I like your mathematical expertise when dealing with problems in golf physics.
However, sometimes you get it wrong.
You wrote-: "Nope.... sometimes forces do no work and when the don't do work they can't contribute power (or store energy)
Lets step back.
Work is Force X Distance (has units of energy)
Power is Work / time."
Your formula is wrong when you state that work = force X distance.
One also needs to consider the work force needed to stay in balance when moving in a circular manner. Centripetal force is constantly operating to keep an object in its circular track while traveling at a constant speed - and if the centripetal force is operant, then it is contributing to work (energy) output by preventing the object from flying off its circular path.
Consider two cars having a 100 miles race. Car A has to travel 100 miles on a straight track. If car A completes the race in 1 hour by traveling at 100mph, then car A has expended a certain amount of energy (work output) to complete the race in 1 hour. Now imagine car B having to travel 100 miles on a circular track. If car B completes the race in 1 hour by traveling at a constant speed of 100mph, then car B has expended much more energy (work output) in the same time than car A. The extra energy was expended in trying to keep the car on the circular track at all times while it was racing around a constantly present amount of road bend at 100mph. That extra energy is the centripetal force energy required to constantly centripetally accelerate the car (to constantly keep the car moving along a circular path, rather than a straight path).
Work ONLY gets done when it causes kinetic energy to change. If D (distance) is zero, no work gets done. Centripetal acceleration does not change the kinetic energy of a rotating body. IT DOES NO WORK.
Originally Posted by Jeff
One also needs to consider the work force needed to stay in balance when moving in a circular manner. Centripetal force is constantly operating to keep an object in its circular track while traveling at a constant speed - and if the centripetal force is operant, then it is contributing to work (energy) output by preventing the object from flying off its circular path.
Consider two cars having a 100 miles race. Car A has to travel 100 miles on a straight track. If car A completes the race in 1 hour by traveling at 100mph, then car A has expended a certain amount of energy (work output) to complete the race in 1 hour. Now imagine car B having to travel 100 miles on a circular track. If car B completes the race in 1 hour by traveling at a constant speed of 100mph, then car B has expended much more energy (work output) in the same time than car A. The extra energy was expended in trying to keep the car on the circular track at all times while it was racing around a constantly present amount of road bend at 100mph. That extra energy is the centripetal force energy required to constantly centripetally accelerate the car (to constantly keep the car moving along a circular path, rather than a straight path).
Jeff.
NO BOTH CARS EXPEND THE SAME ENERGY. However the car on the track's tires would show more wear DUE TO FRICTION (the force that provided the click->centripetal force requirement )
Last edited by no_mind_golfer : 12-24-2008 at 12:21 PM.
Reason: try to get hyperlinks to show
Sorry. I cannot accept your explanation. You eliminate the possibility of using centripetal force as being part of your work output equation by framing your equation in that manner. If you "a priori" exclude centripetal force, then obviously it seems that centripetal force doesn't require energy to become operant. The energy may not be utilised to generate forward momentum (forward kinetic energy) along the race track (in the car example), but energy is required to keep the car on a circular track (and the car's tires know that).
Consider a simple example.
Imagine traveling in a NYC subway car that is traveling at 40mph on a straight rail track. Imagine that you are standing in the center aisle and holding onto a vertical post. Then imagine what happens when the subway car goes around a tight bend at the same speed. You will have to hang onto that vertical post for "dear life" to prevent yourself from being catapulted down the length of the subway car. It requires "energy" to remain stationary in balance and that energy is the energy required to offset a centrifugal force acting on your body. I would imagine that the subway car also needs to expend energy to stay in balance on its circular track, and that energy is centripetal energy.
Sorry. I cannot accept your explanation. You eliminate the possibility of using centripetal force as being part of your work output equation by framing your equation in that manner. If you "a priori" exclude centripetal force, then obviously it seems that centripetal force doesn't require energy to become operant. The energy may not be utilised to generate forward momentum (forward kinetic energy) along the race track (in the car example), but energy is required to keep the car on a circular track (and the car's tires know that).
I'm sorry you're sorry you cannot accept my explanation but I assure you it is the one and only technically correct one. First off the force is called tension (axial load)... and it has two components (one in the normal, perpendicular to path i.e. centripetal direction and one in the tangential direction.) The normal component of the tension force does not move the object closer to the center of rotation and therefore it does NO WORK! The tangential component on the other hand accelerated the object along the path. The tangential component (and this is the one thing BerntR and I can agree on) is what does the work.
Originally Posted by Jeff
Consider a simple example.
Imagine traveling in a NYC subway car that is traveling at 40mph on a straight rail track. Imagine that you are standing in the center aisle and holding onto a vertical post. Then imagine what happens when the subway car goes around a tight bend at the same speed. You will have to hang onto that vertical post for "dear life" to prevent yourself from being catapulted down the length of the subway car. It requires "energy" to remain stationary in balance and that energy is the energy required to offset a centrifugal force acting on your body. I would imagine that the subway car also needs to expend energy to stay in balance on its circular track, and that energy is centripetal energy.
Jeff.
NO it does not require energy! It requires a FORCE (see click here->Centripetal force requirement. This is why we have a variety of words in the lexicon: FORCE WORK ENERGY etc. soforth.... They have different meanings. AGAIN.... SOME FORCES DO NO WORK.
If you are hanging motionless on a jungle gym there is a force in your arms .... but you're not doing work... you're not expending power... You're hanging there motionless. You don't do work until you do a pull up. When you do a pull-up you are moving that force (mass X gravity) through a distance ... THAT is work... THE requires Hp. Hanging Motionless does not..... but there is a FORCE present when you hang motionless... make no mistake about that!
I'm done with this one.... believe what you want to.... makes no difference to me really.... Merry Christmas Jeff
You state-: "'m sorry you're sorry you cannot accept my explanation but I assure you it is the one and only technically correct one. First off the force is called tension (axial load)... and it has two components (one in the normal, perpendicular to path i.e. centripetal direction and one in the tangential direction.) The normal component of the tension force does not move the object closer to the center of rotation and therefore it does NO WORK! The tangential component on the other hand accelerated the object along the path. The tangential component (and this is the one thing BerntR and I can agree on) is what does the work."
You write that tension force has two components - a tangential component and a centripetal component. You then state that only the tangential component does work, because it propels the object along a a path. However, if the path is circular (rather than a straight path), then some other "force" must be doing work to make the object move along a circular path rather than a straight line path. In other words, that other "force" is doing "work" to centripetally accelerate the object (centripetal force is defined in Wikipedia as the force needed to move an object along a circular path rather than a straight path).
You write-: "The normal component of the tension force does not move the object closer to the center of rotation and therefore it does NO WORK! "
That statement makes no sense to me - if a moving object moves from a straight line path to a circular path, then it is moving closer to the center of rotation.
nm golfer
...snip
That statement makes no sense to me - if a moving object moves from a straight line path to a circular path, then it is moving closer to the center of rotation.
Merry Christmas to you!
Jeff.
Quote:
"It took Newton to show us that the Moon is falling to Earth"
By definition there is no point on a circle that is closer to the center of the circle than any other....... I'll leave it at that.
nmgolfer - you wrote-: "there is no point on a circle that is closer to the center of the circle than any other."
That is correct. Imagine that there a million points on that hypothetical circle's circumference, and imagine that an orbiting object (traveling at a constant finite speed) has to move from from one point on the circumference to the next point on the circumference to the next point on the circumference -- and that it has to complete this process one million times to complete one orbit. In each of those movements (from one point to the next point), the orbiting object needs a tangential force to move it at its constant "finite" speed and a centripetal force to keep it moving on the circular path.
After penning post #265, I have come back to this post to add another comment.
My final statement above was "a centripetal force to keep it moving on a circular path." Keeping an orbiting object traveling in a circle requires a restraining force, a force that prevents the orbiting object from moving off into space (in a straight line direction at right angles to the circumference of the circle). That restraining force, which keeps the orbiting object traveling along a circular path, represents centripetal force, and the constant use of a restraining force (centripetal force) requires the constant expenditure of energy - and that represents work.
Jeff.
Last edited by Jeff : 12-24-2008 at 08:10 PM.
Reason: add final commentary
Work is Force X Distance (has units of energy)
Power is Work / time."
Your formula is wrong when you state that work = force X distance.
One also needs to consider the work force needed to stay in balance when moving in a circular manner. Centripetal force is constantly operating to keep an object in its circular track while traveling at a constant speed - and if the centripetal force is operant, then it is contributing to work (energy) output by preventing the object from flying off its circular path.
Jeff,
The formula is 100% correct. Without exception. This is very basic physics.
The efforts to stay in balance, preventing the object from flying off its circular path etc does not produce work in a Newtonian sense. And they do not produce swing speed either. Even though they are required to perform a golf stroke.
Bernt - you wrote-: "If the object is already moving in a circle you don't need the B force vector to move it from A to C. You only need a centripetal force from A to Origo to do that. So your diagram need an invard pointing vector to be complete. Origo is the center of the circle, btw."
You don't understand my position. I have simply divided the model into the two forces that are operant when an orbiting object travels in a circular path - a tangential force (operating always in a straight line direction) that supplies energy to allow the oribiting object to travel at a constant speed; and a centripetal force that causes the orbiting object to centripetally accelerate so that it travels along a circular path instead of a straight line path.
In that diagram - the tangential force vector is directed towards B because a tangential force always operates in a straight line direction. The centripetal force (which is directed towards the center of the circle) deflects the orbiting object so that it ends up at point C instead of point B.
You also wrote-: "The centripetal force doesn't do any work in a Newtonian sense. It doesn't increase the speed. It doesn't overcome any resistive loss if any of such is present. It only changes the direction of the mass movement."
I agree that a centripetal force doesn't increase the speed of the orbiting object and that it only changes the direction of the movement of the mass. However, it requires a Newtonian force to change the direction of a moving object, and when that force is operant it is using energy and therefore doing work.
To make it even easier for anyone to falsify my understanding/explanation of "centripetal force action", I am producing this simple example of forces in action.
This diagram represents a birds eye view of person A pushing a lightweight 10'x10'X10' square box across an ice rink. His plan is to push the square box in a straight line direction towards destination D. He works out that it would take him 20 minutes to accomplish that goal - considering the friction drag of the ice against the undersurface of the box. He pushes in a straight line direction towards destination point D. Because of the 10' height of the box he cannot see where he is going, but imagine that person A has an uncanny ability to always apply his constant push force perpendicular to the surface of the box and that he is always pushing in a straight line thrust action.
After 10 minutes, person A stops for a rest and he notes that he has completed 50% of the distance to destination point D (top diagram) - because he is applying a constant straight line push-force against the box.
Now imagine that person A starts pushing again with the same level of straight line push-force. However, he doesn't realise that person B has arrived and person B is also pushing in a straight line thrust against the surface of the box - at a right angle to person A's straight line thrust action. Person B is less strong than person A and applies less push-force than person B.
What happens after 10 minutes? The box ends up at point C rather than destination point D (bottom diagram). The reason is that there is another force present that deflects the moving mass from its straight line path. In other words, that other force changes the direction of the movement of the moving mass, so that it follows a circular path instead of a straight line path. Note that the other force pushes at right angles to the circumference of the circular path - and that it is therefore directing its force towards the center of a hypothetical circle.
It doesn't take much common sense to interpret what I have described.
Person A is supplying a push-force that moves the mass at a constant speed in a straight line direction - and that represents the tangential force.
Person B is also supplying a push-force that is directed at right angles to the tangential force - and that represents the centripetal force.
It should also be apparent that the centripetal force is a Newtonian-type force that is capable of changing the direction of movement of a mass (that is being moved at a constant speed by a constantly present tangential force), and that it is manifesting energy and performing work.
Since there is friction here, A is doing work even though the speed becomes constant after a while.
If B's contribution is only to turn a linear movement into a circular movement, B is not doing any work. The object is always moving perpendicular to the direction B is pushing. The angle between the object movement and B's pull force need to be different from 90* before any work is done. Changing direction is not the same as doing work - even though it makes a significant difference.
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