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Homework Help: Doubt about a answer on a test

  1. Mar 6, 2015 #1
    Hello everyone,

    A few weeks ago I got a test about physics. One of the questions on the test was about
    a satelite that would go to an other planet, it was doing system tests in orbit, we had to tell if the satelite had weight. I understand it doesn't, because it is falling and so doesn't apply force onto anything, but it was mentioned that the satelite had an engine to fly to the other planet. It wasn't mentioned that the engine was turned on or off when the satalite was in orbit.
    So I filled in: The satelite is weightless when it is in space and when the engine is turned off. When the engine is turned on there is a acceleration, so there is a force acting uppon it, so it does have weight.

    I hope that someone can explain or confirm that this is wrong or right.

    Thanks already!!!
  2. jcsd
  3. Mar 6, 2015 #2


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    Maybe these links will help your understanding:
    http://www.colorado.edu/physics/2000/periodic_table/mass.html [Broken]
    Wikipedia Mass vs. Weight
    Last edited by a moderator: May 7, 2017
  4. Mar 6, 2015 #3


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    Maybe you were thinking too much. I doubt that you were supposed to consider whether the engine was on or off! Perhaps, however, the question is not very good unless you have a clear definition of weight. One definition is the gravitational force acting on an object. In this case, even in orbit, the satellite has weight.

    If you imagine a giant scales attached to the "bottom" of the satellite, then (as the scales would also be in orbit), they would show 0 weight. So, by that definition, while in orbit the satellite is weightless.

    It depends how you were taught to define weight, I'd say.

    In any case, I don't think anyone defines "weight" to be caused by any force other than gravity.
  5. Mar 6, 2015 #4


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    If you jump out of an aeroplane (perhaps a skydiver) while you are falling, before you open your parachute, do you have weight? If not, what makes you fall? (I neglect air resistance: you can choose.)

    In the space station, you are continually accelerating towards Earth (centripetal acceleration, because you are in circular orbit.) If you release an object, it floats next to you, so it too must be accelerating towards Earth. What force is causing that acceleration? (I just thought of the object, because it has no feeling and doesn't know whether it has weight. You the observer have to decide for it.)
  6. Mar 6, 2015 #5
    I was thought that weight is the force acting uppon an object because it is pulled down to eg the earth, so I would say that a satelite and a skydiver are both weightless while falling, but gravity pulls them down.
    The answer was supposed to be that the satelite doesn't have weight, but I also mentioned that the satalite has weight (if the engine is turned on) because it is increasing it's speed And there fore has g forces.
    So could I count the force because of the g forces as weight?
  7. Mar 6, 2015 #6


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    It will depend on how you were taught.

    I was taught that the weight of an object is its "apparent weight" -- the apparent force of gravity in an accelerating coordinate system that you choose to regard as being at rest. Others are taught that the "weight" of an object is its "true weight" -- the actual force of gravity in an inertial coordinate system.

    [There is another layer of complexity if one brings in General Relativity -- let's not do that]

    A space craft in orbit is weightless in the sense of "apparent weight" if you adopt a coordinate system that revolves with the craft. A space craft has weight if you adopt an inertial coordinate system within which the craft is moving in a circular path.

    An astronaut inside his craft is weightless in the sense of "apparent weight" if you use his space craft (with its engines off) as the basis for your coordinate system. If you turn the engines on, his "apparent weight" becomes non-zero.

    An astronaut inside his craft has "true weight" if you consider an inertial coordinate system within which both he and the craft are moving in a circular path. If you turn the engines on, this "true weight" does not change.

    A lump of gold on a scale on the surface of the earth has an "apparent weight" that differs from its "true weight". The difference arises from the centripetal acceleration due to the rotation of the earth. The delta is large enough that, for commercial purposes, an object's "weight" is measured in a way that makes it a synonym for "mass". However, this sense of the word "weight" is one that you will not encounter in the physics classroom.

    None of this changes the physics of the situation. It just affects the words used to describe the physics.
  8. Mar 6, 2015 #7


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    Hello Wouter, welkom bij PF :smile:

    The term 'weight' is usually reserved for the force with which an object is attracted by a much larger object, e.g. a planet.

    The term is not so much suitable for a reaction force the object generates itself by sending away material, e.g. combustion products. For that we have terms like 'inertial forces' or - as you say - acceleration force.

    Einstein became famous for making clear that we can't distinguish between the two by means of physics experiments.

    When orbiting a planet there generally is an attractive force and it serves as centripetal force to keep the object in orbit. Passengers feel weightless, however, because they also need a centripetal force to stay in orbit -- and thereby follow the same trajectory as their space vehicle.

    [edit] this reply is from around 1:30 pm, but it didn't want to post.

    In your shoes I would have answered: has weight because it follows an orbit, so something must be pulling it.

    Don't worry about this one, it's a pretty bad question, didactically.
  9. Mar 6, 2015 #8
    I don't like this question. But it would seem the satellite would indeed be "weightless" according to the Wikipedia articles Borg linked. The satellite is in perpetual freefall with nothing to retard its movement. The phenomenon known as weight is never experienced.
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