# Does weight of an object chance in uniform upwmotionard motion

1. Sep 29, 2013

### Trojan666ru

If i measure a weight on earth without moving in any
direction, it weighs 10kg. Suppose I'm moving upwards in
a uniform velocity 5m/s , how much would object weigh
now ??

2. Sep 29, 2013

### arildno

As the ratio between present gravitational force to initial gravitational force.
If we call r the initial distance from the center of the Earth, r+vt (where v is the velocity you move away with), you should be able to calculate for yourself that ratio of weights, as a function of time.

3. Sep 29, 2013

### Trojan666ru

actually i don't wanna calculate how much it would increase, i wanna make sure the weight increases or not? because I'm moving in an uniform velocity

4. Sep 29, 2013

### arildno

Do you think it would increase or decrease when you move away from the Earth?

5. Sep 29, 2013

### Trojan666ru

it would decrease, but I'm not asking about inverse square law. Imagine I'm moving upto a height of 10 km, consider the attractive force of earth is same at sea level upto 10km, what about now, does the weight of the object increase if I'm moving upwards with uniform velocity of any speed, or the object weigh same 10kg since I'm not accelerating?

6. Sep 29, 2013

### D H

Staff Emeritus
Since you used the colloquial definition of weight, a synonym for mass, the answer is simple: It "weighs" 10 kg, anywhere and everywhere.

That colloquial meaning is not what physicists and engineers mean they talk about "weight". Physicists use the word "mass" to denote mass. Why use the word "weight" when there is already a perfectly good word for mass? Weight in physics has units of force in physics. There are two widely used definitions: The force due to gravitation on the object (mass times gravitational acceleration), and the total of all forces acting on the object except for gravitation.

What arildno is hinting at is mass times acceleration, or Newton's universal law of gravitation. What does that have to say about the force due to gravitation as altitude increases?

7. Sep 29, 2013

### Staff: Mentor

Using "weight" in the precise sense of being what a spring scale underfoot would read....

Aside from a tiny effect from inverse square law as you may move away from the center of the earth, the weight will not change if you're climbing at a constant velocity. It will change if you're accelerating or decelerating, so not climbing at a constant velocity.

Have you ever taken a fast elevator in a tall building? As the elevator first starts moving upwards you feel a slight increase in weight; you're accelerating upwards. As the elevator settles down to a steady upwards climb your weight returns to normal (no acceleration) and then as it slows to a stop at the top of the climb you feel momentarily lighter.

Of course the force of gravity on your body is the same throughout, as is your mass; what's changing is the force between you and the floor of the elevator, what we strictly mean by "weight".

8. Sep 29, 2013

### Trojan666ru

here I'm strictly talking about weight, not mass, consider the mass does not vary because I'm moving with the mass. Weight is the product of mass and g. Here I'm in a lift and the lift goes up with no acceleration, I've an electrical balance which shows exact weight of the object while in not moving and when i move with uniform velocity upwards, i think the the balance will show an increase in weight

9. Sep 29, 2013

### Staff: Mentor

If you are moving at a constant velocity upwards, the electric balance will not show an increase in weight. (Your post #8 landed just one minute after my post #7, they may have crossed).

10. Sep 29, 2013

### Trojan666ru

But according to general relativity gravity is the downward flow of spacetime, if i move opposite to the flow of gravity it must feel an increase in weight

11. Sep 29, 2013

### D H

Staff Emeritus
Then why did you persist in saying the weight is 10 kg? Kilograms are a unit of mass, not force.

That is not the definition of weight used in general relativity (your next post, below). In general relativity, "weight" is what an ideal (spring) scale measures. That's all real forces except gravitation (Newtonian interpretation), or all real forces, period (general relativistic interpretation). Gravitation is not a real force in general relativity.

Where did you get this idea?

12. Sep 29, 2013

### Staff: Mentor

That's not what GR says gravity is, but this doesn't matter because GR itself is basically irrelevant in a situation in which the effects of the Newtonian inverse-square law are small enough to ignore.

This is really just an $F=ma$ problem. We're climbing at a constant velocity, so $a=0$. Therefore $F$ is zero, which can only happen if the downwards force of gravity on our body is exactly balanced by the upwards force on our feet from the scale we're standing on.

13. Sep 29, 2013

### TurtleMeister

10kg. You must accelerate or de-accelerate for the scale reading to change. You can test this in an elevator. Scale reading will change at the beginning and end of the ride.