blike Posted September 5, 2003 Share Posted September 5, 2003 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 Link to comment Share on other sites More sharing options...
IMI Posted September 5, 2003 Share Posted September 5, 2003 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. Link to comment Share on other sites More sharing options...
JaKiri Posted September 5, 2003 Share Posted September 5, 2003 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? Link to comment Share on other sites More sharing options...
JaKiri Posted September 5, 2003 Share Posted September 5, 2003 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. Link to comment Share on other sites More sharing options...
IMI Posted September 5, 2003 Share Posted September 5, 2003 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? Link to comment Share on other sites More sharing options...
Radical Edward Posted September 5, 2003 Share Posted September 5, 2003 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. Link to comment Share on other sites More sharing options...
JaKiri Posted September 5, 2003 Share Posted September 5, 2003 ??????? Link to comment Share on other sites More sharing options...
Radical Edward Posted September 5, 2003 Share Posted September 5, 2003 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. Link to comment Share on other sites More sharing options...
IMI Posted September 5, 2003 Share Posted September 5, 2003 Is the ????? for me or RadEd? Link to comment Share on other sites More sharing options...
JaKiri Posted September 5, 2003 Share Posted September 5, 2003 nm Link to comment Share on other sites More sharing options...
IMI Posted September 5, 2003 Share Posted September 5, 2003 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. Link to comment Share on other sites More sharing options...
blike Posted September 5, 2003 Author Share Posted September 5, 2003 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? Link to comment Share on other sites More sharing options...
JaKiri Posted September 5, 2003 Share Posted September 5, 2003 I said that one above Link to comment Share on other sites More sharing options...
IMI Posted September 5, 2003 Share Posted September 5, 2003 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. Link to comment Share on other sites More sharing options...
JaKiri Posted September 5, 2003 Share Posted September 5, 2003 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. Link to comment Share on other sites More sharing options...
fafalone Posted September 5, 2003 Share Posted September 5, 2003 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. Link to comment Share on other sites More sharing options...
JaKiri Posted September 5, 2003 Share Posted September 5, 2003 It's not accelerating. THERE IS NO RESULTANT FORCE. (also, you can't have a force equal to a mass) Link to comment Share on other sites More sharing options...
IMI Posted September 5, 2003 Share Posted September 5, 2003 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. Link to comment Share on other sites More sharing options...
Kedas Posted September 5, 2003 Share Posted September 5, 2003 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.) Link to comment Share on other sites More sharing options...
fafalone Posted September 5, 2003 Share Posted September 5, 2003 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. Link to comment Share on other sites More sharing options...
JaKiri Posted September 5, 2003 Share Posted September 5, 2003 WHAT DEFINITION OF ACCELERATION ARE YOU USING? It appears different from everyone else's, :lcdelta:v/:lcdelta:t Link to comment Share on other sites More sharing options...
Kedas Posted September 5, 2003 Share Posted September 5, 2003 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 Link to comment Share on other sites More sharing options...
Radical Edward Posted September 5, 2003 Share Posted September 5, 2003 but a=dv/dt .... dv/dt=0 so we have a discrepancy. who is right? Link to comment Share on other sites More sharing options...
fafalone Posted September 5, 2003 Share Posted September 5, 2003 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. Link to comment Share on other sites More sharing options...
JaKiri Posted September 5, 2003 Share Posted September 5, 2003 Acceleration is a result of the NET RESULTANT FORCE. Link to comment Share on other sites More sharing options...
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