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 :

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 :

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 :

 

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

[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.