- #1
Yashbhatt
- 348
- 13
Why is it said that no work is done when I lift a box and walk?
Yashbhatt said:Why is it said that no work is done when I lift a box and walk?
Yashbhatt said:Why is it said that no work is done when I lift a box and walk?
Orodruin said:This is true only when you accelerate. If you are walking at constant speed, the only force you apply to the box is vertical.
Now, in a walk, the box might go up and down a bit, resulting in work actually being done alternatingly on the box and by the box - netting zero work done on the box.
This does not mean that your body is not doing work to generate the required force. However, this work ends up as internal heat rather than providing work on the box. Muscles are not the most efficient source of static forces ...
sophiecentaur said:You just need to realize that "Work done on" (an object) is very strictly defined as the Force exerts on the object times the distance the object moves in the direction of the Force. It will always be different from "Work done by" (you), because practical things are not 100% efficient. If you stand and hold a massive weight for a day and it doesn't move up or down, you will have used a lot of energy but your efficiency will have been 0%.
There is no paradox and no one got anything wrong - it's just where real life and theoretical definitions do not happen to coincide - at first sight.
You apply force to the ground, not to the box. (Other than supporting its weight.)Yashbhatt said:No, every time I walk I have to apply some force.
But you are not exerting a force on the box in the direction of its displacement.When I am moving, I am being displaced. So, the box is also displaced.
Doc Al said:You apply force to the ground, not to the box. (Other than supporting its weight.)
But you are not exerting a force on the box in the direction of its displacement.
Friction is not acting on the box. Friction acts on the ground, which isn't moving.Yashbhatt said:But friction is.
Doc Al said:Friction is not acting on the box. Friction acts on the ground, which isn't moving.
Yashbhatt said:No, every time I walk I have to apply some force.
When I am moving, I am being displaced. So, the box is also displaced.
Yashbhatt said:But friction is.
sophiecentaur said:You are not getting the point that the definition of Work is very strict. You are applying it outside of its proper context. If you move the box up and down, the total displacement is still zero and the Work is zero. If you accelerate it and then slow it down, there is still no net work done 'on' it because it will return any energy it has. And remember - the displacement has to be in the direction that any force is applied, for work to be 'done on'.
You (and many other people, aamof) seem to want to disprove something or find a loophole in this work thing. There are no loopholes - if you stick to the strict definitions. OK, that may not make sense to you but, if you follow Physics further along the line, you will see the point of doing things the Physics way. When you think you have found something which doesn't quite add up - then you need to assume you are probably wrong and find out how, rather than getting cross about silly Physics.
Yashbhatt said:I am not trying ti disprove anything. I am just trying to understand what I am unable to.
Yes, friction acts on you and, yes, that is why you can move. But that friction does no work on you, because the point of application is stationary. And you do no work on the box, as you exert no force in the direction of its displacement (when walking at a uniform velocity).Yashbhatt said:Friction acts on me. That is why I can move.
Drakkith said:Once you are walking at a steady pace, the box will continue to move since it requires a force to stop it (we're ignoring air friction). At this point you are only applying a force upwards against the box to keep it in the air. Since the upwards force you provide is exactly equal to the force of gravity, the box stays at the same height. Since work is force acting through a distance, no work is done in either case.
W = F x D
Forward motion: Zero force applied in the direction of motion, so F x D equals zero, no matter how far the box moves as long as it moves at a steady velocity.
Upward force: The upwards force is counterbalanced by gravity, so the distance the force is applied over is zero, so F x D still equals zero. (Forward motion doesn't count here because the upwards force is perpendicular to the direction of motion)
The key here is to understand that we are talking about work done on the box, not on your body or any part of your body. Obviously it requires work to walk forward, but we don't care about that work because it isn't be done on the box, only on your body. We're also ignoring some real life issues such as not realistically being able to keep the box perfectly steady in your hands and the fact that your body isn't a perfectly efficient machine and has to expend energy (and thus perform work) just to keep the box in the air. Just remember that it's a learning tool, nothing more.
If it helps, replace the person with a frictionless surface. Once the box is accelerated, it requires no work for the box to remain in motion.
sophiecentaur said:All this has nothing to do with the definition of work done on the box. The box doesn't care how you are producing the forces on it or what forces happen to be exerted at any particular time. All that matters, for the purpose of calculating the work done on the box is F.Δx (dot product). If Δx is zero then no work has been done. Why do you keep arguing when all you need to do is to accept the definition and see how it applies here? You will not 'understand' how the definition applies if you do not apply it strictly.
'Work done on' is a tiny subset of the set 'Work'. Arguments involving Work may not apply to 'Work done on'.
how can force be applied just on my body and not on the box
You can always consider you and the box to be the same system, even when you are not touching the box. You can always consider you and the box to be different systems, even when you are not touching the box. The boundary of your system is completely arbitrary. You can even consider half of the box to be a system and the other half of the box to be a different system.Yashbhatt said:I just don't understand the thing that how can force be applied just on my body and not on the box when the box and me are the same system?
And so is the choice of reference frame, which also affects the work done by a certain force. That's another reason why one shouldn't try to understand 'work done' as some objective statement about the situation.DaleSpam said:The boundary of your system is completely arbitrary.
Yashbhatt said:But consider this. As soon as I lift the box, the box and me act as a system and so if force is applied to my body, force is also applied to the box.
And also with the box in my hand I will have to apply a greater force to move forward.
Drakkith said:You can consider you and the box as a single system, but you do not have to. I can easily consider you and the box to be different systems, and in this example it's a requirement if you want to understand. For example, if I drop a hat on your head as you are walking with the box, the hat applies a downward force on you, but not on the box.
When first accelerating, yes. Once you are walking you do not. Remember that we are assuming an idealized, non-realistic version where you can keep the box perfectly steady in your hands, regardless of what actually happens when you walk.
Honestly, you are missing the point here which is that once the box is moving at a steady velocity, no more work is performed on it. It doesn't matter how the box got up to speed or what is holding it up, it's moving and therefore will remain moving until a force is applied to stop it. Your body simply serves as an inefficient machine to keep the box lifted up off the ground.
In order for us to help you, you may need to be more specific about what forces you are referring to because right now people are having to guess.Yashbhatt said:I just don't understand the thing that how can force be applied just on my body and not on the box when the box and me are the same system?
Lsos said:You think you're doing work when walking, but all you're really doing is swinging your heavy legs back and forth as you're falling forward and catching yourself. Not a very efficient way of doing things, when compared to a wheel for example.
russ_watters said:In order for us to help you, you may need to be more specific about what forces you are referring to because right now people are having to guess.
One possibility is that when you walk you do indeed apply forward force to the box...and then you apply backward force to stop it. Both with each step and for the initial acceleration and final deceleration. But these forces (or their works) sum to zero.
If box and your body are considered one object in your analysis, then there is no object named "box" or forces acting on it. Make up your mind how you want to cut the scenario into objects, before you analyze forces and work done.Yashbhatt said:force is acting on the box as my body and box are the same system
Yashbhatt said:Why is it said that no work is done when I lift a box and walk?
Fair enough: my answer applied equally to you and the box: The forces/works sum to zero. If you are under the impression that walking forward involves only forces to push you forward, you are mistaken. Every step except the first and last starts with a push forward and ends with an equal push backwards.Yashbhatt said:My question is that as I walk, friction is the force that helps me in moving forward. So, I can say that friction causes the displacement of my body. So, if force is acting on my body, force is acting on the box as my body and box are the same system.
Yashbhatt said:My question is that as I walk, friction is the force that helps me in moving forward. So, I can say that friction causes the displacement of my body. So, if force is acting on my body, force is acting on the box as my body and box are the same system.
You cannot do an analysis this way. If the system you are analyzing is the box+you then all you can say is that the friction force is acting on the system. You cannot consider the system of the box+you and then make any conclusions about just the box.Yashbhatt said:So, if force is acting on my body, force is acting on the box as my body and box are the same system.
Yashbhatt said:Okay. Now, I get it. It was a moment for me when I made the silliest of mistakes. Sorry of all the trouble especially to @sophiecentaur who got a bit angry.
In physics, work is defined as the amount of force applied to an object multiplied by the distance the object moves in the direction of that force. It is a measure of the energy transferred to an object when a force is applied to it.
Lifting a box requires work because you are applying a force (in this case, the force of gravity) to the box and moving it a certain distance in the direction of that force. This requires energy to be transferred to the box, which is the definition of work in physics.
The amount of work required to lift a box depends on the weight of the box (which is determined by its mass and the force of gravity), the distance the box is lifted, and the angle at which the box is lifted. The steeper the angle, the more work is required to lift the box.
The mass of the box directly affects the amount of work required to lift it. The greater the mass of the box, the more force is needed to lift it, and therefore, the more work is required. This is because the force of gravity is directly proportional to an object's mass.
In addition to the work required to lift the box against the force of gravity, there are other types of work involved. These include the work done by your muscles to apply the force to the box, the work done by friction as the box is lifted, and the work done against air resistance as the box moves through the air.