# Work-Energy Theorem: Is Lifting a Rigid Body Really 0 Work?

• PeterPeter
In summary, the conversation discusses the relationship between work done by a rigid body and the change in kinetic energy. It also brings up the example of lifting a rigid body vertically and the confusion surrounding the net work done in this situation. The work energy theorem is mentioned, as well as the concept of net force and different systems on which work is done. The conversation ultimately questions the validity of the statement that net work is equal to zero when there is no change in kinetic energy.
PeterPeter
I read on the Internet that the work done by a (rigid) body = the change in Kinetic energy.

What if I lift a rigid body slowly and vertically by 1 meter above the Earth's surface so that the initial velocity = final velocity =0?

According to the Work Energy theorem as stated on many sites on the Internet (you can google these for yourself) the net work done = 0 because the change in KE =0.

Yet I have done work against gravity. Clearly something is wrong! It's all very confusing!

jfmcghee
PeterPeter said:
Yet I have done work against gravity.
You have done positive work, gravity has done negative work, the net work is zero. Alternatively, you have done positive work that went into gravitational potential energy.

PeterPeter said:
the net work done = 0 because the change in KE =0.

Yet I have done work against gravity.
Is the work you did equal to the net work?

scottdave
It is helpful (necessary) to specify the system on which you are doing work. You can't just say "I did 10 J of work," it has to be "I did 10 J of work on the object." Then like @scottdave said, you have to take into account all of the forces doing work on the object.

When you lifted the object which system were you doing work on? The object? How about the object-Earth system?

## 1. What is the Work-Energy Theorem?

The Work-Energy Theorem is a fundamental principle in physics that states that the work done on an object is equal to the change in its kinetic energy.

## 2. How does the Work-Energy Theorem apply to lifting a rigid body?

When lifting a rigid body, the work done by the person lifting is equal to the change in the body's potential energy. This is because the person is applying a force to the body, causing it to move against the force of gravity, and therefore increasing its potential energy.

## 3. Why is the work done when lifting a rigid body considered to be 0?

According to the Work-Energy Theorem, the work done is equal to the change in kinetic energy. In the case of lifting a rigid body, the body does not have any initial or final velocity, therefore its change in kinetic energy is 0. This results in the work done being 0.

## 4. Does this mean that no energy is being used when lifting a rigid body?

No, energy is still being used when lifting a rigid body. The person lifting the body is using their muscles to apply a force, which requires energy. However, the work done is 0 because there is no change in the body's kinetic energy.

## 5. Are there any real-life applications of the Work-Energy Theorem in lifting objects?

Yes, the Work-Energy Theorem is commonly used in mechanical engineering and construction to calculate the amount of work needed to lift heavy objects. It is also used in sports and fitness to understand the energy requirements for lifting weights and performing other physical activities.

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