Acceleration

[fafalone]

So basically you're saying the idea of a downward force and normal force are bunk?

[JaKiri]

What?

[Radical Edward]

so everything is being accelerated all the time then?

[fafalone]

I fail to see what your problem is with what I'm saying.

 

| = ma (a=g, not zero)

v

[] = 0, the TOTAL SUM OF ALL FORCES IS ZERO, HENCE *TOTAL* a = 0

^

| = -ma (a=g, not zero)

 

The object has TWO NON-ZERO ACCELERATIONS FROM FORCES ACTING ON IT, THESE ARE EQUAL AND OPPOSITE SO THE *TOTAL* ACCELERATION IS ZERO.

[Radical Edward]

it has two forces on it, but no acceleration.

[JaKiri]

Acceleration is the result of, well, a resultant force.

 

You can't just split it down.

[fafalone]

F=ma, how does it have a force acting on it but not an acceleration?

 

:sum:F = 0,

:sum:a = 0

[Kedas]

I just think this should also be mentioned in this thread:

F=:lcdelta:(M.v)/:lcdelta:t

 

F=M.a is only valid when M=cte

 

Could someone explain to me how you can do formulas in this forum?

can we do vectors?

 

edit:

Thanks for the info

[fafalone]

Click 'Get More' in the smilies box for our selection of math symbols

[JaKiri]

fafalone said in post #32 :

F=ma, how does it have a force acting on it but not an acceleration?

 

:sum:F = 0,

:sum:a = 0

 

a = :sum:F/m (effectively)

[Kedas]

fafalone said in post #32 :

F=ma, how does it have a force acting on it but not an acceleration?

 

:sum:F = 0,

:sum:a = 0

 

It's a bit the same question as having a sound and an anti-sound (opposite sound).

You can also ask yourself why can't I hear anything when I know there are two sounds.

 

It's only the total sum that counts and that's it.

[JaKiri]

If it's acting on the same point, otherwise you'll get moments and stress.

[YT2095]

forgive my simplistic veiw, but I`ve read so far and only understand 1 or 2 posts (never could get my head around Physics).

but I remmember something about the ball changing energy from Kinetic (moving) and Potential when it`s at its peak (0 velocity). so the telefone has Potential energy as the ball does.

or that something entirely different?

[Kedas]

Potential and Kinetic energy don't say anything about acceleration.

Although kinetic energy has everything to do with movement (incl. velocity)

while potential energy has everything to do with distances/lengths/heights.

[YT2095]

perhaps it`s just me being over simplistic, but I figured through conservation laws stating that energy cannot be created or destroyed only changed, that in effect your ball would be like a "capacitor" or rechargable battery, you throw the ball up with say units 10 of energy and barring energy losses through air friction as heat, you`ll get exactly 10 units back out as a whack when it hits your hand after catching it. to accelerate I understood as the constant application of force over time. the ball to me would only actualy be accelrating while it`s still in your hand up until the point you let go, then it has all the energy it`s going to have. My point being, that it will only accelerate at the transition point of potential to kinetic energy.Gravity in this instance can be treated as rubber band or the like, Air friction ignored, and Time is arbitrary also as specific units.

[Kedas]

I think you should better start a new thread about potential and kinetic energy

[YT2095]

LOL, but then folks will start talking about me, saying I ask too many questions

 

I can`t see how you anyone could rule out potential and kinetic energies and the transtitions between them in this acceleration question, to my understanding, the ball only accelerates while it`s in your hands and also while it begins to fall from it`s max height. perhaps I don`t know enough? I`m not really all that into physics, but his question intrigued me, so I sought to solve it with my limited but not non-existant knowledge (I love stuff like this)

so I see:

ball in hand arm accelerating the ball upwards (kinetic)

hand lets go, ball decelerates (still kinetic)

ball reaches max height and stops (kinetic becomes potential)

potential them becomes kinetic after and undefined moment and accelerates again.

ball hits hand and transfers the energy of the original arm swing back into the hand and arm.

acceleration occurs twice to my understanding.

[Kedas]

about your example:

actually the ball doesn't stop accelerating a single moment.

but the kinetic energy does reach zero at highest and lowest point.

(your arm/body absorbed the energy when you caught it when the kinetic energy was at its high)

[YT2095]

how do you work that out?

acceletartion is a function of increasing velocity over time. some famous guy said something that a body in motion will remain in motion until acted upon by another force (or something like that?)

in this ball idea, it`s a closed system, there are no external forces acting upon it. so it can never increase it`s velocity (accelerate) over time. what you put in, is what you get out. it can only accelerate while it`s in your hand (your putting something IN). and when it begins to fall again (transitionalising between potential to kinetic).

I hope we`re not talking about the Universe expanding at an increased rate or some such thing? I was looking at this as a closed system and not a trick question!?

[Kedas]

there are no external forces acting upon it

 

Uuhm, the force of gravity is unknown to you ?

 

no trick question, unless you see earth as a big trick

[YT2095]

but the force of gravity was previously overcome by throwing the ball up against it in the first place, if it goes UP with 10 units of energy then it won`t come down with 11 units because of gravity? forget gravity, think of it as an elastic band instead, or a spring. as I said, it`s a closed system, no other external input of energy is used to make it accelerate.

[Kedas]

The force of gravity is only 'overcome' when you had the ball in your hand the moment you release it there is notting to 'overcome' gravity and the bal will start to slow down (neg. acceleration)

 

 

This could help you understand PE and KE.

Press the "Potential and Kinetic Energy Experiment"

at this page: http://www.visionlearning.com/library/module_viewer.php?mid=46

[YT2095]

I will certainly have a look at that site, thnx

 

the ball will indeed slow, as the transition shifts from the kinetic to the potential (deceleration), gravity won`t slow? the ball will though, as it makes this transition.

 

My conjecture, is that in a closed system, acceleration can only be born of the transtion of potential to kinetic.

 

try reversing the experiment, imagine you have the ideal rubber ball and no air friction etc... you drop that ball from 1 meter above a solid imovable surface. while you hold this ball before you drop it, it has only Potential energy, when you drop it, the transition is made from potential to kinetic, it hits the floor, compresses to a maximum point (remmember it`s an IDEAL rubber so no heat loss etc...) at this stage, it`s full of potential energy again.

then it makes the transition Potential to Kinetic again and accelerates

a complete 180 degrees from the 1`st experiment, with the same results

I contend that Acceleration is a function of potential to kinetic energy transferance in all models.

[Kedas]

acceleration can only be born of the transtion of potential to kinetic.

 

if the energy in this ideal system is constant and the kinetic energy is defined by it's velocity then obviouly the PE can't change if the KE isn't changing and the KE can only change when it's velocity changes therefor only when it is accelerating can the PE and KE change.

 

This isn't true for the link I gave because the man is adding energy in the system while he is pulling the iron up to make the PE increase.

[YT2095]

I`ll stand by that statement for a closed system actualy

open systems then kinetic to kinetic acceleration is perfectly acceptable too, IE/ I shoot a moving ball bearing with bullet and make it move even faster, but that`s external input and not a closed system as the ball argument is.

even the energy in your arm to propell the the ball is potential, and stored as glucose/ATP or something similar. If you put a small rocket motor on the ball, it`s potential energy Before you light it is stored as chemical energy. or ever a little propeller and motor with a battery, the battery`s still the "Potential" energy... it`s only when it makes that transition from potential to kinetic that acceleration is possible in a closed system.

I`m no Physicist though, so I maybe wrong, but my "gut" tells me I`m not