Acceleration

[blike]

If I throw a ball straight up into the air, is it still accelerating when its velocity is 0 (at the peak of its trajectory)?

I say yes...but I just want to be sure

[IMI]

If you throw the ball straight up it will come straight back down. Therefore, it won't have a trajectory. Trajectory is associated with curved travel. Aside of that, if it's velocity is 0 how can it still be accelerating? If only for an instant it stops moving all together and begins falling. In fact, as it moves upwards from the point of launch it is losing speed vice accelerating. It won't begin accelerating until it starts falling.

[JaKiri]

It is still accellerating. At the peak of its trajectory, the velocity is instantaneously 0.

IMI: Why would gravity stop when its at the peak?

[JaKiri]

An additional argument: If someone isn't accelerating when it's stationary, how would anything EVER move?

ps

:lint:0dt can still equal a constant, for the maths.

[IMI]

I see, so you are saying that although something is travelling upwards, and losing speed in that direction, it is in the process of accelerating downwards?

[Radical Edward]

IMI said in post #2 (http://www.scienceforums.net/forums/showthread.php?s=&postid=21210#post21210):
If you throw the ball straight up it will come straight back down. Therefore, it won't have a trajectory. Trajectory is associated with curved travel. Aside of that, if it's velocity is 0 how can it still be accelerating? If only for an instant it stops moving all together and begins falling. In fact, as it moves upwards from the point of launch it is losing speed vice accelerating. It won't begin accelerating until it starts falling.

of course it is accelerating when it is at zero velocity, it's just derivitives and stuff.

[JaKiri]

???????

[Radical Edward]

IMI said in post #5 (http://www.scienceforums.net/forums/showthread.php?s=&postid=21214#post21214):
I see, so you are saying that although something is travelling upwards, and losing speed in that direction, it is in the process of accelerating downwards?

yeah, it is changing it's velocity from +v to -v acceleration is a change in velocity dude.

[IMI]

Is the ????? for me or RadEd?

[JaKiri]

nm

[IMI]

Radical Edward said in post #8 (http://www.scienceforums.net/forums/showthread.php?s=&postid=21217#post21217):


yeah, it is changing it's velocity from +v to -v acceleration is a change in velocity dude.

I never looked at it that way. Very cool. Makes sense. If something is moving up, and losing speed, it is getting closer to coming back down and hence accelerating.

[blike]

Thanks for the answers.

This question came up in physics class, and the physics professor said "i'll leave that for you to look into". I instantly thought the answer was yes, it is still accelerating, but the majority of the class agreed it was not. I used the argument that if it wasn't accelerating, and its velocity was 0, then why would it move?

[JaKiri]

I said that one above :cool:

[IMI]

So, is the phone on my desk accelerating towards hitting the floor? It isn't moving but if it will ever be knocked to the floor in the future then every second it is closer to that happening.

[JaKiri]

it's not accelerating at the moment, because there's no resulting force.

On an object in mid air, there is a resulting force (gravity) so there's an acceleration. (F=ma)

The desk is providing a normal reaction to your phone, so there's no force.

[fafalone]

Yes, it's acclerating, but the table is exerting an upward force on it equal to the mass of the phone * acceleration due to gravity.

[JaKiri]

It's not accelerating. THERE IS NO RESULTANT FORCE. (also, you can't have a force equal to a mass)

[IMI]

I know, sorry, I was just playing devil's advocate. Good thread though, I learned something today...a new way to look at things. Thanks all.

[Kedas]

You know the equation F=M.a ??
Well, if F isn't 0 then it's accelerating.

About your phone you have F of gravity but you have also -F from the table the sum is 0 meaning acceleration is 0.

edit:
I should press the reload button before pushing the sent button :)
(damn, you all respond fast.)

[fafalone]

The downward force on the object is mg, f=ma, A IS NOT ZERO. THe normal force is ma in the +y direction, =mg, so the NET acceleratation is zero, but there is an acceleration.

[JaKiri]

WHAT DEFINITION OF ACCELERATION ARE YOU USING?

It appears different from everyone else's, :lcdelta:v/:lcdelta:t

[Kedas]

acceleration has the same effect as gravity but that doesn't mean that what is attracted by gravity is accelerating.

edit:
if I could just ones post faster than MrL_JaKiri ;)

[Radical Edward]

but a=dv/dt .... dv/dt=0 so we have a discrepancy. who is right?

[fafalone]

It all goes back to there being two equal and opposite forces. v=0i+(mgt+-mgt)j. There are two forces which cause two accelerations that make to total accleration 0.

[JaKiri]

Acceleration is a result of the NET RESULTANT FORCE.

[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 (http://www.scienceforums.net/forums/showthread.php?s=&postid=21248#post21248):
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 (http://www.scienceforums.net/forums/showthread.php?s=&postid=21248#post21248):
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 :-P

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 :)

[Kedas]

for a completely closed AND ideal system you are right like I said in my previous message.

[YT2095]

Kedas, LOL, a new page already mate :)
we`re doing well :) I`ve gotta hand it to Blike, he sure knows how to start off a good topic :)

[spacetime1]

If I throw a ball straight up into the air, is it still accelerating when its velocity is 0 (at the peak of its trajectory)?

I say yes...but I just want to be sure


YOU ARE RIGHT

www.geocities.com/physics_all/index.html

[Nirav]

If I throw a ball straight up into the air, is it still accelerating when its velocity is 0 (at the peak of its trajectory)?

I say yes...but I just want to be sure


Well... if i tell u honestly.... the ball even upon reaching its peak( where its speed is relatively zero to us) it can be considered to be accelerating........
this is given by the principle of equivalence( of A.Einstein) .... which tells that every object within the space- time curvature can be considered to b accelerating.
so there 's ur answer man!

[ydoaPs]

it is accelerating. it's acceleration is -9.81m/s/s

edit: for future reference, don't use geocities as a rescource.

[pi_of_9]

I have a question that deals with acceleration but did not want to start a new thread as the question has probably already been addressed.

If there was a large hole that extended from the North Pole through the center of the Earth to the South Pole and a person fell in...would he fall all the way through or stop at the center? If he stops at the center would he move up and down like a bungie jumper until he comes to rest?

[Callipygous]

I have a question that deals with acceleration but did not want to start a new thread as the question has probably already been addressed.

If there was a large hole that extended from the North Pole through the center of the Earth to the South Pole and a person fell in...would he fall all the way through or stop at the center? If he stops at the center would he move up and down like a bungie jumper until he comes to rest?

im guessing were assuming he survives things like pressure and heat and all that? he would pass though and start moving "up" on the other side untill the gravity slowed him enough to pull him back, like the bungie jumper. he would eventually stop due to friction with air.

[Cap'n Refsmmat]

If you did it on the Moon, however, with no air friction, he would just keep going from one end to the other without stopping.

[YT2095]

last 2 posts are wrong. in the center (use a stone instead, it gets around the survivability probs) it`ll reach the middle , pass along it, be drawn back and eventualy reach equlibrium, with the graviation field, pulling equal in all directions.
it will have effectively acheived gravitational rest mass.

[Callipygous]

how does that make my post wrong?

and actually, with no air resistance it wont. just like with no air resistance a ball thrown straight up(or up at all, it doesnt even need to be straight) will come back down at exactly the same speed.

[YT2095]

we`re talking the effects of gravity here, air resistance plays no significant part when falling to the center of a planet (Life Insurance may do though) :)

[Cap'n Refsmmat]

last 2 posts are wrong. in the center (use a stone instead, it gets around the survivability probs) it`ll reach the middle , pass along it, be drawn back and eventualy reach equlibrium, with the graviation field, pulling equal in all directions.
it will have effectively acheived gravitational rest mass.
Say what?
In the middle gravity will not affect it at all, since it will balance out. As you get farther out from the core gravity slows you down until at the top you stop. Then you fall right back down again.

[Callipygous]

we`re talking the effects of gravity here, air resistance plays no significant part when falling to the center of a planet (Life Insurance may do though) :)

air resistance is the only thing that makes it so he will eventually stop. air resistance creates this thing called terminal velocity, without which he would build enough speed on his plunge to overcome the gravity at the center and "fall" back up to exactly the same height again.

[swansont]

Oscillate. Damped oscillation, in the presence of air.

Fg=-GMm/r2, but for a uniform density, M varies as r3, since only the enclosed volume of mass contributes to the pull (Gauss's law)

So you end up with something of the form F=-kx, which is Hooke's law, which applies to springs, etc. Simple harmonic motion.

[Callipygous]

Oscillate. Damped oscillation, in the presence of air.

Fg=-GMm/r2, but for a uniform density, M varies as r3, since only the enclosed volume of mass contributes to the pull (Gauss's law)

So you end up with something of the form F=-kx, which is Hooke's law, which applies to springs, etc. Simple harmonic motion.

super...

english?

[jordan]

Assuming no friction, the person with fall back and forth forever taking the same time on each trip back and forth.

[[Tycho?]]

Wouldn't some energy be lost due to gravitational waves?

[swansont]

super...

english?

No, American. Why do you ask? ;)

The person would always feel a restoring force toward the center, if they are not at the center. And the force gets larger as they move away. In the absence of friction, or other dissipative force, this means that they would oscillate back and forth.

[RICHARDBATTY]

At the stop point the inertial forces and gravity cancel so there is no acceleration.

[jordan]

Don't you mean there is no velocity. Acceleration would be constant in this.

[Callipygous]

no, there would be no velocity at either end, there should be no acceleration for one instant in the middle, when no forces are acting on him, then his velocity carries him through and he starts accelerating back toward the center again.

[cyeokpeng]

Just think it like this.

Though at the peak of its path its intstantaneous velocity is zero, take an elemental time after that. Its velocity will start to increase but in the opposite direction, since it is under the influence of gravitational acceleration g. So in effect, it still is accelerating at the peak, or else it would stay stationary there and not fall down, which you wil never see it happening like this.

[Panic]

yes there is accelleration .....

Acceleration is continuous and constant.

just because a function has a maximum or minimum does not mean it stops there.

the object reaches 0 m/s for only and infinitly small amount of time... it does not hang there like jordan!

[Callipygous]

acceleration is not continuous. there is no acceleration for one instant at the very bottom. it hits 0m/s at the peak(after falling back up), but it hits 0m/s/s at the very middle, when there is an equal gravitational force on all sides.

note that there are two situations in question here. the first post about throwing a ball straight up and the second question about falling through the center of the earth. just incase we are talking about different things here. im talking about falling through the middle.

[JaKiri]

there is no acceleration for one instant at the very bottom.

Net acceleration anyway.

[Mart]

Net acceleration anyway.

Did you mean not acceleration any way - like in any direction?

[Callipygous]

Did you mean not acceleration any way - like in any direction?

net acceleration. there is acceleration from gravity, its just cancelled out by an exactly opposite force, therefor 0 net acceleration.