Your weight from the speed of 100 km/sec?

[hellwing]

what is your weight if you have the speed of 100 km/sec?

[insane_alien]

the same as if your were at 0 km/sec

[ajb]

You mean your "relativistic mass" as measured by an observer comoving with respect to you at 100 km/sec ?

[iNow]

This sounds to me like a "how much do you weigh on the moon" type of problem, except with greater velocity than 9.8 m/s/s as opposed to less. Is that an appropriate way to frame the OPs question, or am I missing something simple?

[Cap'n Refsmmat]

9.8m/s/s is an acceleration, not a velocity, so that doesn't really apply here. I think he's assuming you're in space traveling at Ludicrous Speed^TM.

[iNow]

So I was, indeed, missing something simple. Thanks Cap'n.

 

 

We can't stop. It's too dangerous. We have to slow down first.

[hellwing]

You mean your "relativistic mass" as measured by an observer comoving with respect to you at 100 km/sec ?

 

hmm,what i mean is while traveling or if you throw down 2 kilos socks of rise while you are in 4th floor building,do you think it would double the weight?

[honestdude14]

Weight itself, is an attribute given to a body relative to it's mass and gravitational acceleration. On earth that acceleration is constant at 9.81m/s/s. Weight as we know, is the body's mass times the earths gravitational acceleration(w=m*a). I think you are asking what kind of force will be exerted, rather than its weight.

[mooeypoo]

Actually, allow me to be the smartass and add that your weight CAN change with change in speed (acceleration), if your acceleration is not horizontal (say, you're moving up or down a hill, or up/down and elevator)...

 

Weight is the force that is exerted.. F=ma.. usually it's F=mg, but if you're accelerating up or down or diagonally, it will change that force, and hence change your weight.

 

</slightly smartass>

 

~moo

[Mr Skeptic]

Let me be even more of a smartass and point out that accelerating horizontally will change both the magnitude and direction of the normal force we call "weight".

 

As for a relationship between velocity and weight, I would say that without a normal force (something to stand on), there is no weight. Therefore, traveling at sufficient velocity to maintain an orbit will make you weightless. Traveling at a suborbital speed will decrease your weight in a similar manner, but not to zero. This, of course, is because orbits have an acceleration in the vertical direction, like mooey said.

[mooeypoo]

Let me be even more of a smartass and point out that accelerating horizontally will change both the magnitude and direction of the normal force we call "weight".

 

As for a relationship between velocity and weight, I would say that without a normal force (something to stand on), there is no weight. Therefore, traveling at sufficient velocity to maintain an orbit will make you weightless. Traveling at a suborbital speed will decrease your weight in a similar manner, but not to zero. This, of course, is because orbits have an acceleration in the vertical direction, like mooey said.

 

That's a good point, I didn't think abou tit.

 

But here's a question: Orbit can be achieved at relatively high distance above the earth. Since the earth is round and not flat, won't this make the "horizontal" speed non-horizontal (but rather centripetal)?

[swansont]

That's a good point, I didn't think abou tit.

 

But here's a question: Orbit can be achieved at relatively high distance above the earth. Since the earth is round and not flat, won't this make the "horizontal" speed non-horizontal (but rather centripetal)?

 

Tangential, perhaps? Centripetal would be along the radius.

 

As Mr Skeptic has already implied, the thing we call weight, as measured with a bathroom scale or equivalent, is really the normal force. We should try and keep clear what is asked, and what is being measured.

[elas]

Tangential, perhaps? Centripetal would be along the radius.

 

Is there a need for a new word here, like circupetal, or concentripetal.

[honestdude14]

perhaps the "smartass"(he said it, i didn't) would like to KINDLY explain to me how actual weight can change. If weight is directly proportional to mass.. and both mass and the gravitational pull are constant..then shouldn't weight be constant as well? I think we are considering two different definitions here. Actual weight and apparent weight are two different things.

[Sisyphus]

perhaps the "smartass"(he said it, i didn't) would like to KINDLY explain to me how actual weight can change. If weight is directly proportional to mass.. and both mass and the gravitational pull are constant..then shouldn't weight be constant as well? I think we are considering two different definitions here. Actual weight and apparent weight are two different things.

 

No, they're talking about gravitational pull changing as a result of change of position. As in, the pull is less high up in the air because you're farther away from the Earth's center of mass. I know: annoying.

[honestdude14]

wait.. what are we talking about again.. oh yeah.. a vague question about weight where the only detail known is a constant speed. I'm going to sit this one out.. sorry guys.

[Mr Skeptic]

perhaps the "smartass"(he said it, i didn't) would like to KINDLY explain to me how actual weight can change. If weight is directly proportional to mass.. and both mass and the gravitational pull are constant..then shouldn't weight be constant as well? I think we are considering two different definitions here. Actual weight and apparent weight are two different things.

 

Well, the thing is, weight is a force, specifically the force you exert on whatever it is you are standing on. Actually, what I said earlier about weight being the normal force is not exactly accurate, as the normal force is of equal magnitude, but in the opposite direction, to the force you exert on the floor.

 

Force = mass times acceleration. The only way to change the force is to change either mass and/or acceleration. When people talk of weight, they usually refer to the force exerted by a mass due to the gravitational acceleration g = 9.81 m/s². But so long as there is a difference in acceleration between yourself and what you are standing on, you will exert a weight on the floor, and the floor will exert a normal force on you, to keep you from going through the floor. You are used to accelerating at 9.81 m/s² with respect to the floor, but you must have some experience with a different acceleration. For example in a car that is turning, a roller coaster, or a merry-go-round. With a different acceleration, you will feel a different weight.

[swansont]

With a different acceleration, you will feel a different weight.

 

But again, our perception of weight is actually the normal force exerted on us. Astronauts on the shuttle or ISS feel weightless, but the gravitational force on them is not zero.

[mooeypoo]

That's because the "sum" of forces applied *is* zero. Otherwise, they'd be moving.

Well, there's the tangential speed (so force towards the center of the 'circle') but .. meh.. how DO we defined "weight"? The forces, (like "Sum of all F") or the "subjective feeling" ?

 

~moo

[Cap'n Refsmmat]

No, the sum of the forces applied is not zero. Otherwise they'd be moving in a straight line tangential to their orbit.

 

Astronauts in orbit are being constantly accelerated because they're traveling in a circle. They're not just floating there with no forces being applied on them.

[Pete]

what is your weight if you have the speed of 100 km/sec?

If you are moving perpendicular to a gravitational field then your weight would be W = mg = m₀g/sqrt(1 - v²/c²)

 

The derivation of this is found on my website here

http://www.geocities.com/physics_world/gr/weight_moving_body.htm

 

I derived this in two ways. The first one is heuristic and thus easy to follow. The second is a rigorous derivation using general relativity.

 

Pete