Work done by a constant Force: questions

In summary, when lifting an object, the system can be defined as the object alone or the object plus the Earth. When considering the object alone, the work done by gravity is negative, transferring energy from the object to the gravitational field. However, when considering the object and the Earth as a system, the work done by gravity is already accounted for in the potential energy of the system. Therefore, lifting the object results in a transfer of energy from the person to the object and no change in the kinetic energy of the system. It is important to carefully define the system when dealing with gravity to avoid confusion.
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SebastianRM
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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.
 
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SebastianRM said:
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.
 
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SebastianRM said:
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.
 
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Dale said:
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 :wink: ).

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.
 
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FAQ: Work done by a constant Force: questions

What is work done by a constant force?

The work done by a constant force is the product of the magnitude of the force and the displacement of the object in the direction of the force. It is a measure of the energy transferred to or from an object by the force.

How is work calculated?

Work is calculated by multiplying the magnitude of the force by the distance the object moves in the direction of the force. The formula for work is W = Fd, where W is work, F is force, and d is displacement.

Can work be negative?

Yes, work can be negative if the force and displacement are in opposite directions. This indicates that the force is doing work against the motion of the object, resulting in a decrease in the object's energy.

What is the unit of work?

The unit of work is joule (J), which is equivalent to a newton-meter (N*m). This unit represents the amount of work done when a force of one newton is applied over a distance of one meter in the direction of the force.

How does work relate to power?

Work and power are related through the formula P = W/t, where P is power, W is work, and t is time. Power measures the rate at which work is done, or the amount of work done per unit of time.

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