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When does a ball enter free fall? 
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#1
Nov2512, 06:57 PM

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So in a physics lab, we threw a ball upwards and recorded its acceleration.
If you look at the graph there is actually a period of time where it goes from positive acceleration to 9.8 m/s2. But how is that possible? Before I let go of the ball, it experiences a positive acceleration because I am applying a force causing the net force to be up, but as soon as I let go, the only force acting on it is gravity, so wouldn't the acceleration have to jump from some positive number to g? 


#2
Nov2512, 09:16 PM

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#3
Nov2612, 09:06 AM

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#4
Nov2612, 10:57 AM

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When does a ball enter free fall?



#5
Nov2612, 11:15 AM

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The acceleration does not have to be a continuous function of time. It can change discontinuously. Only the velocity and displacement need to be continuous functions of time.



#6
Nov2612, 11:51 AM

P: 265

I would imagine that as the ball leaves the hand, the upward force on it goes to zero very quickly, but not instantaneously  the hand isn't rigid.
So, the acceleration will change very quickly from positive to negative, but not instantaneously. 


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Nov2612, 12:30 PM

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#8
Nov2612, 12:54 PM

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If you model the hand as a rigid body, then I can see that the force from it would drop to zero instantly giving a discontinuous acceleration.
But this is a consequence of our idealised model. I don't see how the force/acceleration could be discontinuous in a real situation. 


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Nov2612, 12:57 PM

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#10
Nov2612, 02:01 PM

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#11
Nov2612, 02:46 PM

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#12
Nov2612, 03:34 PM

P: 273

So at what acceleration does the ball enter free fall? My TA said when the acceleration becomes negative. Maybe because at zero acceleration there is no net force?



#13
Nov2612, 03:43 PM

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At what acceleration does the ball enter free fall? That's easy: by definition, free fall occurs when only gravity is affecting it, and you already know that answer. 


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