# Question about relative motion: 3 Golden Retrievers in deep space

## Main Question or Discussion Point

Gravitational effects from large bodies of mass are nonexistent. These golden retrievers are in deep, deep space.

They are named Mr. Squabbles, Alfredo, and Billy.

Mr. Squabbles is suspended in space (at rest)--he has not been acted upon by any force. (If that's possible.)

Alfredo has a rocket booster, and assuming deep below him in space there is marked pavement (something to aid in measuring distance--it can be grass, whatever), he is moving at 100 mph.

Billy has a rocket booster and is going around in circles (around Alfredo and Mr. Squabbles) at varying rates (35-40, 45-50, 20-25mph).

My question: Will Billy, who is "accelerating", feel a "jerk" when he changes his rates of motion ("accelerates" of "decelerates"? If so, why?

I know we--or the pups--can SEE the relative motion, but in deep space, how can any of these goldens actually feel the difference between being at rest or accelerating?

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jtbell
Mentor
in deep space, how can any of these goldens actually feel the difference between being at rest or accelerating?
If they are accelerating, they can feel their rocket boosters pushing against them.

jtbell:

okay, but can you define "push" a little more? Perhaps tell me the significance of the "push"?

Nugatory
Mentor
Gravitational effects from large bodies of mass are nonexistent....
I know we--or the pups--can SEE the relative motion, but in deep space, how can any of these goldens actually feel the difference between being at rest or accelerating?
Proper acceleration (and therefore jerk, which really is the scientific technical term for a change in acceleration with time - that is, the "acceleration of the acceleration") is not relative. You can detect it in an isolated room without any windows or external reference point using an accelerometer: basically hang a heavy object in the middle of the room using six springs, one to each wall. Now if you apply forces with a rocket engine or some such to accelerate the lab in empty space, the springs will stretch as they transfer the force of the rocket engine from the walls to the object.

(Note that such an accelerometer will NOT detect coordinate acceleration, as when an object is free-falling relative to the surface earth. Check the PM reply I sent you a few days ago for example, and consider how the accelerometer will act when on the surface of the earth and when in freefall)

It is weird and to many people somewhat disturbing that speed cannot be defined in any absolute sense while acceleration can be... But it seems to be that way, and space travel has given us plenty of chances to confirm it under free-fall conditions (not perfect as there are still gravitational forces present, but pretty good - no reason to expect greatly different results for astronauts in deep interstellar space).

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Nugatory:

Yes, I'm finding it very disturbing that acceleration can differ from speed in such a way. This is perhaps what is so hard for me to understand.

russ_watters
Mentor
Haven't you ever ridden in a vehicle and experienced the difference between speed and acceleration?

PeterDonis
Mentor
2019 Award
I'm finding it very disturbing that acceleration can differ from speed in such a way. This is perhaps what is so hard for me to understand.
IMO the key is to carefully distinguish between coordinate acceleration and proper acceleration. Proper acceleration is the kind that can be felt, i.e., measured directly. But the term "acceleration" is often used in a way that implicitly means coordinate acceleration, not proper acceleration. For example, a falling rock near the surface of the Earth is said to be "accelerated", but this is only coordinate acceleration: the rock does not feel any acceleration. We, standing on the surface of the Earth, feel acceleration, even though we are at rest with respect to the Earth: this is proper acceleration.

Another way to say it is that the falling rock is weightless, whereas we, standing on the Earth, feel weight. If you want to know whether proper acceleration is present or not for a given observer, ask yourself whether that observer is weightless or feels weight. For your three golden retrievers, as you posed the scenario in the OP, Mr. Squabbles is weightless, Billy feels weight, and I'm not sure about Alfredo because you mentioned that he has a rocket booster but you also mentioned that he is moving at a constant speed relative to a marked pavement, which is in deep space so it is presumably floating freely; this would indicate that Alfredo's rocket booster can't be firing, so he should be weightless as well.

Nugatory
Mentor
Nugatory:

Yes, I'm finding it very disturbing that acceleration can differ from speed in such a way. This is perhaps what is so hard for me to understand.
Ever tried holding a cup of coffee in a moving car? It doesn't matter how quickly the car is moving as long as the road is straight and smooth... But do something to change the cars velocity, such as tapping the brakes or turning the steering wheel, and the coffee will slosh.

ghwellsjr
Gold Member
Gravitational effects from large bodies of mass are nonexistent. These golden retrievers are in deep, deep space.

They are named Mr. Squabbles, Alfredo, and Billy.

Mr. Squabbles is suspended in space (at rest)--he has not been acted upon by any force. (If that's possible.)

Alfredo has a rocket booster, and assuming deep below him in space there is marked pavement (something to aid in measuring distance--it can be grass, whatever), he is moving at 100 mph.

Billy has a rocket booster and is going around in circles (around Alfredo and Mr. Squabbles) at varying rates (35-40, 45-50, 20-25mph).

My question: Will Billy, who is "accelerating", feel a "jerk" when he changes his rates of motion ("accelerates" of "decelerates"? If so, why?

I know we--or the pups--can SEE the relative motion, but in deep space, how can any of these goldens actually feel the difference between being at rest or accelerating?
You have a very incomplete description of your situation. You haven't told us where each retriever is in relation to the others. You haven't told us if Alfredo's rocket is firing. You've just said Billy is going around the other two in circles but why did you include both of them? Couldn't you have just had one retriever going around in circles and experiencing acceleration? When you include extraneous factors with no apparent reason, it makes us wonder what you are thinking or why you bothered to include these other two retrievers. I also don't see the point of the marked pavement.

A simple answer to your last question: when you are sitting at rest at a stop light in your car, you feel only the force of gravity pressing you down in your seat. When you step on the gas, you also feel the force of acceleration pressing you back in your seat. Or if you are traveling at some speed down the road and you make a turn, you feel yourself being pushed to one side of the car. In a weightless environment, you wouldn't feel the downward force but you would feel the backward force when you fire your thruster and it would go away when you turn the thruster off. Do you really have a problem with this?

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Dale
Mentor
My question: Will Billy, who is "accelerating", feel a "jerk" when he changes his rates of motion ("accelerates" of "decelerates"? If so, why?
Yes. Because the rocket pack exerts an unbalanced real force on Billy.

The Great Peter Donis, DaleSpam, and GHWells explained what I was missing. Great job, guys, and thanks! If a mentor wants to close the thread, feel free to do so.

ghwellsjr
Gold Member
The Great Peter Donis, DaleSpam, and GHWells explained what I was missing. Great job, guys, and thanks! If a mentor wants to close the thread, feel free to do so.
It's good that you got clarity but it would be helpful to know what exactly you were missing.

pervect
Staff Emeritus