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Location:
Melbourne
Registered:
October 2003
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Physicist please read.
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Thu, 07 April 2005 05:26
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Barry throws a ball into the air. First consider the ball while it remains in Barry's hand. (ignore air resistance)(and barry is on earth)
Kim says barry has exerted a greater force on the ball than the ball exerts on Barry and hence the ball accelerates. Barry says kim has got it wrong and the ball exerts the same force as his hand. Secretly Barry is worried because he knows the ball shouls remain stationary or at constant velocity if the forces are equal. Kathy says they are both wrong, and that the reaction force to Barry's hand is gravity so as long as Barry exerts more force than gravity does he can throw the ball.
Who is right, and who is wrong?
this should seperate the men from the boys.
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Location:
Campbelltown
Registered:
November 2003
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Re: Physicist please read.
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Thu, 07 April 2005 05:28
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Newton's Third law for evrey action there is an equal yet opposite reaction, hence there will be an equal force actng on his arm as there is on the ball.
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Location:
Melbourne
Registered:
June 2004
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Re: Physicist please read.
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Thu, 07 April 2005 05:34
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Barry is right, however since He is pushing the ball (and the ball is pushing back) much harder than gravity, the ball will get airborne for a short time.
The ball's reaction force is being transmitted to Barry's hand, which then transmits it through Barry to the Earth, which actually moves very very slightly away from the ball.
Yup - Bazza's moving the earth!
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Location:
Tamworth
Registered:
October 2002
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Re: Physicist please read.
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Thu, 07 April 2005 06:09
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wow just reading that makes my head hurt let alone thinking about it
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Location:
Land of Oz
Registered:
June 2004
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Re: Physicist please read.
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Thu, 07 April 2005 06:19
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Hi,
Kim is wrong, Barry and Kathy are only partly correct.
As RT142 mentioned, Newtons 3rd law states that the ball is exerting the same energy back into Barry as he throws the ball.
Gravity has nothing to do with Newtons 3rd law. So, in effect, yes he is exerting more energy than gravity (as Kathy said) to get the ball into the air. However, the energy Barry used to get the ball into the air is also directed back into him.
Kim is wrong because of the 3rd law and Barry, having much more mass than the ball, probably doesn't feel the "opposite and equal" effect on him - but it's there nonetheless.
Barry, would be correct if he was in a zero-gravity situation. Once the ball was thrown it would reach its terminal velocity and maintain it (that is, the velocity of his hand at the moment of release).
seeyuzz
river
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Location:
Brisbane
Registered:
October 2004
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Re: Physicist please read.
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Thu, 07 April 2005 06:38
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WTF?!?! this is stupid....
the ball has a mass which is not important since we're not considering air resistance... and a velocity at any point in time... barrys hand also has a velocity at any point in time, but it's only important up to the point where the ball leaves barrys hand...
gravity is constant 9.8m/s/s (meters per second, per second) on earth, air resistance is nill...
newtons 3rd law is only true for systems in equilibrium, IE, stationary, or constant speed. as soon as forces are unbalanced ie, barry hurls the ball up, then obviously there's accelletarion for a time, say barry accelerates the ball to a speed of 20m/s immediatly after the ball leaves barrys hand it starts to slow down at the rate which gravity acts on it, 9.8m/s/s there for it will reach it's maximum height of 20.29m at t=2.04sec (where t=0 is where the ball left barrys hand)
the moral of the story is:
Kim is right, but she's talking about when barry throws the ball,
Barry is right, but he's talking about the ball stationary in his hand.
kathy is both right and wrong, she's wrong 'cause she says the others are wrong, but she's right 'cause in this case the reactive force to barrys hand is the weight of the ball multiplied by the force due to gravity. hence, as kim stated if barry overcomes this force, then he'll accelerate the ball.
basically your all right, but your talking about the senario at different stages, or saying it in a different way...
anyone care to argue that, can go and unbalance their forces over a cliff
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Location:
Melbourne
Registered:
October 2003
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Re: Physicist please read.
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Thu, 07 April 2005 15:02
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| hickoz_bro wrote on Thu, 07 April 2005 16:38 |
WTF?!?! this is stupid....
the ball has a mass which is not important since we're not considering air resistance... and a velocity at any point in time... barrys hand also has a velocity at any point in time, but it's only important up to the point where the ball leaves barrys hand...
gravity is constant 9.8m/s/s (meters per second, per second) on earth, air resistance is nill...
newtons 3rd law is only true for systems in equilibrium, IE, stationary, or constant speed. as soon as forces are unbalanced ie, barry hurls the ball up, then obviously there's accelletarion for a time, say barry accelerates the ball to a speed of 20m/s immediatly after the ball leaves barrys hand it starts to slow down at the rate which gravity acts on it, 9.8m/s/s there for it will reach it's maximum height of 20.29m at t=2.04sec (where t=0 is where the ball left barrys hand)
the moral of the story is:
Kim is right, but she's talking about when barry throws the ball,
Barry is right, but he's talking about the ball stationary in his hand.
kathy is both right and wrong, she's wrong 'cause she says the others are wrong, but she's right 'cause in this case the reactive force to barrys hand is the weight of the ball multiplied by the force due to gravity. hence, as kim stated if barry overcomes this force, then he'll accelerate the ball.
basically your all right, but your talking about the senario at different stages, or saying it in a different way...
anyone care to argue that, can go and unbalance their forces over a cliff
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not to sure about this. I think you've got a good idea, but get lost using the wrong terms.
for starters, Kim is never right at any time. A quick look at the 3rd law will show this.
Corona RT142 statement is correct.
St184 makes a notable point about the reaction of the earth moving away from the ball.
And River (who I was secretly hoping would join this discussion) is also correct.
However I think barry just confused: "the ball should remain stationary or at constant velocity if the forces are equal" this is true if the SUM of all forces acting on an object are equal. The force barry initially mentions is the 'Action (hand on ball) - Reaction (ball on hand) force. This action-reaction pair of forces aren't the only forces in the 'isolated' system.
Its kim's statement that doesn't quite make sense.
"the reaction force to Barry's hand is gravity", I'm not convinced. The reaction to barry's hand moving up is the earth moving away from his hand. However if talking about the action of barry's hand applying a force to the ball and moving it up, then the REaction is the ball applying a force that moves barry down (effectively this is so minute it's considered not to exist). But by using [b] "so" [b] in "so as long as Barry exerts more force than gravity does he can throw the ball" it would appear that she uses part one of her sentence (which is correct) to back up part two. Even though part two is correct (I'm pretty sure) part one doesn't prove.
All barry has to do is apply a force with a greater magnitude than Gravity to throw the ball up.
hence, Kim is also right (dependent on the interpretation of the queston)
I invite people to submit there suggestions. I won't of course give everyone the same attitude as I gave hickoz_bro...but if you submit with the same arrogance as hickoz-bro...what do you expect.
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Location:
Melbourne
Registered:
June 2004
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Re: Physicist please read.
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Fri, 08 April 2005 00:54
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| Quote: |
"Kim says barry has exerted a greater force on the ball than the ball exerts on Barry and hence the ball accelerates."
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Close: Barry has exerted a greater force on the ball than GRAVITY exerts on the ball, hence the ball accelerates.
i.e. Barry exerts a NET force on the ball
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"Barry says kim has got it wrong and the ball exerts the same force as his hand. Secretly Barry is worried because he knows the ball should remain stationary or at constant velocity if the forces are equal."
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Barry's statement is correct, but Barry is confused about the difference between reactionary force (the ball pushing back against Barry's hand) and the NET force (which determines acceleration)
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"Kathy says they are both wrong, and that the reaction force to Barry's hand is gravity so as long as Barry exerts more force than gravity does he can throw the ball."
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Kathy stopped taking her medication a week ago.. . Don't listen to anything Kathy says.
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Toymods Social Secretary
Location:
Sydney
Registered:
July 2002
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Re: Physicist please read.
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Fri, 08 April 2005 03:33
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Location:
Tamworth
Registered:
October 2002
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Re: Physicist please read.
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Fri, 08 April 2005 03:51
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oh now i get it that picture helped out heaps 
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Location:
Melbourne
Registered:
June 2004
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Re: Physicist please read.
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Fri, 08 April 2005 04:07
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"The WITZL"s post is too good, so I had to report him - even though He's a moderator:
| Dobbing in teh WITZL |
It's not fair that Moderators steal the thunder on threads, especially when they obviously have the time to spend making a post this funny, as well as actually explaining the issue that started the thread.
What's wrong with leaving us plebs to fight it out amongst ourselves, unenlightened by moderator-wit?
Thankyou 
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Location:
Brisbane
Registered:
October 2004
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Re: Physicist please read.
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Fri, 08 April 2005 09:24
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T APLUS 22:- firstly i defend my arrogance, by saying, i never stated that "stupid" is bad... lol... i love stupid, it's what i live for, that's why i bothered replying,
anyhow, i didn't quite follow what you said about KIM,
okay, lets break this up into teeny tiny bits... we have the ball, barrys arm, and the earth..
the ball exerts a force on barrys hand (downwards) this force is supplied by the earth (gravity). it also exerts gravitational forces of its own, but having such a small force we call this force... tiny, little... insignificant, same is the case with barrys hand.
barrys hand exerts a force on the ball, (supplied by his copius muscles ) this force must be equal to the WEIGHT* (not mass) of the tennis ball AND his hand/arm, in order to hold the ball steady. this is called equilibrium. all the down forces equal all the up forces.
gravity x (mass of arm + mass of tennis ball) = Force exerted by bazza's arm.
now, in order for the tennis ball to leave bazza's hand he needs to create an unstable system, here we have two options he can either increase or decrease the force applied to the ball. in this case he'll increase the force, but what happens here, is as he increases the force, the reaction force increase, since the downward force on the ball increases (gravity + resistance to accelleration) so in order to accellerate the ball barry needs to overcome the new system of forces, which changed the system of forces, and this is why we have an unstable system, basically he needs to supply a force greater then the combined gravity and resistance forces so as to make the ball go fast...
once the ball leaves bazza's hand, there remains only one force on it, gravity, which means the ball slows down, comes to a stop, then accellerates up again, until it hits barry.
the key to newtons laws are that they work ONLY in a stable system, ie equilibrium. if every force had an equal and opposite reaction force, then nothing would accellerate ever, and we'd never enjoy such delights as twin turbos 
THE WITZL:- lol... i hadn't seen any "Kath & Kim" connection until you pointed it out then 
*Weight is defined as the MASS multiplied by the Force due to Gravity... Mass is constant, reguardless of environment (moon, or earth)
apologies to all those who are acctually worse off after reading that
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Location:
Land of Oz
Registered:
June 2004
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Re: Physicist please read.
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Fri, 08 April 2005 10:36
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Hi,
| hickoz_bro wrote on Fri, 08 April 2005 19:24 |
the key to newtons laws are that they work ONLY in a stable system, ie equilibrium. if every force had an equal and opposite reaction force, then nothing would accellerate ever, and we'd never enjoy such delights as twin turbos
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Absolute Rubbish!
If there "wasn't an equal and opposite force" then nothing would accelerate.
I suggest you go and have a look at the following site that will illuminate you in Newtons Three Laws.
http://www.glenbrook.k12.il.us/gbssci/phys/Class/n ewtlaws/u2l1d.html#balanced
seeyuzz
river
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Location:
Brisbane
Registered:
October 2004
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Re: Physicist please read.
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Sat, 09 April 2005 08:10
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| river wrote on Fri, 08 April 2005 20:36 |
If there "wasn't an equal and opposite force" then nothing would accelerate.
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okay... ummm... i got no idea what your on about there, lets just say there's a gross misunderstanding...
if i get what your saying here, you mean that if we have a body, with a force supplied from the left, and a lesser (or no) reaction force (not opposite and equal) as in CASE A then the object WON'T accellerate??? where as if there IS an equal and opposite force, then the object WILL accellerate???
just take a squizz at the image, and tell me which way the object in CASE B will accelerate...
btw, the "hand of god" force is just a simplified way of saying there's a force applied somehow
in the LINK you supplied, there are a few key point:
| Care of "The Physics Classroom" |
There is an unbalanced force; and as such, the book changes its state of motion. The book is not at equilibrium and subsequently accelerates.
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| Care of "The Physics Classroom" |
Since these two forces are of equal magnitude and in opposite directions, they balance each other. The book is said to be at equilibrium.
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i'm sure you could bring that info to an argument like this, and expect to be right, i'm sure we've just been misunderstood.
later
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Location:
Melbourne
Registered:
October 2003
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Location:
Brisbane
Registered:
October 2004
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Re: Physicist please read.
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Sun, 10 April 2005 10:47
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"Some idea of mechanics" hahahha... you might say that.... 4th year Mech eng... yeah, we get a bit of that kinda stuff....
anyhow, the reaction force on the block experience during acceleration is known as Inertia, basically the blocks desire to stay still. on top of this you have a building resistance caused by things like friction between the block and the surface as well as air friction, once the applied force equals the resistance forces we have equilibrium once again, at this time inertia = 0.
and please note, like i said earlier 99% of laws etc rely on the system being in equilibrium, while a system is unbalanced then your friction forces don't have a single number to represent them, all reaction forces need equations for representation. i have no idea what your background of physics/mechanics is, but newtons 1 and 3 rely on the system being in equilibruim,
2 is really the key to this argument in a way... F=MA. since M (mass) is constant, you need F (force) to get A (accelleration). however as we mentioned before you have the forces I (inertia)......... *thinking thinking*
hmmm... perhaps you could say that the opposite force during accelleration IS inertia, but then the point with that i fail to see is this inertia would then make the system of forces balanced... therefore we'd have no accelleration 
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I Supported Toymods
Location:
south Melbourne/KL
Registered:
June 2004
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Re: Physicist please read.
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Mon, 11 April 2005 01:49
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damn u guys, im in uni taking a break and looking up toymods just to get more velocity, acceleration, g=9.81 crap on here!!! 
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Location:
Melbourne
Registered:
October 2003
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Re: Physicist please read.
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Mon, 11 April 2005 08:43
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"Engineers solve problems, Physicists stop asking the wrong questions."
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Location:
Land of Oz
Registered:
June 2004
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Re: Physicist please read.
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Tue, 12 April 2005 06:35
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Hi,
| hickoz_bro wrote on Sun, 10 April 2005 20:47 |
and please note, like i said earlier 99% of laws etc rely on the system being in equilibrium, while a system is unbalanced then your friction forces don't have a single number to represent them, all reaction forces need equations for representation.
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I still don't understand why these laws apply to only systems in equilibrium. Can you point me to somewhere on the net that specifically states this is the case?
seeyuzz
river
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Location:
Melbourne
Registered:
October 2003
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Re: Physicist please read.
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Tue, 12 April 2005 13:35
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| river wrote on Tue, 12 April 2005 16:35 |
Hi,
| hickoz_bro wrote on Sun, 10 April 2005 20:47 |
and please note, like i said earlier 99% of laws etc rely on the system being in equilibrium, while a system is unbalanced then your friction forces don't have a single number to represent them, all reaction forces need equations for representation.
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I still don't understand why these laws apply to only systems in equilibrium. Can you point me to somewhere on the net that specifically states this is the case?
seeyuzz
river
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Maybe he means
"99% of the questions in the textbook rely on the objects being
in equilibrium."
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