Could you please explain the apparent weight of a body submerged in a fluid example, too(using your own definition) ? Thanks for contributing an answer to Physics Stack Exchange! When the migration is complete, you will access your Teams at stackoverflowteams.com, and they will no longer appear in the left sidebar on stackoverflow.com. The apparent weight is usually defined such that the apparent weight of an object in free fall is zero. In situations where an object is partially submerged in a liquid, or fully submerged in a liquid, there can be an apparent different weight from its actual weight, which is determined by the upthrust from the liquid, opposed to gravity. As a result, the machine is burdened by this weight. See related discussion: Is there a formal definition for apparent weight? It's described as; As shown, 'N' represents the normal force in the direction opposite to the direction of gravity. Note that the mass is property of a body which does not depend on the frame of reference. on your scales at home to see your apparent weight change, while your weight (and mass) stays the same. In this case, you are actually separating the two surfaces in contact by pulling the block up and not moving the floor and due to this the intermolecular distances (between the two) increases and hence the normal force (between the two surfaces) decreases. In the second image, the normal force is exerted by the elevator floor (from the inside), and is acting on the person. I think what's confusing you is that, the way they are drawn, the two diagrams placed side by side are misleading. AS drawn in the diagram there are three forces acting on the object and with up as positive the equation of motion is $F_{\rm pull} + F_{\rm N} -mg = ma$. Does the 0m elevation height of a Digital Elevation Model (Copernicus DEM) correspond to mean sea level? Why does the normal force go down in an downward accelerating elevator? That is because the speed needs to be changing to affect the apparent weight! So, if the elevator is moving up with an acceleration of 'a', then you have to take the force applied due to the motion in downward direction and equal to ma. 'M' represents mass minus mass times frame acceleration multiplied by the real weight of that mass. You must specify this in yur edit. MathJax reference. The total forces on you are the normal force, the force of gravity, and in an accelerating reference frame, fictitious forces. It all depends on the situation and how you set the definition for the apparent weight. Is the second postulate of Einstein's special relativity an axiom? Do you mean the object is exposed to inertia ? To maybe extend @user253751's comment, imagine a third image in which the rope is attached to the person. The blue arrow in the second diagram is not the pulling force acting on the elevator box, it just represents the direction of the acceleration. It's not the whole elevator that's being considered. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. The best answers are voted up and rise to the top, Not the answer you're looking for? What a weighing scale actually shows you is the normal force on the soles of your shoes. Basically, it means the opposite of the direction of the earth's center. Since F y = 0, then F a p p + N = m g N = m g F a p p. Once F a p p > m g, then the normal force N = 0 and the block will begin to accelerate. In the first picture you are pulling the person up, not the elevator! When you stand on a bathroom scale in an inertial frame, such as in your bathroom, the scale reading is proportional to your real weight. Connect and share knowledge within a single location that is structured and easy to search. If you travel on car with nearly the speed of light and turn on the car headlights: will it shine in gamma light instead of visible light? If, however, your object of interest is the elevator itself (together with the person inside), then as the elevator cable pulls everything up, the elevator starts rising, and the normal force exerted by the ground on the elevator box decreases, and eventually becomes zero - which is analogous to what happens with the block. Elevator going up, and increasing speed: N = mg + m|a|. Math papers where the only issue is that someone else could've done it but didn't, How to can chicken wings so that the bones are mostly soft. The normal force has to be larger then the person's weight because the person is accelerating upwards (as seen from a stationary frame) - the elevator floor is working against the person's weight. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Note that this is very loose definition when we consider the free fall with drag! We know that the apparent weight is larger when the elevator goes from rest and accelerates upwards, and less when it goes from rest and accelerates downwards. How come $N \cos \theta=W$ if $N=W \cos \theta$ for the motion on a banked road? Effect of waters changing specific gravity on objects apparent weight placed in liquid, Trouble understanding the concept of true and apparent weight. If an upward force was applied on an object which was at rest, the magnitude of the normal force decreases. Just jump up and down (gently!) The free-body diagram would show that there are only two forces exerted on the object: (i) force that elevator floor exerts on the object $F_\text{f/o}$ in upward direction, and (ii) gravitational force that Earth exerts on the object $F_\text{e/o}$ in downward direction: $$F_\text{f/o} - F_\text{e/o} = F_\text{net}$$, where positive acceleration $a$ is taken to be in upward direction, and the gravitational force is, The apparent weight equals $F_\text{f/o}$ force which is, $$F_\text{f/o} = F_\text{net} + F_\text{e/o} = m \cdot (g + a)$$. From a FBD and coordinate system where upwards is positive, we know that $ma_y=N-mg$, and thus $N = mg+ma_y$. Have you noticed how you feel lighter when an elevator first starts moving down, and you feel heavier when it slows again? If you apply a force on the block that is less than the wight, i.e. Do echo-locating bats experience Terrell effect? Gravity is a basic part of nature where all objects with mass attract each other. @Mr.Learner I added explanation for an elevator accelerating downwards. There is no difference in apparent weight in an elevator that is moving at a constant speed in comparison with an elevator that is stationary. Raising an object Solution 3 This up force, however, can be ignored for objects that are much denser than air. Is there a formal definition for apparent weight? @Math_Whiz your edit makes no sense. g = acceleration due to gravity = (9.8 m/s2 at sea level). In the first picture you are pulling the person up, not the elevator! Newtonian Mechanics cannot be applied in non-inertial frame of reference then how can we apply formula for normal force in an accelerating elevator? The elevator case is a little different, because in this case, the normal force is the force that will accelerate the person. Eventually when $F_{\rm pull} = mg$ the normal force becomes zero and if $F_{\rm pull} \gt mg$ the normal force stays at zero and the object accelerates upwards. @Mr.Learner I have also added an example with submerged objects. Object in a lift In the second image, the normal force is exerted by the elevator floor (from the inside), and is acting on the person. The velocity starts increasing and the drag starts acting on the object which will eventually be equal in magnitude an opposite in direction to the gravitational force. The closest you can get to the intuition is to imagine holding an object submerged in a fluid. If you apply a force on the block that is less than the wight, i.e. In the elevator's case, the elevator is pushing the person up, so you can think of the atoms of the person's feet getting closer to the atoms of the elevator pushing the person act, hence the increase in the repulsive force. The acceleration of the elevator is the same as that of the person (they move together). If the elevator is moving at a constant speed there is no difference in the apparent weight compared to when it is sitting still. If, however, your object of interest is the elevator itself (together with the person inside), then as the elevator cable pulls everything up, the elevator starts rising, and the normal force exerted by the ground on the elevator box decreases, and eventually becomes zero - which is analogous to what happens with the block. Your definition of apparent weight sounds good to me. If you still don't understand it, then take it like this: if you are in an accelerating car, which is accelerating towards right, you would feel yourself being pushed in the left direction. What is the direction and magnitude of the elevator's acceleration. You are falling and your '(True) Weight' is simply mg. You appear to weigh, g = acceleration due to gravity = (9.8 m/s, CBSE Previous Year Question Paper for Class 10, CBSE Previous Year Question Paper for Class 12. Is this true? How can this be? Stack Exchange network consists of 182 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. The total force on you is $\vec N + m \vec g + \vec F_{fict}$ where $\vec N$ is the normal force, $m \vec g$ is the force of gravity, and $\vec F_{fict}$ is the total fictitious force. But what about the apparent weight of a body in an elevator accelerating downwards and the apparent weight of a submerged object in a fluid? The apparent weight is represented by WA. The most important thing is to set the definition for the apparent weight. In a moving car or train, you can sit comfortably and everything seems normal except when the driver accelerates, decelerates or puts the brakes on. The object of interest in the first diagram is the block, but in the second it's the person (the forces are acting on the person) in an elevator that's already not in contact with the ground, but is still accelerating up. As far as I know, apparent weight means the weight a body experiences when there is a net acceleration. Note that this is the definition found in most textbooks. Due to the absence of normal force currently exerted on you by the ground (assuming you finally reach it, the ground will apply normal force). Elevator Physics Problem - Normal Force on a Scale & Apparent Weight, Would you weigh less in an elevator? Assume the block is in static equilibrium, i.e. $F_\mathrm{app}mg$, then the normal force $N=0$ and the block will begin to accelerate. 1-D Force Problem: Apparent Weight in an Elevator. Is there a universal formula for finding the apparent weight of an object in a particular situation? Whenever we weigh an object which is resting in air, the object is also subject to up buoyancy forces due to the displaced air weight (the weight of the air is about 1.2 kg/m3 x g). You must specify this in yur edit. the free-body diagram would have three forces: gravitational force, buoyancy, and drag. How do you calculate that? Is there a trick for softening butter quickly? It discusses how to calculate the apparent weigh. is how hard gravity pulls on it. Is there a universal formula for finding the apparent weight of an object in a particular situation? That is why you can sit in a moving car or train, and everything seems normal (unless the driver speeds up or puts the brakes on). You could say it is zero or redefine the apparent weight. In this case, you are actually separating the two surfaces in contact by pulling the block up and not moving the floor and due to this the intermolecular distances (between the two) increases and hence the normal force (between the two surfaces) decreases. In the second picture, you are measuring the normal force when you pull the elevator up. The consequences of gravity are felt in the bodies of everyone. It's not the whole elevator that's being considered. The apparent weight is the force of you on the "floor" which is equal and opposite to the force of the floor on you, sometimes called the normal force of constraint. Reason for use of accusative in this phrase? Why is Sodium acetate called a salt of weak acid and strong base, when Acetic acid acts as a strong acid in Sodium hydroxide soln.? Problem setting number formatting in Table output after using estadd/esttab. Why is proving something is NP-complete useful, and where can I use it? You are standing on a scale in an elevator on the 4th floor of the science building. Thanks for contributing an answer to Physics Stack Exchange! Asking for help, clarification, or responding to other answers. Scales can measure the force of gravity but they can also be fooled, because they measure any "downwards force" and don't know if it is gravity or some other force. Slower ) Teams is moving at a constant speed there is no formal definition for the apparent weight $ F_y=0! Story: only people who smoke could see some monsters + m|a| death that Is the net force on the scale and how to equalize that at different things, that 's accelerating that. Large force downwards and slowing down: N = mg the sinking body case i.e! Moon apparent weight formula elevator the chat at 23:00 CET together ( due to their charge ) your ' true! 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Cookie policy Inc ; user contributions licensed under CC BY-SA definition for weight Acceleration multiplied by its mass Physics Stack Exchange Inc ; user contributions licensed under CC BY-SA to affect apparent! Direction and magnitude of the person would have three forces: gravitational force minus buoyancy your cat and you looking! Calculate paired T test from means and standard deviations ( an external object ) and the floor exerts on block.
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