What is definition of weight? Does weight of an object change under water, or the weight remains the same, but the: 'apparent weight' = 'weight' - 'buoyant force' ?
Same question for object submerged in air, or object in free fall?
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$\begingroup$ Not general definition, only in context of the question. $\endgroup$– CorneliusCommented Mar 4 at 11:30
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4 Answers
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Strictly speaking, weight is the force experienced by an object due to its presence in a gravitational field. In a uniform gravitational field (which the surface of the earth approximates), it is the product of the mass of the object and the gravitational field strength (also commonly called gravitational acceleration) at its location (commonly denoted as g; On earth, g is roughly 9.8m/s^2). Simply put, it is the force acting on an object, solely due to the gravitational field it is present in.
The concept of apparent weight arises when there are other forces acting on the object. Due to these forces, the object 'feels' a different amount of pull/push in a particular direction, than the pull due to gravity alone.
An example is a person in an accelerating elevator. When the elevator is, say, moving with increasing speed upwards, the person inside feels the floor 'pushing' on him/her more than if he/she were on the ground. This push (the normal force between the person and the floor of the elevator) is equal in magnitude to the apparent weight of the person. Note that the real weight of the person (gravitational force between the person and the earth) has not changed, since the gravitational field of the whole setup is considered to be uniform. If the person was standing on a weighing scale in the elevator, the reading would be the apparent weight. In this particular example, the apparent weight is the normal force between the floor and the person.
An object in free fall has no normal force acting on it, and thus feels no push against the gravity of the earth. Thus it's apparent weight is zero.
Another place where apparent weight would turn up would be while using a spring balance to measure the weight of an object submerged in a liquid. Here the reading on the scale is the apparent weight again, but rather than normal force, here it manifests as tension in the spring of the spring balance. Here again, the real weight of the object has not changed. It is effectively still in the same gravitational field (that of the earth).
So apparent weight is sort of an 'effective weight' that the object 'feels' due to various forces, and finding it depends on the context of the situation.
Note: If you went to the moon, your REAL weight WOULD change, since you are now present in an entirely different gravitational field from that on earth.
Hope this helps.
Edit: the introductory(2nd) paragraph in wikipedia, which you have linked initially describes real weight, then apparent weight (reaction force...). The context depends on the situation.
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Weight is just another word for gravitational force caused ny another object. Your weight is the gravitational force you feel due to the Earth's pull.
Typically, though, the word weight is only used when the other object is unaffected by the gravitational force itself. For example, the gravitational force between Earth and moon is large enough to cause disturbances in both. Not just does the moon orbit, but the earth simultaneously "wobbles" and feels tidal effects etc. to a significant extend. So we wouldn't use the word weight for the gravitational force on the moon. But the gravitational force on an apple by the earth is much more influential than the simultaneous gravitational force on the earth by the apple. So here we will use the word weight for for the gravitational force felt by the apple.
If you go to the moon, then your weight would be 6 times smaller since the moon's gravity is about 6 times weaker. This is not apparent weight - this is your weight because this is how much the moon pulls in you.
The concept of apparent weight is about the "feeling" or "appearance" of a different weight, even though your weight has not changed. When submerged, then another force - the buoyant force - is pushing on you as well. So you feel different. But not because gravity changed.
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Many people define weight as the force of gravitational attraction on a body whilst some others define it as the reading on a weighing scale.
Using the first definition the weight does not change in your example but with the second definition what you have called apparent weight is the weight.
Using the first definition an orbiting satellite has a weight whereas using the second definition it does not.
I use the first definition.
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$\begingroup$ What is the weight of an astronaut in orbit? In your opinion it is the same as on earth, whereas most people including myself would view the astronaut as weightless. $\endgroup$– my2ctsCommented Mar 9 at 18:14
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$\begingroup$ @my2cts I would say that the Earth still attracts the satellite with a gravitational force so the satellite has a weight but the satellite would appear weightless, ie the reading on a spring balance would be zero. Like a lot of terms there can be different definitions but the important thing is that it is known to others which definition I are using. $\endgroup$– FarcherCommented Mar 9 at 22:41
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$\begingroup$ Would you then also say that microgravity experiments are superfluous because the weightlessness is only apparent? $\endgroup$– my2ctsCommented Mar 9 at 23:18
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$\begingroup$ No (to your question). I am making a point that I use the gravitational attraction definition of weight whereas others use the "spring balance" definition of weight. It boils down to semantics and not Physics. As long as the definition used by a person is known to me I can cope with either definition. The word apparent is used by me to enable me to differentiate between the two definitions. $\endgroup$– FarcherCommented Mar 10 at 8:52
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$\begingroup$ 'Einstein argued that inside a windowless elevator, a person cannot tell whether the elevator is at rest in a gravitational field or is instead being hauled up with constant acceleration. He then conjectured that the laws of physics themselves must be identical in both situations.' This means there is no such thing as 'apparent' weightlessness. scientificamerican.com/article/…. $\endgroup$– my2ctsCommented Mar 10 at 9:33
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Weight is simply the gravitational acceleration multiplied by the mass of the object. It is essentially the magnitude of the force acted upon on it by the earth( in case of us).
- Objects weight remains same if it's distance from the centre of the planet is constant.
- The apparent weight in the case of water is due to a phenomenon called the buoyant force. It is simply the mass of the fluid displayed times the gravitational acceleration.
- In the case of free fall we can have two explanation - first, Einstein's equivalence principle and second, from Newton's equations.
- That phenomenon is called weightlessness.The person falling towards the gravitational field follows geodesic paths.
