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We have all heard people saying that to lift an object of mass $m$, you have to apply a force $F$ equal to its weight $mg$. But isn't it getting the force equal to its weight from the surface to which it is attached to (normal force). Why it is willing to change that equilibrium state by getting the same force from us as from the surface? (Consider the situation devoid of any resistance) . I think we must be applying slightly more force to it in order to move it even with constant velocity at least at the start and balancing the force of gravity afterwards.

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    $\begingroup$ The ancient Egyptians [while building the pyramids] used the weight of an object to lift something of far greater weight. $\endgroup$ – Turk Hill Oct 28 at 20:06
  • $\begingroup$ @Turk How? Also can you attach any proof for your claim. $\endgroup$ – The Last Airbender Oct 29 at 6:09
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    $\begingroup$ It's called a lever :) Of course, the force acting on the object to be lifted still needs to be exactly equal to the weight of that object - otherwise it will accelerate (up or down). Lifting usually means applying a larger force for a while (to get the object to move), and then maintaining constant force equal to its weight (to either keep moving it upwards or keep it at a given "height" after a short drop in force to stop it). $\endgroup$ – Luaan Oct 29 at 8:20
  • $\begingroup$ @Ramanujan_π I saw it in a video somewhere. When I find the source I will gladly let you know. $\endgroup$ – Turk Hill Oct 29 at 15:43

13 Answers 13

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There are two points to be clarified here.

  • The normal reaction force from the surface is a self-adjusting force. In particular, it can take any value so as to prevent the object in contact from penetrating. So, if an object resting on a surface has a weight $w$ then the normal reaction force would be $w$ in the upward direction. Now, if you apply an external upward force on the object (with your hand, say) of a magnitude $w/2$ then the normal reaction force from the surface would change its value to $w/2$. Now, if you apply an external force of a magnitude $w$ in the upward direction then the normal reaction force from the surface would reduce to zero.
  • However, as you correctly notice, when the upward external force is exactly the same as the weight in magnitude, the object is still in perfect equilibrium. And since the initial velocity of it was zero, its velocity would still remain zero because equilibrium means no acceleration. So, there would be no movement. So, in order to actually lift the object, you do need to provide an upward force which is at least slightly greater than the weight of the object. Once you apply such a force even for a tiny amount of time, the object would pick up an upward velocity because it would have been subjected to an upward acceleration for that tiny amount of time. Once this is accomplished, you can reduce the magnitude of the upward force to be exactly the same as the magnitude of weight and the object will continue to move in the upward direction, in equilibrium, but now, with a constant velocity (that it picked up during that tiny amount of time of acceleration).
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    $\begingroup$ Not that I disagree with your answer (the second bullet is essentially the same as my answer), but technically one doesn't need to apply a slightly greater upward force. All that is needed is to apply a sufficient net downward force before reaching the desired height so that the velocity is zero at the desired height. Bottom line: as long as the net change in kinetic energy is zero between the start and end points, the net work will be zero and the object will strictly have gravitational potential energy at the desired height. $\endgroup$ – Bob D Oct 27 at 16:42
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    $\begingroup$ @BobD Probably I misunderstood your point but I am only addressing how to lift the object. In order to lift the object who is initially resting on the surface, you need to apply a slightly larger force for at least a tiny amount of time. As you say, you already say this in your answer. I think what you are pointing out is the mechanism with which you'd halt the object at a certain height without doing any net work on it. That I agree that we'd also need to apply a net downward force for the appropriately tiny fraction of time in the last stage of motion. $\endgroup$ – Dvij Mankad Oct 27 at 17:25
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    $\begingroup$ @BobD Oh yes, absolutely. I should emphasize that. I only meant at least a small net upward force for at least a brief amount of time. Yes, but it is nice to point out that no irreversibilities creep in even if the net force is large and for a good amount of time. $\endgroup$ – Dvij Mankad Oct 27 at 17:37
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    $\begingroup$ In effect, gravity has taken the energy you supplied to the object and stored it as gravitational potential energy in the earth-object system. The work you did on the object is passed along to the earth-object system. The key point to understand is an object by itself does not “possess” gravitational potential energy. Gravitational potential energy is the energy of the earth-object system. $\endgroup$ – Bob D Oct 28 at 15:09
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    $\begingroup$ @DvijMankad I beg to differ. The first bullet was relevant and I neglected to address it. Therefore, you deserve the "accept" as well as all the up votes! $\endgroup$ – Bob D Oct 29 at 21:29
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In real situations, when a mass is sitting on a table, its weight will slightly flex the table. So lifting with a force that equals the weight will remove normal force pressure from the surface, the surface will flex back to it's former position giving the slight increase in net upward force to start acceleration. This can be visualized easier if you imagine the mass resting on a spring. So in most real situations applying a force equal to it's weight would lift it.

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  • $\begingroup$ I agree in a perfect situation where an equal and opposite force (to the weight) could be immediately applied at once, then there would be some force from the surface lasting only long enough for the surface to "relax". During this relaxation, there would then be a non-zero, upward, very-short lived impulse and if neither the applied force nor the force of gravity changed at all, then the object would indeed continue to move at a constant speed upward. I estimate a 1kg object compressing a surface "spring" only 1 um would have about 3 mm/s velocity upward... not negligible. $\endgroup$ – C Perkins Oct 27 at 19:04
  • $\begingroup$ But I disagree in a "real" situation an equal and opposite force could not be applied exactly and instantaneously. Either the force would overshoot the weight, even momentarily, to induce its own acceleration that it would be difficult to distinguish between its influence and the normal force... OR to apply exactly the correct amount would take at least some time (not instantaneous) to avoid overshooting the weight the force is ramped up. Over the time it takes to ramp up the force to the correct value, it would be relaxing the normal force enough that any acceleration would be negligible. $\endgroup$ – C Perkins Oct 27 at 19:13
  • $\begingroup$ @C Perkins It would not have to be instantaneous, relaxation of stresses from normal force would not be instant. $\endgroup$ – Adrian Howard Oct 27 at 21:52
  • $\begingroup$ But what I mean is that if the applied force (the force that is supposed to counteract the gravitation force of the object) where applied gradually (not immediately equal to the weight), then the relaxation of the normal force could go slowly enough that by the time the other forces matched exactly there would be zero normal force, so there would then be no additional force to accelerate the object and it would simply remain next to the surface. $\endgroup$ – C Perkins Oct 27 at 23:37
  • $\begingroup$ More specifically I was addressing the "most real situations". In order to actually cause lift without exceeding the weight, the applied counter-force would have to be applied quickly enough at exactly the right magnitude (but without exceeding the weight threshold) to cause lift due to the normal force as you described. That is actually very unrealistic, so most real situations would result in the applied force varying too much, thus resulting in either no acceleration/movement upward or it would be excessive and go far beyond the equality point specifically mentioned in the question. $\endgroup$ – C Perkins Oct 27 at 23:40
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You are correct that there must initially be a net upward force, no matter how small and how brief, to get the object going. But in addition to balancing the forces immediately after to achieve constant velocity there must be a net downward force just before reaching the height such that the object will come to rest, if it is to possess ONLY gravitational potential energy at that height. Otherwise the object will possess both gravitational potential energy and kinetic energy when it reaches that height.

Hope this helps

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    $\begingroup$ What height? There is no mention of height in the problem and I perceive that the problem implies constant gravitational force so that it doesn't change with height. Saying "just before reaching the height" doesn't really match the question and is a bit confusing. And if you simply mean the nearly infinitesimal height from the surface (as the normal force relaxes), well to reach that height it would have had to experience a net upward force so the explanation about a "net downward force" doesn't make sense either. $\endgroup$ – C Perkins Oct 27 at 19:29
  • $\begingroup$ @CPerkins I know there was no mention of height. The OP reference to "We have all heard people saying that to lift an object of mass m, you have to apply a force equal to its weight" refers to the statements frequently made that in order for an object to gain only gravitational potential energy, at any arbitrary height, the net work done on the object has to be zero so that there is no change in kinetic energy. For there go be no net change in kinetic energy, the object must start at rest and end at rest. If it needs a little acceleration to get started, it needs an equal deceleration to stop. $\endgroup$ – Bob D Oct 27 at 20:01
  • $\begingroup$ @CPerkins In order to decelerate and come to rest before reaching the end point, there needs to be a net downward force on the object. That's what I meant by a net downward force. You may find that confusing, but most others don't. $\endgroup$ – Bob D Oct 27 at 20:03
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    $\begingroup$ If describing the overall path and motion which you describe, then sure that all sounds right. But the question sets up a context of the forces relevant just as an object is lifted off a surface. Your answer doesn't even discuss the surface or normal forces, so once again any significant height or later deceleration is irrelevant to the actual question. $\endgroup$ – C Perkins Oct 27 at 20:25
  • $\begingroup$ @CPerkins The question, as I interpret it, is how can we lift an object off a surface unless we apply a net upward force. And the answer is you can't. I simply put in the context of the issue many people have with the requirement of no net force in order to attain only gravitational potential energy. That's how I wanted to answer. You are certainly entitled to your own opinion. I leave it at that. $\endgroup$ – Bob D Oct 27 at 20:31
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All of the above answers make a false assumption. They assume that the object is not currently moving vertically. This is not given in the question. So the correct answer should be it depends on whether the object is moving vertically or not. If the object had already been propelled upwards and we "caught" it midflight the "very small force for a very small time in order to overcome equilibrium argument" doesn't hold.

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  • $\begingroup$ I do appreciate the pedantry of this answer. small details are important sometimes $\endgroup$ – Alex Robinson Oct 28 at 16:16
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On the surface of a planet with an atmosphere, you have two things helping you: The buoyancy of the object (basically the weight of the air that it replaces), and assuming the planet is rotating and you are not at one of the poles, the centrifugal power.

So to lift an object, you only need to apply a force slightly less than the weight, because there are two effects that help you.

This will be very noticable if you try to lift a balloon filled with air. The air weighs a few grams, but you don't need to supply a force for that. With 100 kg of iron, only about 12 liters of air weighing about 15-16 grams are replaced, so the difference is tiny.

PS. See Adrian's answer for a third force helping you.

PS. Someone could claim that all these forces should just be added to the force that you apply yourself. Centrifugal power is different. It's not a force, it's just an effect of the object trying to move in a straight line, when due to earth rotation for example all objects resting on the surface actually move at very high speed along a circle, not a straight line.

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Assuming the object is on the surface of the earth there is a slight gravity gradient, so applying a constant force equal to the weight of the object at that initial position results in an unstable situation.

A slight movement of the object upward (from thermal motion, vibration or whatever) will reduce the force of gravity by about 0.3 mGal/m -- a gradient of 3,080 E (eotvos), so absent any other significant forces (a rather big assumption) it would tend to eventually fly off into space.

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    $\begingroup$ Also, 'at STP'. When you fail to quantize a hypothetical situation, you get pedantic answers like this, +1. $\endgroup$ – Mazura Oct 29 at 3:30
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From a computational point of view, a force is a floating point number, not an integer. With floating point numbers, there is not much point in saying things like x equals y, because x and y may differ a very small amount, so small that some consider this as equal, and others may consider this as different. If you program:

float x;
....
if (x == 1.23) ...

then the if-clause may never get true, because x may get very close to 1.23, but never equal to it. Normally you would instead code something like:

if (x <= 1.23)

or:

if (x > 1.22 and x < 1.24) ...

For your question, if you want to 'lift' an object with mass m with a very small accelleration a, the required upward force F = m x a may be very small, where some may consider this as "zero" or "no" force and others may consider this as a considerably non-zero force.

It all boils down to what you mean with "equal".

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Your conclusion is correct. To make the object move you must initially apply a force greater than the weight of the object in order to accelerate it from rest. After that you can reduce the force to match the object's weight and it will continue to rise at whatever speed it had reached while you were accelerating it.

In practice it will be impossible to apply a force that is at all times exactly equal to the weight of the object, so what will actually happen is that the object's speed will vary and you will continually have to make small corrections to the force.

If you want a more precise answer, you should also take into account:

1) Air-resistance, the effect of which which will depend on the size and shape of the object, its speed relative to the air through which it is moving, and the pressure and moisture content of the air, etc.

2) Buoyancy, the effect of which which will depend on the volume of the object and its density.

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Maybe you've heard of Isaac Newton's second law of motion...

$F=ma$

where

$F=$ net force on an object

$m=$ mass of object

$a=$ acceleration of object

In your question, you ask about a situation where the net force on the object is zero. The force due to gravity (weight) is equal and opposite to your lifting force. You can see using the equation that if $F=0$, then also $a=0$.

The object cannot begin to move with an acceleration of zero. Therefore it will remain stationary. If your lifting force is greater than the weight, then the net force will be greater than zero and the object will begin accelerating upward. The magnitude of the acceleration will depend on both the net force and the mass of the object.

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It really depends on your definition of "lift".

If lift is taken to mean that the object should cease applying any pressure to the surface underneath it, then the force that you need to apply needs to be exactly equal to its weight, no more.

If lift is taken to mean that the object should not only cease applying any pressure to the surface underneath it, but also move away from the surface, then of course you will need to apply such a force that not only overcomes the weight of the object, but also accelerates it by a tiny little bit so that it may start to move away. Without at least a tiny bit of initial acceleration the object will not move.

(In the actual, real™, physical world the object will of course move away from the surface even if you try the first scenario, partially because of air currents, vibrations, etc. and partly because you cannot actually apply a force which is exactly equal to a weight: it will necessarily vary slightly over time due to vibrations in your lifting apparatus. So, while the force that you are applying is less than the weight of the object, you will see no effect, but at the first moment that the force ever so slightly exceeds the weight of the object, the object will move.)

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Simplistically but hopefully usefully:

Apply Newtonian principles strictly.

Assume ideal conditions but include as many ideal factors as needed to be 'happy' with the result. eg

  • include or exclude buoyancy as desired - it is a real world factor but typically amounts for around 0.1% of the total "weight". eg for water air-buoyancy is about 1.2 grams per kg of water at sea level.

  • include or exclude drag losses when moving as desired - drag in air is about 0.6 x A x V^2 in Si units. (A - projected frontal area, V - velocity). At say 1 m/s that's around 0.5 newton per square metre of frontal area - not something you will easily notice in many scenarios. At 10 m/s it's around 50 newton and 'you may begin to notice'. t 100 m/s it's around 5 kN and will dominate in many real world scenarios.

A "lifting" force exactly matching a supporting force replaces it seamlessly. Newton says so.
Believe him.
He knew what he was about :-)

Any 'upwards' forces greater than the 'downwards' weight force will accelerate the body using standard Newtonian expressions.

However - If we deal separately with the above weight versus "supporting force" balance, even though it is 'correct enough', it actually complicates what seems to happen. So once we are happy with the separate "hovering" and acceleration forces metaphors, you can then sum together all the components and deal with the 'vector sum'. So sum weight (= m.g), buoyancy (if considered, = volume a air-density), air-drag (gets messy quickly - best left ideal at zero to start with ~= 0.6 x Area x velocity squared) and the results will be exactly as you'd expect. (That's assuming you don't want to add in solar radiation pressure or any other 3rd 4th ... order effects).

QED?

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As has been mentioned in @Adrian's answer in the real world there is nothing rigid, neither the support where the mass m is resting on be it a table or cement bench nor the mass itself.

So there is always a certain amount of energy reserved in the support due to its deflection/settlement under the weight of the mass.

Therefore there are two forces here, let's call them,

$$ F_1 = \text {the force we exert} \\ and, \quad F_2= Kx $$

With K as spring constant of the support and x as the settlement of the support under the m*g.

So we actually need to apply just enough force (slightly less than the weight, mg) to give the spring action of the support a chance to push the object up act as a projectile moving up. And we can calculate the F1 just short of weight enough so that$$ W*H= F_1*H +\frac{1}{2}Kx^2 \\ F1= W-\frac{1}{2H}Kx^2 $$ With W as weight and H as height. And we inspect that the F1 is smaller than the wight.

And therefore we do not need any downward transient force at the top end of the trajectory.

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Yes we apply a slight greater amount of force at the beginning and then we lift it up in equilibrium condition.In fact in extremely small amount of time the object gains some velocity when we apply a force just greater than its weight.After that extremely small period of time we can lift the particle up in equilibrium.

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    $\begingroup$ I don’t recall seeing gravitational potential energy being defined that way. Even if an object has a velocity at a given height it also possesses gravitational potential energy. In order for it to ONLY possess gravitational potential energy at a given height, the change in kinetic energy must be zero $\endgroup$ – Bob D Oct 27 at 9:31
  • $\begingroup$ I already said this in the last sentence $\endgroup$ – VK_fan Oct 27 at 10:51
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    $\begingroup$ Maybe it’s just me, but I found the last sentence confusing $\endgroup$ – Bob D Oct 27 at 11:00

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