Trying to understand Work and Energy

In summary: This makes sense since the friction is directed opposite the direction the ground is moving.In summary, work is a transfer of energy that occurs when a force is applied to an object causing it to move. When the force is applied in the opposite direction of the object's displacement, work is done by the force and energy is removed from the object. This can be seen when a box is placed on a table and then falls, gaining kinetic energy as it moves downward. The work done by the force of gravity on the box removes energy from it. However, work can also be done by the box on the ground as it falls, transferring energy to the ground. This is an example of how work done by a system, such as the falling
  • #1
Jazz
103
5
I understand that:

1) For work to be done there must be a force (or a component of it) exerted upon an object, causing it to move. Work done on a system puts energy into it.

2) If the force is applied in the opposite direction to that of the object’s displacement, then the work done by that force removes energy from it.

3) If work is done by a system, energy is removed from it as well.

But here is where I get confused:

If I put a box on a table, it gains PE. If I let it fall from that height, it will lose its PE. My question is, when falling what force in the upward direction is being applied to the box, removing energy from it? On what is my system (the box) doing work?

My textbook asks for giving an example where the work done by the system removes energy from it. I've been thinking in a car toy after having pushed it. It’s easy to think about it as friction doing work on the car with a direction opposite to its motion, and hence removing KE from it; but if my system is the car toy, on what is this doing work? On the ground?

And ultimately (and my today’s silly question), Can a system do work on itself? I guess it can’t, because Newton’s Third Law would be violated. If this conclusion isn’t wrong, then what about the act of eating? A person eating is putting energy on themselves, but how is this interpreted in the context of work?

Thanks !
 
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  • #2
When the box on your table falls, a downward force is applied to your box and your box moves downward. As a result work was done on your box and it gains kinetic energy. Associate work not with potential energy or energy in general, but rather specifically with kinetic energy. As an aside, your falling box is also doing work on the Earth while it is falling.
 
  • #3
DocZaius said:
When the box on your table falls, a downward force is applied to your box and your box moves downward. As a result work was done on your box and it gains kinetic energy. Associate work not with potential energy or energy in general, but rather specifically with kinetic energy. As an aside, your falling box is also doing work on the Earth while it is falling.

Aah, true.

About the car toy, the only work done is by friction against its motion, right? Can there be an explanation where the car is the thing doing work on something (like the ground) removing energy from it?
 
  • #4
Kinetic energy is not an invariant (aka absolute) quantity. It depends on your choice of reference frame. If you choose a reference frame in which the ground is moving and in which the car is [initially] at rest then friction between the two will remove energy from the ground while imparting energy to the car.

There is an invariant quantity here -- the difference between the total starting kinetic energy and the total ending kinetic energy does not depend on the choice of reference frame. This lost kinetic energy appears as heat due to the friction.

Note that when the car slows the ground down, the amount of work it is doing on the ground is negative.
 
  • #5


Dear reader,

Thank you for your interest in understanding work and energy. I can provide you with a response to your questions and attempt to clarify any confusion you may have.

First, it is important to understand that work and energy are closely related concepts. Work is the transfer of energy from one object to another, and energy is the ability to do work. In your first statement, you correctly state that work is done when a force is exerted on an object, causing it to move. This is because the force is transferring energy to the object, giving it the ability to do work.

Now, let's address your confusion about the box on the table. When you lift the box onto the table, you are exerting a force on the box in the upward direction. This force is doing work on the box, transferring energy to it and increasing its potential energy (PE). When the box falls from the table, the force of gravity is acting on it in the downward direction, causing it to lose PE. This is because the force is now doing work on the box in the opposite direction, removing energy from it.

To answer your question about what force is acting on the box in the upward direction when it falls, it is the normal force from the table. This force is exerted on the box by the surface of the table and is equal in magnitude and opposite in direction to the force of gravity, balancing it out. In this case, the normal force is doing work on the box in the upward direction, removing energy from it and decreasing its PE.

In your example of the car toy, you are correct in thinking that friction is doing work on the car in the opposite direction of its motion, removing energy from it. In this case, the system is the car and the ground is the external object on which the car is doing work. This is an example of the work done by the system removing energy from it.

Your question about a system doing work on itself is an interesting one. In the context of physics, a system is defined as a collection of objects or particles that are being studied. In this case, the system cannot do work on itself because it is not considered an external object. However, in the context of biology and the act of eating, the body can be considered a system. In this case, the muscles and organs in the body are doing work on the food, breaking it down and transferring energy to the body. So,
 

What is work?

Work is defined as the product of force and displacement. In simpler terms, it is the transfer of energy from one object to another, resulting in displacement or motion.

What is energy?

Energy is the ability to do work. It is a fundamental concept in physics and is measured in joules. Energy exists in many forms, such as kinetic, potential, thermal, and electromagnetic.

How are work and energy related?

Work and energy are directly related. Work is the transfer of energy, and energy is what is needed to do work. In other words, work done on an object results in a change in its energy. This relationship is described by the work-energy theorem.

What are the different types of work?

There are three types of work: positive, negative, and zero. Positive work is done when a force is applied in the same direction as the displacement. Negative work is done when a force is applied in the opposite direction of displacement. Zero work is done when there is no displacement, or when the force and displacement are perpendicular.

How is work calculated?

Work is calculated using the equation W = Fd, where W is work, F is force, and d is displacement. The unit of work is joules, which can also be written as newton-meters. It is important to note that work is a scalar quantity and does not have a direction.

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