Accelerometer and inertial/non-inertial frames of reference (1 Viewer)

[CHUDYMASTER]

I was not present the day my class did the accelerometer experiment. I get the gists of what they did, but I don't understand how an accelerometer can show/distinguish inertial and non-inertial frames of reference.

Could someone please explain?

 

[professor]

what on earth is an accelerometer?

 

[kini mini]

Hi Chudymaster

A quick reminder - in an inertial frame of reference there is no acceleration, there is in a non-inertial frame. An accelerometer measure acceleration and can therefore distinguish between the two.

You could make a crude accelerometer in your car by hanging a cork from your rear view mirror. If you have one of those Elvises who do the pelvic thrusts, they only do their thing when the car accelerates or you touch them, so you could use one of those too

 

[CHUDYMASTER]

Then is it correct to say that because there is increasing displacement with an increase in weight of the pendulum, the cart is in a non-inertial frame?

 

[kini mini]

Originally posted by CHUDYMASTER
Then is it correct to say that because there is increasing displacement with an increase in weight of the pendulum, the cart is in a non-inertial frame?

No - increasing displacement can also result from constant speed .

Taking your example of a pendulum on a cart, if displacement is increasing at an increasing rate , then there will be a force on the pendulum (F = ma) causing it to move from the vertical and showing that the cart is in a non-inertial frame of reference.

 

[CHUDYMASTER]

Oh yeah. So basically, in an inertial frame, if the cart was moved horizontally with uniform speed, then the pendulum would progressively be displaced at EVEN intervals as the mass was incremented.

HOWEVER...if it is in a non-inertial frame of reference (Which it is because of gravity in this example), then the pendulum will be thrust forward due to the force of gravity making IT experience a force. This will result in progressively longer distances each time as more mass is added.

FASCINATING!!! (correct me if I am wrong, please)

 

[Mathematician]

noo..

Im sorry , but i think ur wrong lol.

I can't explain it right now , ill tell u when i can but i don't think its the force of gravity making it move is it? (W=mg)- This force acts on all objects.(clear up kini mini lol)

But answering ur problem , basically the aim of ur class experiment was probably something like : Confirm General relativity.(ie No mechanical experiment can detect motion in an non accelerated environment, that is an inertial frame of reference)

U had to move a trolley cart with a pendulum bob tied to it?

U notice that when ur moving with uniform speed, the pendulum bob doesnt move ( which means u cant distinguish between being at rest or moving with uniform speed)


The pendulum bob(the accelerometer) is equipment used for the mechanical experiment to attempt at detecting motion in a inertial frame of reference. It failed(couldnt distinguish between being at rest or uniform speed). Hence u have confirmed the principle of general relativity.

Example1: U are in the back of a delivery truck(moving with uniform speed) with no windows and the doors are locked.

If u couldnt hear the sound of the engine u couldnt tell the difference between being at rest or with uniform speed(ur body can only detect accelerations)


kini mini:Optical experiment isnt a mechanical experiment lol. So aether model doesnt even apparently violate the principle of general relativity lol.(Its an experiment to do with light.)

Also does the accelerometer experiment really confirm it? Does this account for all mechanical experiment?

CHUDYMASTER: Why do u bother trying to make it sound complicated? u might confuse urself, lol. It wont make ur answers really better. U just need the key words like (accelerometer, Uniform speed etc..) to get the marks.

 

[kini mini]

Originally posted by CHUDYMASTER
Oh yeah. So basically, in an inertial frame, if the cart was moved horizontally with uniform speed, then the pendulum would progressively be displaced at EVEN intervals as the mass was incremented.

No, if the cart were to move at a constant speed the pendulum wouldn't move at all. It's the acceleration that makes it move, i.e. the little shove you give the cart.

HOWEVER...if it is in a non-inertial frame of reference (Which it is because of gravity in this example), then the pendulum will be thrust forward due to the force of gravity making IT experience a force. This will result in progressively longer distances each time as more mass is added.

Gravity can basically be ignored here because it has nothing to do with the cart's motion along the table - the cart is exerting a force on the table and vice versa due to gravity of course, that's true to some extent of every particle in the universe. It's the thrust forward that provides the acceleration we detect using the pendulum. The effect of gravity on the pendulum is simply that it usually hangs straight down.

Originally posted by mathematician

kini mini:Optical experiment isnt a mechanical experiment lol. So aether model doesnt even apparently violate the principle of general relativity lol.(Its an experiment to do with light.)

I don't know what you're referring to in telling me this, the aether model supposes a universal frame of reference for which we have no evidence...

Also does the accelerometer experiment really confirm it? Does this account for all mechanical experiment?

Does it really confirm what?

This thread seems unnecessarily confusing to me, the concept is not too difficult yet people are tying themselves in knots thinking about it. Refer to your textbook(s) as well .

 

[CHUDYMASTER]

Mathematician: I like my english.

Kini Mini and Mathematician: Methinks I get it now. Thanks.

 

[wogboy]

Yep the pendulum will only remain upright while there's no acceleration (i.e. at rest or moving with constant speed). If you hang the pendulum on the ceiling inside the moving object (e.g. a car) and want to measure the horizontal acceleration (a) of the car, you can use the equation:

a = g tan@

where @ is the angle of the pendulum from the vertical.