Work and Constant Speed: Exploring Kinetic Energy

[Bashyboy]

I am reading this article on the concept of work,http://www.lightandmatter.com/html_books/lm/ch13/ch13.html#fig:tractor-pulling-plow , but the paragraph immediately below the section "Calculating work as force multiplied by distance" appears to implicitly say that some sort of work is being done, even though in the supposed situation they are assuming that speed is constant, which would mean there is no force. Could someone try to explain what exactly the paragraph is saying? Especially what they mean by "kinetic energy of the weight"?

 

[Doc Al]

I am reading this article on the concept of work,http://www.lightandmatter.com/html_books/lm/ch13/ch13.html#fig:tractor-pulling-plow , but the paragraph immediately below the section "Calculating work as force multiplied by distance" appears to implicitly say that some sort of work is being done, even though in the supposed situation they are assuming that speed is constant, which would mean there is no force. Could someone try to explain what exactly the paragraph is saying? Especially what they mean by "kinetic energy of the weight"?

Since the tractor exerts a force through a distance, work is done. If the object being pulled is moving at constant speed, it just means that other forces are acting so that the net force is zero. But the tractor is still doing work. Depending on the details, that work could go into increasing PE or KE, or breaking up the soil.

 

[Naty1]

Suppose you drag a plow thru the ground at constant speed. Are you doing any work??

Yes. constant speed does not mean no work is being done. It takes effort [work] to drag a plow at constant speed. That's why a car burns gas going down the highway at a constant speed...friction of air floq and tires, for example, must be overcome even at constant speed.

When you increase the speed of a mass, you also increase it's kinetic energy...an energy of motion. This is measured by 1/2mv² and it takes work to increase such kinetic energy...analogous to the work it takes to lift a weight increasing it's potential energy. When lifting a weight it is often assumed the weight is not accelerated, that is, it's velocity is constant, so that only potential energy is changed; if you rapidly lift a weight increasing its speed, and hence it's kinetic energy, such a weight is capable of doing more work because you have put more work into it. That's why, for example, wood chippers have a heavy flywheel so when something thick and solid has to be cut, the energy in the flywheel can briefly do the work so the engine doesn't slow down too much...any maybe stall.

 

[Bashyboy]

Since the tractor exerts a force through a distance, work is done. If the object being pulled is moving at constant speed, it just means that other forces are acting so that the net force is zero. But the tractor is still doing work. Depending on the details, that work could go into increasing PE or KE, or breaking up the soil.

Oh, okay, so the NET FORCE is zero, but the definition of work doesn't deal with the the forces summed together (net force), it deals with each individual force acting on an object. So, for instance, if we had a tractor pulling a trailer at a constant velocity, work would be done by friction acting against the tires and the hitch pulling on the trailer.

Now, I am having a little troubling seeing how friction can do work, and how work can it accumulate over a distance. Is it because at the moment of contact, between some object and the ground, once the force is applied the two surfaces are moving relative to one another?

 

[Doc Al]

I'm unclear what you are asking. Getting back to the tractor: If it exerts a force as it moves through a distance then it does work. Regardless of whether it's moving at constant speed or accelerating.

 

[CWatters]

Now, I am having a little troubling seeing how friction can do work

Friction typically converts one type of energy (mechanical) to another (heat).

http://en.wikipedia.org/wiki/Friction

Friction is not a fundamental force but occurs because of the electromagnetic forces between charged particles which constitute the surfaces in contact. Because of the complexity of these interactions, friction cannot be calculated from first principles, but instead must be found empirically.