car moving at a constant velocity....and many other questions on KEby leright Tags: constant, moving, velocityand 

#1
Sep306, 10:19 AM

P: 1,194

This is NOT hw...just a semantics inquiry.
Say you have a car moving at a constant velocity. Is the engine doing work on the object since it is exerting a force on the car in the direction of the motion? I am thinking not, since the frictional force balances the force the engine is exerting. I can calculate the work the engine does, which is the force the engine exerts in the direction of the motion over a distance (integral of the force dotted with a differential length, which becomes FD since force is constant) and I get a positive number. Now, if I calculate the work the friction force does, which is negative (the friction force is in the opposite direction of the length, so the dot product is negative but the magnitude is the same since the car is moving at a constant velocity and the forces balance) and the same magnitude as the work the motor does. This results in a net work of zero. This is consistent with the workKE theorem. No work done means no change in kinetic energy, and there is no change in kinetic energy of the car as it is moving at constant speed. A lecturer (AT UNIVERSITY EVEN) explained work by pushing a piece of chalk on the table work is done, and since the force is constant, the work is simple the pushing force times the distance. But really, the only work done was the KE. Therefore, the only time work was done was the tiny moment of time (and tiny distance) when the pushing force was greater than the static friction force...but then when the friction became larger (kinetic friction) the pushing force was balanced and then no work is done, and there is no change in KE. Now, this brings me to my next question. I know KE is relative. Now say I have a car moving at a constant velocity and I am standing still. Then I move to a constant velocity relative to the car. The KE of the car is going to change (say decrease, since I move in the same direction as the car) and my kinetic energy is going to change. The kinetic energy of the car decreases and the kinetic energy of me also increases. However, energy needs to be put into me that is equal to my kinetic energy, right? So my kinetic energy balances the energy put into me, so energy is conserved. But, the kinetic energy of the car DECREASES. There is no longer a balance in energy. Kinetic energy is not conserved, it seems! Can someone explain this? What am I not accounting for? Now, can someone explain rotational kinetic energy? Total KE is the translational KE + rotational KE, right? Well, people seem to say that, for instance, centripetal forces cannot do work on objects because they do not change the kinetic energy (and the force is perpendicular to the movement, so, by definition of work, no work is done), but now, it seems they CAN do work on objects, because they change the rotational kinetic energy, and total KE is the rotational KE + translational KE, and if we have a change in KE we do work. So, do centripetal forces do work or do they not do work? I suppose that circular motion occurs because a centripetal force, in one frame, causes a displacement in the direction of the centripetal force and work is done, which causes a change in rotational KE. Now, In the next frame, that same centripetal force (not translated to the next frame) is perpendicular to the motion. But the centripetal force is in fact in the same direction as the motion, so it DOES do work. Also, people say that Bfields don't do work. They do do work even though they cause a centripetal force as they change rotational KE of an object, and the object gains a component of displacement in the direction of the centripetal force. However, is the centripetal force causing this displacement component in the same frame, or in the following frame. I suppose if it is the following frame then the centripetal force does no work. HOWEVER, THE CENTRIPETAL FORCE HAS TO DO SOME INITIAL WORK TO PROVIDE AN ANGULAR VELOCITY. 



#2
Sep306, 10:40 AM

Sci Advisor
HW Helper
P: 2,004

Okay, one thing at a time...
A centripetal force does no work, it simply maintains circular motion of the particle (changes direction, not the speed) 



#3
Sep306, 10:57 AM

P: 1,194

let's say we have a point mass moving at a constant velocity. Then, we have a centripetal force act on the particle which causes it to move on a circular path of radius r, say. All of the translational energy is converted into rotational kinetic energy, but the total kinetic energy is the same. Therefore, the centripetal force does no work.
ok, got it. Thanks. And also, magnetic forces can do work. what about a mass being lifted by an electromagnet, or two wires carrying current in the same direction? 



#4
Sep306, 02:15 PM

Sci Advisor
HW Helper
P: 2,004

car moving at a constant velocity....and many other questions on KEIn the case of an electromagnet lifting a piece of metal (like the big ones they have in a car scrap junkyard) the work is really done by the agancy supplying the electromagnetic with power. Someone has to keep the current through the electromagnet going, so an electric potential has to be maintained. It's sometimes hard to figure out what agency is doing the work, but it will never be just the magnetic force. 



#5
Sep306, 02:39 PM

P: 1,194

If the magnetic dipole moment doesn't diminish, then where does the energy come from then? 



#6
Sep306, 02:43 PM

P: 1,194

so why do people call things magnetic energy, and why is there a magnetic energy density if magnetic fields can never do work? Why am I learning about magnetic work in my thermal physics class?
So, really, magnetic fields don't have energy, because they cannot do work, but can only take other forms of energy and convert then into energies that can do work? 


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