Author Topic: Acceleration  (Read 5978 times)

Offline benparry

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Acceleration
« on: June 28, 2018, 04:53:51 AM »
Good Morning All

can i just ask a question.

i read on one of the threads here and cannot bloody find it now that an object in space never stops accelerating.

is that correct.

i thought an object needed a force acting upon it to keep accelerating. i have read that gravity could be a force but can it keep accelerating it forever.

cheers

Ben

Offline Halcyon Dayz, FCD

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Re: Acceleration
« Reply #1 on: June 28, 2018, 05:42:54 AM »
Isn't that what a periodical orbit is?
An object continuously and perpetually accelerating towards its primary?
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Offline benparry

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Re: Acceleration
« Reply #2 on: June 28, 2018, 05:53:58 AM »
I'm not sure. as an example if i were in space and fired a bullet out of a gun the power of the gun would allow it to accelerate for a bit, presumably until it reached its top speed but would it accelerate forever.

Offline Northern Lurker

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Re: Acceleration
« Reply #3 on: June 28, 2018, 06:17:38 AM »
Is someone confusing falling and acceleration? Object in orbit is perpetually falling towards primary but because of it's huge speed, surface of primary curves away from object before it hits the ground. Also in non-circular orbit the object is accelerating (trading potential energy to kinetic energy) from apogee to perigee and decelerating (trading kinetic energy to potential energy) from perigee to apogee.

Lurky

Offline benparry

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Re: Acceleration
« Reply #4 on: June 28, 2018, 06:20:12 AM »
Sorry Lurky that has gone straight over my head lol

so are you saying that objects cannot constantly accelerate without a force acting on it

Offline Trebor

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Re: Acceleration
« Reply #5 on: June 28, 2018, 06:24:14 AM »
Sorry Lurky that has gone straight over my head lol

so are you saying that objects cannot constantly accelerate without a force acting on it

There is still gravity in space, and gravity would still apply a force.

Offline Northern Lurker

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Re: Acceleration
« Reply #6 on: June 28, 2018, 07:38:50 AM »
Sorry Lurky that has gone straight over my head lol

so are you saying that objects cannot constantly accelerate without a force acting on it

Yes, acceleration of mass always requires force.

as Trebor said, the force of gravity acts on our object. The object has both kinetic and potential energy. Kinetic energy is from the speed of the object and potential energy is from the height of the object from it's primary's center of gravity. An ellipse shaped orbit has a point of maximum distance called apogee and point of minimum distance called perigee.

At the perigee, the object has minimum amount of potential energy and maximum amount of kinetic energy. After passing the perigee, the height (potential energy) starts to increase and the speed (kinetic energy) starts to decrease until the apogee is reached. There the potential energy is at maximum and kinetic energy is at minum. After passing the apogee potential energy starts to decrease and kinetic energy to increase until perigee is reached again.

Total energy of the system stays the same but the ratio between kinetic and potential energy changes. Also notice that while the object is always under acceleration, only the "downhill side" of orbit increases the speed and the "uphill side" of orbit is deceleration or slowing down.

I hope this helps and sorry, English is not my primary language

Lurky

Offline benparry

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Re: Acceleration
« Reply #7 on: June 28, 2018, 09:08:50 AM »
ah ok so basically without gravity the object would simply move at the same speed not accelerate whereas if the object were in an orbit it would accelerate but not forever.

Offline JayUtah

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Re: Acceleration
« Reply #8 on: June 28, 2018, 10:04:00 AM »
Without the context it's impossible to determine what may have been meant by the prior statement you refer to.  Usually when we say "in space" we mean in a purer Newtonian environment unaffected by nearby bodies or air resistance.  In that case a bullet fired from a gun would accelerate to its muzzle velocity by the propulsion of the expanding gas and the continue in a straight line at a constant velocity.  (There would be a force on the gun too, because of conservation of momentum.  The gun would accelerate in the rearward direction until the bullet left the muzzle.)  But in practical terms, anywhere in the solar system, motion is governed by orbital mechanics, of which gravity is a big part.  In most cases a bullet fired from a gun anywhere in the solar system would enter some kind of solar orbit.  And has has been explained, orbits involve constantly changing velocity.

When we fly actual spacecraft we differentiate between orbital flight and "accelerated" flight because different computer models are needed to maintain the spacecraft's state vector, its understanding of position and velocity.  But velocity changes in both models.  In orbital flight there is no reading on the ship's accelerometers because their reference masses are affected by the same orbital forces as the surrounding ship.  Velocity changes are deduced based on where we compute the ship is along the orbits modeled in its flight plan, largely a function of time.  In accelerated flight, the engines are used.  The ship accelerates according to a force that is coupled to the reference masses in the accelerometers only in a way that allows acceleration to be measured, then integrated over time to obtain velocity.
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Offline benparry

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Re: Acceleration
« Reply #9 on: June 28, 2018, 10:25:32 AM »
Without the context it's impossible to determine what may have been meant by the prior statement you refer to.  Usually when we say "in space" we mean in a purer Newtonian environment unaffected by nearby bodies or air resistance.  In that case a bullet fired from a gun would accelerate to its muzzle velocity by the propulsion of the expanding gas and the continue in a straight line at a constant velocity.  (There would be a force on the gun too, because of conservation of momentum.  The gun would accelerate in the rearward direction until the bullet left the muzzle.)  But in practical terms, anywhere in the solar system, motion is governed by orbital mechanics, of which gravity is a big part.  In most cases a bullet fired from a gun anywhere in the solar system would enter some kind of solar orbit.  And has has been explained, orbits involve constantly changing velocity.

When we fly actual spacecraft we differentiate between orbital flight and "accelerated" flight because different computer models are needed to maintain the spacecraft's state vector, its understanding of position and velocity.  But velocity changes in both models.  In orbital flight there is no reading on the ship's accelerometers because their reference masses are affected by the same orbital forces as the surrounding ship.  Velocity changes are deduced based on where we compute the ship is along the orbits modeled in its flight plan, largely a function of time.  In accelerated flight, the engines are used.  The ship accelerates according to a force that is coupled to the reference masses in the accelerometers only in a way that allows acceleration to be measured, then integrated over time to obtain velocity.

great stuff

thanks a lot Jay

Offline Jason Thompson

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Re: Acceleration
« Reply #10 on: June 28, 2018, 11:03:54 AM »
i thought an object needed a force acting upon it to keep accelerating. i have read that gravity could be a force but can it keep accelerating it forever.

Just worth a reminder here that, while 'acceleration' is commonly used to refer to increasing speed, acceleration is properly defined as any change in velocity, not speed. Velocity is a speed and a direction. Given this, slowing down is an acceleration, and so is changing direction. Essentially what that means is that any object not on a straight line path at a fixed speed is accelerating. An object in circular orbit is constantly accelerating even if its speed is constant because it is moving in a circle, i.e. constantly changing direction. In space, due to the gravitational infuences on every object, nothing really travels in a straight line at fixed speed.

This is one of those cases where lay use and actual definition vary, so care is needed when describing it.
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Offline benparry

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Re: Acceleration
« Reply #11 on: June 28, 2018, 11:16:52 AM »
Ah well maybe thats what the original quote i remember actually meant Jay. thanks Jason.

Offline JayUtah

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Re: Acceleration
« Reply #12 on: June 28, 2018, 12:07:41 PM »
Could be.  As Jason points out, the engineering use of "accelerated" flight is a misnomer.  Acceleration, in the pure mathematics of physics, is nothing more exotic than the first-order time derivative of the velocity vector in some reference frame.  It can be computed just as easily for pure orbital motion as it can be for powered flight.  So from a pure physics standpoint some of those thought experiments still make sense.  If the reference frame is, say, some part of the ISS, then some object moving within it and reckoned according to it could exhibit certain "pure" Newtonian motion -- if the measurement is casual and air is ignored -- even though the whole kit and kaboodle is whipping around the Earth in orbital motion.  That's why any question regarding velocity and acceleration is met with the retort question, "...reckoned according to what?"

For powered flight very near some large astronomical body, the body is often considered the fixed point against which velocities (and their accompanying accelerations) are measured, because orbital motion around that body is what dominates your dynamics and engine maneuvers are most often designed to affect that orbit.  You ignore the effects of other, more distant, orbiting bodies, or bodies around which your entire system is further orbiting.  For orbits around the Earth, Earth can be considered fixed.  Same for the Moon.  For transfer orbits, you need a multi-body solution.  In Apollo this was too much heavy lifting for the AGC, so it used an approximation of what's called the "restricted" three-body problem.  In that model, the two principal bodies (Earth and Moon) together determine the orbital path of the third body (the spacecraft), and the spacecraft's mass is assumed to be negligible -- i.e., the paths of Earth and Moon are not measurably affected by the third body.  In practice this was implemented first as an Earth-fixed model with adjustments for the increasing effects of the Moon's gravity, then as a Moon-fixed model with adjustments made for the decreasing effects of Earth's gravity.  There comes a point at which the computer switches models and, due to the errors in approximation, seems to "jump" instantly from one reckoned position to another.  Then of course you have observations from Earth of the spacecraft's position and velocity along the vector from the receiving antenna, so that provides a toehold for Mission Control mainframes to more finely fix the spacecraft's state vector, which can then be "poked" as needed into the AGC.

It's important in understanding Apollo engineering to realize that the computer had to use these various synthetic models deduced from orbital mechanics, and then separately a a closed-loop measured method when under power.  The orbital model needed to update the state vector only once every several seconds in order to maintain an accurate knowledge of where it was in space.  But under "accelerated" flight, you needed to poll the accelerometer data ten times a second or so in order to capture any vicissitudes of engine operation and update the state vector.  They're qualitatively and quantitatively very different solutions -- both necessary in order to fly in space.  But it masks the purity of what we sometimes consider in thought experiments carried out in space.
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Offline ka9q

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Re: Acceleration
« Reply #13 on: June 29, 2018, 07:38:48 AM »
And then there's Einstein's view of gravity, which is that it exists as an upward acceleration while you're standing on the earth, but not when you're free-falling in an orbit around it...

Offline bknight

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Re: Acceleration
« Reply #14 on: June 29, 2018, 10:07:31 AM »
And then there's Einstein's view of gravity, which is that it exists as an upward acceleration while you're standing on the earth, but not when you're free-falling in an orbit around it...

Upward?  Shouldn't that be downward?
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