Work done by a constant Force: questions

[SebastianRM]

I was reviewing physics stuff and I've managed to confuse myself.
I was reading about work (principles of ohysics, serway&jewtt):
-work is an energy transfer
-if work is done on a system and W is positive, energy is transferred to the system; if W us negative, energy is transferred from the system
So it makes sense for me when I lift an object to think I am doing positive work, thus it's potential energy increases. But how do I make sense of the negative work by gravity on the system.

 

[Dale]

But how do I make sense of the negative work by gravity on the system.

You have to be VERY careful about what you define as "the system". You can do the analysis with any definition of "the system" but you have to be consistent.

Scenario: I lift a rock from the ground to a higher location.

The system = the rock + earth: There are two forces on the rock, the contact force from my hand and the gravitational force from the gravitational field of the earth. My hand does positive work, transferring energy from me to the system, and gravity is an internal force within the system so it does not do work on the system. The role of gravity is not to do work on the system but rather it simply stores the energy as potential energy within the system rather than kinetic energy.

The system = the rock: There are two forces on the rock, the contact force from my hand and the gravitational force from the gravitational field of the earth. My hand does positive work, transferring energy from me to the system, and gravity does negative work, transferring energy from the system to the gravitational field.

Unfortunately, we often speak of "the rock's potential energy" but that is sloppy and misleading. The potential energy is a property of the rock + Earth system not the rock itself. Usually this is OK, but if you do a careful analysis you can get confused if you don't recognize this.

 

[Doc Al]

So it makes sense for me when I lift an object to think I am doing positive work, thus it's potential energy increases. But how do I make sense of the negative work by gravity on the system.

Careful. When you use gravitational potential energy you are already accounting for the effects of gravity. So you do not ask about the work done by gravity, as you've already included that. (An increase in gravitational PE is equivalent to gravity doing negative work.)

You could avoid using potential energy and treat gravity as just a force. In which case you do positive work and gravity does negative work (on the rock) and the net is zero (assuming you lift with a force equal to the weight), thus no change in kinetic energy.

But take heed of @Dale 's advice regarding the definition of "system" when dealing with gravity.

 

[cianfa72]

Scenario: I lift a rock from the ground to a higher location.

The system = the rock: There are two forces on the rock, the contact force from my hand and the gravitational force from the gravitational field of the earth. My hand does positive work, transferring energy from me to the system, and gravity does negative work, transferring energy from the system to the gravitational field.

Unfortunately, we often speak of "the rock's potential energy" but that is sloppy and misleading. The potential energy is a property of the rock + Earth system not the rock itself. Usually this is OK, but if you do a careful analysis you can get confused if you don't recognize this.

Yes, I believe this is a key point often source of confusion (at least for me ).

In your example the Earth's gravitational field is an "external" system just like your hand and the "external" gravity force basically transfers energy from the "system rock" to the external "gravitational field system".

It is worth noting that the "system rock" does not change energy at all when it is lifted since the 'net external work' is zero.