Questions about Gravity

[Maulik]

This is elemantary physics problem at best. I contend my weight is larger as I accelerate down an elevator.

If I am accelerating upward at any speed > 0 m/s**2 is my weight > || < my mass at an elevation of 0 meters?

Additionally, if I'm on a rollercoaster as I'm reaching the uppermost point what is my mass in regard to the journey upward and downward? Specifically, if my weight is decreasing is this the result of my body wanting to continue moving in a linear motion as I move downward?

[BrickTamlin]

You have used mass and weight interchangeably in your questions, making them nonsensical.

[Phil153]

Mass: How much stuff something has got in it
Weight: How much force that something exerts on something else (usually a set of scales).

If you're floating freely in space, your mass is (say) 75kg, but your weight is 0kg. On the moon (http://www.exploratorium.edu/ronh/weight/) your mass would be 75kg but your weight would be 12.5 kilos.

To answer your questions:

-If you stand on a set of scales on a an elevator accelarating downloads, you will weigh less.
-Vice versa for going up
-At the high point of a rollercoaster, it's possible to be weightless for a short time (as measured by a scale on your seat.)

[Matt R.]

[ QUOTE ]
Specifically, if my weight is decreasing is this the result of my body wanting to continue moving in a linear motion as I move downward?

[/ QUOTE ]

It is the result of you accelerating. A scale measures the normal force it is applying to an object to keep it from accelerating downward relative to the scale. Don't think of weight as an absolute property -- only rest mass is absolute. Weight is simply the force vector F = mg when you are at rest relative to the earth -- but this will change once you start accelerating. This is because what weight really is, the normal force, changes when you add acceleration to the picture.

From Newton's 2nd Law, the sum of all force equals ma. So, when you're in an elevator the 2 forces acting on you are mg (your mass times the gravitational acceleration) and n (the normal force -- this is what the scale is measuring. Your weight.) Since the sum of the forces is ma, if you take "up" to be the positive direction, you have n - mg = ma. So the normal force, your weight, is measured to be ma + mg.

From this you see that if a is positive -- you are accelerating upward in the elevator -- n > mg. So you weigh more. And vice versa.

Also, note that when you are NOT accelerating ma = 0.
So, F = n - mg = 0 and n = mg. This is why your weight when you are not accelerating relative to the earth is F = mg and it only depends on your mass.

[benfranklin]

There is no gravity. The world sucks.

[Mik1w]

I thin technically, your weight does not change. The force on you due to the gravitational field you're in is exactly, the same, because the gravitational field is uniform in the region of space you're travelling, and your mass is constant. You just feel lighter because you feel a force lighter than your weight from the floor, and the main way we judge our weight is by "feeling" the reaction force of the ground we are standing on.

Mass is a fundemental physical property of an object, weight is dependant on mass and the gravitational field. The gravitational field is the ratio of your weight to your mass. On the surface of the Earth, this is approximately ten Newtons of weight for every kilogram of mass you have.

[Trantor]

[ QUOTE ]
This is elemantary physics problem at best. I contend my weight is larger as I accelerate down an elevator.

[/ QUOTE ]

Not sure what you mean "accelerate down an elevator". If you mean you are in a closed elevator which, in the earth's frame, is accelerating downward then your weight would be larger if it was accelerating downwards at more than 2G in the earth's frame. You would be on the elevator ceiling, of course.

[ QUOTE ]
If I am accelerating upward at any speed > 0 m/s**2 is my weight > || < my mass at an elevation of 0 meters?

[/ QUOTE ]
Your weight is more, ie as measured in your accelerating frame of reference.

[ QUOTE ]
Additionally, if I'm on a rollercoaster as I'm reaching the uppermost point what is my mass in regard to the journey upward and downward? Specifically, if my weight is decreasing is this the result of my body wanting to continue moving in a linear motion as I move downward?

[/ QUOTE ]

Your mass is constant(unless you are scared shitless by the experience). Your weight changes though, as experienced by the rider.

weight is mass times the accelrating force acting on it in your frame of reference. Acceleration due to gravity and other forces are indistinguishable in an accelerating frame of reference (good old Einsteins theory. The decrease in weight experienced as you go over the top is becausethe roolercoaster no longer applies a force to your body resisting gravity in the earth's frame.On the roiller coaster this manifests as a reduction oin the acceleration experienced ie your weight decreases, in your frame.

The key is to talk about a given frame and what is experienced in that frame, and not to mix frames up.

[microbet]

You are right. The closer you get to the surface of the earth, the stronger gravity will pull on your mass and the greater your weight will be.

[hmkpoker]

Gravity doesn't exist. There is no scientific evidence of gravity existing. It's just a load of propaganda from the liberal atheist community who want to promote immorality and homosexual activity.

Praise Jesus.