Work Done by gravity

[crazyasian]

I read a few other posts and I just wanna make sure I'm understanding this concept correctly. So work CAN be done by gravity as long as gravity is not acting perpendicular to the object. Thus if an object slides down an incline gravity does force on the object because it is acting at an angle that is not purplendicular. So assuming an incline, here's where I get confused. If I were to calculate work done, would I use the vertical change in height (mgh) or would i use the distance traveled down the incline (which would be larger than the vertical change)? Thanks for any help.

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[Rabolisk]

What is work, qualitatively? If a force does work on an object, what changes in that object? How does this relate to gravity?

 

[fizzgig]

this confused me too. conservative forces to not do work. EK says, then, if a question asks for the work done by gravity, consider gravity to be outside the system.

normally if an object falls (straight down or down an incline, IF theres no friction) it's PE loss is completely conserved in KE. that is why gravity is a conservative force.

so if you ask for the work done by gravity as an object falls (F*d, mgh), don't consider the other half (the energy it gains, KE). i guess consider it to halt at the bottom.

 

[coyfish]

Conservative forces do not depend on the path taken. So incline or not makes no difference. Height is the only thing that matters. Gravity can do work.

Anything that changes PE or KE does work. Magnets don't do force to moving particles because the force is always perpendicular to the motion. Thus the KE never changes . . . just direction.

Nonconservative forces like friction do depend on the path taken because energy is lost that can't be recovered. Friction loses energy as heat. It is directly influenced by the Normal force which depends on the angle of incline and the distance traveled.

W=PE+KE really answers any doubt.

 

[seraphkz]

Wait...What are you talking about?

Work = F*d

So, if a thing is just sitting on a table and not working, it's experiencing a gravitational force, but that gravitational force is not doing any work because the thing is not moving.

Now if something is falling (with an angle or without), it is traveling a distance, therefore there is work done by the gravity....

Is my understanding wrong?

 

[Pons Asinorum]

Wait...What are you talking about?

Work = F*d

So, if a thing is just sitting on a table and not working, it's experiencing a gravitational force, but that gravitational force is not doing any work because the thing is not moving.

Now if something is falling (with an angle or without), it is traveling a distance, therefore there is work done by the gravity....

Is my understanding wrong?

It all depends on whether gravity is the only force acting, or if there are other, non-conservative forces acting. If something is falling (with angle or without) in the absence or air resistance (friction), there is no work being done because delta(KE) will always equal delta(PE) regardless of how far it travels. It's all about properly setting up the system to answer the question that MCAT is asking. This is good advice:

EK says, then, if a question asks for the work done by gravity, consider gravity to be outside the system.

 

[seraphkz]

Right I see. But isn't that the work done by the system. There is still work done by the gravitational force. It's just canceled out by the deltaKE when considering the whole system.

 

[Rabolisk]

The work done by the gravity only depends on the change in height and mass of the object. This is true whether an object falls in vacuum, in air, or if an external force lowers it.

 

[Pons Asinorum]

The work done by the gravity only depends on the change in height and mass of the object. This is true whether an object falls in vacuum, in air, or if an external force lowers it.

That's false, but for the narrow argument of an MCAT question that is phrased as "what is the work done by gravity," consider it to be true.

 

[crazyasian]

The work done by the gravity only depends on the change in height and mass of the object. This is true whether an object falls in vacuum, in air, or if an external force lowers it.

That makes sense.

 

[dougkaye]

pretty much all of these MCAT problems can be solved by applying the following formula, which folks have already mentioned:

W = F(net) * dcos(theta) = dK

and then remember that dK = -dU, and therefore |dK| = |dU|, where d = delta. This allows us to solve pretty much all of the sliding block, falling object questions at least in theory.

If there is friction or any other forces (such as a second applied force pushing in the opposite direction of another applied force), make sure you use the NET force (which can include friction), and then apply the same formula above.

 

[PeterPesto]

Work done by gravity depends if you use the work-energy principle or conservation of mechanical energy.

Under conservation of mechanical energy, the system is the object and the earth. Because gravity is a conservative force, it does negative work on the way up and positive work on the way down. The net work done by gravity = 0.

Under work-energy, your system is the object. Thus, gravity can do work on an object if it increases its kinetic energy.

Anyone correct me if I'm wrong.

 

[TXKnight]

I read a few other posts and I just wanna make sure I'm understanding this concept correctly. So work CAN be done by gravity as long as gravity is not acting perpendicular to the object. Thus if an object slides down an incline gravity does force on the object because it is acting at an angle that is not purplendicular. So assuming an incline, here's where I get confused. If I were to calculate work done, would I use the vertical change in height (mgh) or would i use the distance traveled down the incline (which would be larger than the vertical change)? Thanks for any help.

W done by gravity think Work-Energy theorem. Regardless of path taken Work done by gravity is mgh....Work done by other forces then might depend on angle and what not, it depends if its conservative or non-conservative.