Is there work for object moving at a constant speed?

[miyayeah]

Homework Statement

An object of mass 100kg is acted on by a constant force as it moves through a displacement at a constant speed of 7.2m/s. What is the net work done on the object?

Homework Equations

Wnet = ΔK
Fnet = 0

The Attempt at a Solution

I know that the answer is zero because the equation, expanded, is: ½mv_f²-½mv_i². Since final and initial velocity is the same, the net work will be zero... but I'm having difficulty trying to conceptualize the idea. So is kinetic work only present if the object is accelerating, either speeding up or slowing down? How does net work differ from work?

 

[stockzahn]

Imagine a car driving on a (horizintal) highway. To keep it at constant speed, you need to burn gas by pushing down the pedal. But the energy supplied by the gas and transformed into work by the engine does not increase the kinetic energy (which is approximately constant, when neglecting the mass reduction of the car due to the burned gas). The kinetic energy also is present, if the car is proceeding at constant speed, the difference of the kinetic energy at two different times indicates the variation of speed and/or mass. Net work is the difference of the work done on the system and the work done by the system and it and any net work done corresponds to a variation of the stored energy of the system. Since the energy of the car on the highway in our case is constant (constant speed, constant height, ...), the net work equals zero. That means all the work done by the engine does not remain in the car, but is expended again and exits the system. But what for?

 

[haruspex]

An object of mass 100kg is acted on by a constant force as it moves through a displacement at a constant speed of 7.2m/s

It seems to suggest a nonzero constant net force, as a vector, and a nonzero time, but in that case I do not understand how the speed can be constant. So the question is either flawed or deceptive.

 

[vela]

So is kinetic work only present if the object is accelerating, either speeding up or slowing down?

What's kinetic work? Do you mean the change in kinetic energy? If so, that's not work.

How does net work differ from work?

The same way a force differs from the net force on an object.

 

[Chestermiller]

Homework Statement

An object of mass 100kg is acted on by a constant force as it moves through a displacement at a constant speed of 7.2m/s. What is the net work done on the object?

Homework Equations

Wnet = ΔK
Fnet = 0

The Attempt at a Solution

I know that the answer is zero because the equation, expanded, is: ½mv_f²-½mv_i². Since final and initial velocity is the same, the net work will be zero... but I'm having difficulty trying to conceptualize the idea. So is kinetic work only present if the object is accelerating, either speeding up or slowing down? How does net work differ from work?

If a body has several forces acting on it, if its velocity is constant, each individual force does work on the body, but the sum of these amounts of work must be zero (since the sum of the forces is zero). So even though the individual forces do work on the body, there is no net work done by the combined forces on the body.

 

[Ray Vickson]

Homework Statement

An object of mass 100kg is acted on by a constant force as it moves through a displacement at a constant speed of 7.2m/s. What is the net work done on the object?

Homework Equations

Wnet = ΔK
Fnet = 0

The Attempt at a Solution

I know that the answer is zero because the equation, expanded, is: ½mv_f²-½mv_i². Since final and initial velocity is the same, the net work will be zero... but I'm having difficulty trying to conceptualize the idea. So is kinetic work only present if the object is accelerating, either speeding up or slowing down? How does net work differ from work?

The only way the scenario make sense is that if the object is subject to something like friction, so a force is needed to maintain its constant speed. If the force is applied (for example) through a cable pulled by a motor that burns fuel (or by you tugging on the cable), the motor (or you) expends energy in keeping the object moving. So work is being done on the "system", which is object + its environment.

 

[miyayeah]

What's kinetic work? Do you mean the change in kinetic energy? If so, that's not work.The same way a force differs from the net force on an object.

Yes, sorry, I meant kinetic energy.