# General physics Question

Discussion in 'Physics' started by dthx, Jun 8, 2013.

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1. ### dthx Thread Starter Member

May 2, 2013
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when something like a rocket leaves the earth, does it get lighter and lighter as it gets farther away from earth.
And is that due to the acceleration of the rocket....
Or....is it due to moving farther from the earth's gravitional field.

2. ### circuitfella11 Member

May 10, 2013
56
5
yes it gets lighter and lighter.. its not due to its accelerations, its due to its distance of the object..

newton's law of universal gravitation..

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3. ### paulktreg AAC Fanatic!

Jun 2, 2008
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A large percentage of the rocket weight is fuel so that will have a significant impact as it is burned off.

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4. ### MrChips Moderator

Oct 2, 2009
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It depends on your meaning of "lighter".

An object has both mass and weight. The two are very different.
The mass of an object is fixed regardless of gravitational forces and it's location in space.
The weight of an object is the force exerted on the object due to gravity. Thus an object in outer space away from any other large massive object is "weightless", i.e. has no weight.

The answers provided by previous posters are all correct. The rocket loses both mass and weight as it is propelled away from the earth.

1) The rocket loses mass as it burns up its fuel.

2) The rocket loses weight because as it moves away from the earth the gravitational attraction between the rocket and the earth is reduced.

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5. ### SinewaveMan New Member

Apr 21, 2013
16
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Not being a physicist or rocket scientist I would agree with MrChips, adding that the mass (not weight) would tend to increase as its velocity increases (Mr Einstein et al). That however is "mind bending" stuff and we should hope it runs out of fuel or other acceleration long before the mass gain becomes noticeable.

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6. ### Wendy Moderator

Mar 24, 2008
21,439
2,958
As I understand it, the Soviet rockets actually accelerate as they burn off fuel, the G forces for the passengers increase.

The Saturn V burned something like 2.5 million gallons of fuel in 30 seconds. I'm not sure of this fact, it may have been a couple of minutes. However, they turned off the center engine of the 5 shortly after the launch.

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7. ### dthx Thread Starter Member

May 2, 2013
194
14
A very good explaination of the diff between mass and weight.
Great topic over a backyard burger tomorrow.
thank you all...

8. ### SplitInfinity Member

Mar 3, 2013
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In order for a Rocket to leave the majority of Earth's Gravitational Effect in order to say...Travel to the Moon...the rocket must travel at least at 7 Miles per second...which is the EARTH'S ESCAPE VELOCITY.

Any velocity less than this will result in the rocket either being captured in an orbit around earth...and such orbit's could be unstable resulting in a decaying orbit where the rocket will orbit for a specific time then fall back to Earth.

The Rocket get's lighter as it travels farther away from Earth but it is the Rocket's Mass that determines it velocity dependent upon applied force of propulsion.

Split Infinity

May 2, 2013
194
14
dthx

10. ### WBahn Moderator

Mar 31, 2012
23,398
7,109
You could travel from hear to the Moon or anywhere else you wanted and never travel faster than an arbitrarily small fraction of the escape velocity.

"Escape velocity", in general, is the velocity that something would have to have if it were to be "fired" from the center of mass of an object and then proceed away in freefall but have sufficient velocity to not return. Another way of saying it is that it is the velocity that an object would have as it passed through the center of gravity were it started from rest at an infinite distance at rest and then fell to the other object.

In common usage regarding Earth, it is generally used to mean the velocity that an unpowered projectile would have to have if, fired straight out from a non-rotating Earth (or fired from one of the poles) to escape Earth's gravity (i.e., travel an arbitrarily far distance from Earth).

To the degree that something is not in freefall (e.g., it has a rocket engine), the notion that it has to exceed the escape velocity in order to leave an object loses meaning. Case in point was the Apollo 10 capsule, which achieved the current record for fasted manned flight relative to Earth at 11.08km/s on its way back. This is close to, but still below, the escape velocity. But, more to the point, it is not comparable to escape velocity because it was not straight out/in relative to the Earth.

The moon missions didn't "escape" the Earth's gravity. After all, the moon is an Earth satellite and, therefore, well within Earth's gravity well. All the moon missions did was transfer to a sufficiently high orbit to place them near enough to the moon to place them within the moon's sphere of influence, which is the region (within the Earth's sphere of influence) in which the moon's influence is actually greater.

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