Work+conservation of energy

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In summary, the conversation discusses the concept of work done when sliding a block on a floor with friction. It is mentioned that the points A, B, C, and D make a rectangle and the block is moved at a constant speed. It is then questioned if the work done by the closed loop is zero. It is explained that the individual forces' work is not zero, but the net force is zero, resulting in no change in kinetic energy. It is also mentioned that for a frictionless surface, energy is conserved while for a friction surface, work must be done by an applied force and goes to heat. A formula for calculating work done in this scenario is also provided.
  • #1
UrbanXrisis
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Sliding a block on a floor that has friction. Going from points A to B to C to D then back to A. points ABCD make a rectangle. Moving the block at a constant speed. What is the work done by the closed loop?

Since speed is a constant, then there isn't a change in KE, so then work done has to be zero right?

What if I did this on a frictionless table? Would the work done be zero as well?
 
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  • #2
UrbanXrisis said:
Sliding a block on a floor that has friction. Going from points A to B to C to D then back to A. points ABCD make a rectangle. Moving the block at a constant speed. What is the work done by the closed loop?
The work done by what force? To move that block a force must be applied to overcome friction. That force does work which ends up increasing the internal energy of the block + floor. (They heat up.)

Since speed is a constant, then there isn't a change in KE, so then work done has to be zero right?
The work-energy theorem says that the work done by the net force on an object will equal the change in KE. But the net force is zero, thus no change in KE.

What if I did this on a frictionless table? Would the work done be zero as well?
If you applied the same force, the block would accelerate and the work done would equal the change in KE of the block. To move the block at a constant speed requires no force (ignoring those corners!).
 
  • #3
"Since speed is a constant, then there isn't a change in KE, so then work done has to be zero right?"

The work of the NET force is zero, but this doesn't mean that the work from the individual forces (friction and pushing force) are each zero.
The individual forces' work is most certainly not zero, only their SUM.
 
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  • #4
"Since speed is a constant, then there isn't a change in KE, so then work done has to be zero right?"

Work isn't zero for individual forces just like urban said. On a frictional surface conservative energy isn't conserved thus W(neoconservative)=change(mgh)+0 KE...The force is neoconservative in this case I think thus W(nc of friction)=mgh if you have those variables of h and m.
 
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  • #5
Doc Al said:
If you applied the same force, the block would accelerate and the work done would equal the change in KE of the block. To move the block at a constant speed requires no force (ignoring those corners!).

Does this mean there is no change in KE?
 
  • #6
UrbanXrisis said:
Does this mean there is no change in KE?
If the net force is zero, no work is done and the KE does not change.
 
  • #7
so there is no change in KE for frictionless or friction surfaces?

How do you know if energy is conserved or not?
 
  • #8
UrbanXrisis said:
so there is no change in KE for frictionless or friction surfaces?
If the object moves at constant speed how can the KE change? :smile:

How do you know if energy is conserved or not?
I'm not sure what you mean:
(1) For the frictionless surface, no energy is added (no work is done by an outside force) so energy is certainly conserved.
(2) For the friction surface, work must be done by an applied force. That work goes to heat. But no net work is done on the object, so its mechanical energy is conserved.
 
  • #9
Hey, couldn't you just use this formula:
[tex]
W = ( \mu mg \cdot \vec{F}_\textrm{normal} ) ( AB + BC + CD + DA )
[/tex]
This depends of course on what information you've got.
Edit: made a correction to the formula
 
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  • #10
The formula [tex]W = ( \mu mg \cdot \vec{F}_\textrm{normal} ) ( AB + BC + CD + DA )[/tex]does indeed work. But remember that work equals
force times displacement, not distance. The displacement (net distance traveled) in this case is zero, so the work is zero. If you want to show this, make CD and DA negative distances.
 
  • #11
OK, I see. Thanks for clearing that up.
 
  • #12
gschjetne said:
Hey, couldn't you just use this formula:
[tex]
W = ( \mu mg \cdot \vec{F}_\textrm{normal} ) ( AB + BC + CD + DA )
[/tex]
This depends of course on what information you've got.
Edit: made a correction to the formula
What's this formula supposed to be? What do you mean by [itex]\mu mg \cdot \vec{F}_\textrm{normal}[/itex] ?
 

1. What is the concept of work in relation to conservation of energy?

Work is defined as the transfer of energy from one system to another. In the context of conservation of energy, work refers to the amount of energy used or transferred in order to change the state or position of an object or system.

2. How does the principle of conservation of energy apply to work?

The principle of conservation of energy states that energy cannot be created or destroyed, but can only be transformed from one form to another. This means that the total amount of energy in a closed system remains constant. In the case of work, the energy used to perform work is transformed into other forms, such as kinetic or potential energy, but the total amount of energy within the system remains the same.

3. Can work be done without any energy input?

No, work cannot be done without any energy input. According to the first law of thermodynamics, energy cannot be created or destroyed, only transferred. Therefore, in order to perform work, some form of energy must be used or transferred from one system to another.

4. How is the concept of work related to the law of conservation of energy?

The concept of work is directly related to the law of conservation of energy. Work is a form of energy, and according to the law of conservation of energy, the total amount of energy in a closed system remains constant. This means that the energy used to perform work must come from another form of energy within the system, and the total amount of energy within the system will not change.

5. What are some real-world examples of the conservation of energy in relation to work?

Some real-world examples of the conservation of energy in relation to work include the use of a pendulum, where potential energy is converted into kinetic energy and then back into potential energy, and the use of a battery to power a light bulb, where chemical energy is converted into electrical energy and then into light and heat energy. Additionally, the law of conservation of energy applies to everyday tasks such as lifting objects, where the energy used to lift the object is transformed into potential energy, and using a car, where the chemical energy in fuel is converted into kinetic energy to move the car.

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