of . . . . . . this is pretty basic physics.

[iNow]

The acceleration due to gravity near earths surface is [math] 9.8 \frac{m}{s^2} [/math] down at all times on any body that is allowed to fall freely. This doesn't change even if the object has an upward velocity. I don't know where people (you and iNow) are saying that there is a change in acceleration at the apex of . . . . . . this is pretty basic physics.

Acceleration is a vector quantity with both magnitude and direction. Yes, acceleration is always positive, but there comes a point where the piston is no longer accelerating... It has magnitude zero as it transitions from the upward direction to the downward direction. What is so confusing about this for you?

[Xittenn]

Acceleration is a vector quantity with both magnitude and direction. Yes, acceleration is always positive, but there comes a point where the piston is no longer accelerating... It has magnitude zero as it transitions from the upward direction to the downward direction. What is so confusing about this for you?

 

There is no transition when it comes to gravity. The acceleration acting on a body, regardless of its velocity, due to gravity is very much [math] - 9.8 \frac{m}{s^2} [/math], period. I don't know what the acceleration through the peak of a pistons turn about is, but I can say with some certainty that in an idealized model of this situation it is not 0. I'm really not the person to make this any clearer.

[TonyMcC]

The acceleration due to gravity near earths surface is [math] 9.8 \frac{m}{s^2} [/math] down at all times on any body that is allowed to fall freely. This doesn't change even if the object has an upward velocity. I don't know where people (you and iNow) are saying that there is a change in acceleration at the apex of . . . . . . this is pretty basic physics.

 

I would say that an instantaneous time of zero acceleration occurs at the mid point of the piston's travel and instantaneous speed of zero occurs at the top and bottom of the pistons travel. i.e a sine and cosine relationship. ( Neglecting the fact that even with a long connecting rod and small crank displacement there will be a little distortion of the sine and cosine waveforms)

Edited January 10, 2012 by TonyMcC

[iNow]

There is no transition when it comes to gravity. The acceleration acting on a body, regardless of its velocity, due to gravity is very much [math] - 9.8 \frac{m}{s^2} [/math], period.

I wonder if part of the disagreement here is the frame of reference we're choosing for our positions.

 

To be clear, I'm not saying that the piston is not accelerating through space, or that the earth is not orbiting around the sun... of course the piston is also accelerating around the sun, or through the galaxy, or through the supercluster... or whatever.

 

However, I thought we were agreed here that the frame of reference under discussion is being limited to the cylinder or head of the engine... in which case, yes... It stops at each pivot point for at least an instant before reversing the direction of travel.

 

Additionally, I'm talking about the piston itself... not the crank, or the cam shaft, or the fly wheel or any other component... Just the piston.

 

Perhaps this gets us closer to being on the same page???

 

 

 

 

[Xittenn]

I would say that an instantaneous time of zero acceleration occurs at the mid point of the piston's travel and instantaneous speed of zero occurs at the top and bottom of the pistons travel. i.e a sine and cosine relationship. ( Neglecting the fact that even with a long connecting rod and small crank displacement there will be a little distortion of the sine and cosine waveforms)

 

- Piston Motion Basics - Travel, Velocity, Acceleration, Vibration

 

There are charts and everything!

 

Notice in particular the graph that includes both velocity and acceleration.

 

I wonder if part of the disagreement here is the frame of reference we're choosing for our positions.

 

To be clear, I'm not saying that the piston is not accelerating through space, or that the earth is not orbiting around the sun... of course the piston is also accelerating around the sun, or through the galaxy, or through the supercluster... or whatever.

 

However, I thought we were agreed here that the frame of reference under discussion is being limited to the cylinder or head of the engine... in which case, yes... It stops at each pivot point for at least an instant before reversing the direction of travel.

 

Additionally, I'm talking about the piston itself... not the crank, or the cam shaft, or the fly wheel or any other component... Just the piston.

 

Perhaps this gets us closer to being on the same page???

 

 

I hear they have great correspondence programs for high school coarses these days. A good many of them are now done online. You might want to do a review of the basics before entering into discussions about matters you clearly have no properly functioning knowledge of.

 

I would like to make an additional note, observe momentum against acceleration. The extremes of each coincide with the others 0 values. When there is no acceleration there is max momentum and vice verse.

[iNow]

I hear they have great correspondence programs for high school coarses these days. A good many of them are now done online. You might want to do a review of the basics before entering into discussions about matters you clearly have no properly functioning knowledge of.

I appreciate the tip. Would you be willing to respond to the question I put forth now?

[Xittenn]

Perhaps this gets us closer to being on the same page???

 

No, we are not on the same page. Question answered I am through with this thread now.

 

Have fun!

[iNow]

Where do we differ?

[A Tripolation]

...I never knew classical mechanics was so hotly disputed.

[JustinW]

I just asked to see where the conversation would go in the first place. I've noticed when talking with smart people that you have to be specific or the conversation could go anywhere. It's fun reading though. It's interesting to see how many different sides there are to a simple arguement that seems cut and dry.