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
You wrote regarding centrieptal force-: "NO it does not require energy! It requires a FORCE (see click here->Centripetal force requirement. Thi s 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."
That's nonsense. If a force is operant, then it must be using energy and therefore doing work.
You equate work only with displacement of an object in space. You do not consider it work to keep an object stationary in the face of opposing forces. However, it does require energy to keep an object stationary if there is another force acting to displace that object. For example, if person A is pushing a three foot X three foot diameter block of steel on an ice rink, then it requires energy to move the block against the resistance of the ice. If person B then stands opposite the block and pushes in the opposite direction with the same degree of force as person A is exerting to push it forward, then person B is using energy, and person B is doing work, even though the block of steel is now stationary (not moving, not being displaced).
Therefore, in your example of a person hanging from a jungle gym by his arms, he is doing work even though he is stationary. He is constantly using muscle energy (doing muscle work) to overcome the effect of gravity on his body mass. If he didn't constantly perform that muscular work, gravity would pull his body down to the ground. You wrongly believe that he is only working when he he does chin-ups and pulls his body up.
You sent me to the following website, which demonstrates that a centripetal force must be using energy, and the constant expenditure of energy represents work.
In that first animation, the ball travels in a straight line if there is no force (requiring energy expenditure) to keep it stationary. In the second animation, a block provides a force that keeps the ball stationary - and that represents centripetal force. Any force that is constantly operant requires energy and therefore centripetal force must be performing work. In other words, energy is required to keep the ball stationary (from traveling forward in a straight line) and the restraining force that provides the "energy for restraint" is performing work.
Hopefully, you are wise enough to realise that your car seat belts (or car airbag) will be performing work to keep you from being ejected through the windscreen if you hit another car head-on at 100mph. The work it is performing is to prevent you from becoming an instantaneous human projectile!
Hybrid Mode
