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July 15th, 2016, 01:46 AM  #1 
Member Joined: Jan 2016 From: United States Posts: 61 Thanks: 7  Quick question about instantaneous velocity I did part a and b. Although I need a little help with part c. I know you're supposed to solve for t^2 but I don't know what value to use for acceleration. The book says to use the formula: After that I'd use the quadratic formula. I'm just not sure which values to use... At first I thought the initial velocity would be zero and the acceleration would be zero but then that left me with: 1/2 * (4.8 m/s^2) = 0 which doesn't lead to the right answer. Could you guys point me in the right direction? 
July 15th, 2016, 02:07 AM  #2 
Senior Member Joined: Apr 2014 From: UK Posts: 914 Thanks: 331 
Differentiate the distance to get speed, I make it t = 13 1/3

July 15th, 2016, 02:08 AM  #3 
Senior Member Joined: Dec 2013 From: some subspace Posts: 212 Thanks: 72 Math Focus: real analysis, vector analysis, numerical analysis, discrete mathematics 
Use the definition of velocity. See e.g. wikipedia.

July 15th, 2016, 02:18 AM  #4 
Member Joined: Jan 2016 From: United States Posts: 61 Thanks: 7 
well yea I got that far you differentiate to get 2bT  3cT^2 or 0.36T^2 + 4.8T. but then what would C be in the quadratic formula? 0? that doesn't get the right answer =/ that's the part I'm stuck on I ended up just doing 0.36T^2  4.8T = 0 > (0.36T^2) / (0.36T) = (4.8t)/ (0.36T) which gave the right answer, but I wanted to know if there was a way to use the quadratic formula and the above equation since thats how the example in the book was done. I guess at this point its kind of an algebra question than a physics question. Last edited by GumDrop; July 15th, 2016 at 02:32 AM. 
July 15th, 2016, 03:03 AM  #5 
Member Joined: Jan 2016 From: United States Posts: 61 Thanks: 7 
never mind, I figured it out. Acceleration isn't constant so it can't be done.

July 15th, 2016, 03:09 AM  #6 
Senior Member Joined: Apr 2014 From: UK Posts: 914 Thanks: 331 
I would still consider it a physics question as you need to know the relationship between distance and speed. Physics leads to maths, but maths doesn't have to lead to physics 
July 15th, 2016, 03:42 AM  #7 
Member Joined: Jan 2016 From: United States Posts: 61 Thanks: 7 
well... I "know the relationship" as in I'm comfortable with the definitions like in the labs I answer all the analysis questions right I just need to put it into practice by doing problems and memorizing the formulas, which is what I'm doing right now. Ironically if you heard me explaining the concepts to my lab partners you'd think I'd know this stuff until you put a basic problem in front of me and see how long it takes me to work through it lol. Also thanks for the help, I appreciate it. I'm going to be working through a lot of problems today (I've set all of today aside for this stuff.) so I'll probably end up making another thread for general questions I have. That way I don't end up spamming with like 10 different threads. Today's physics day 

Tags 
instantaneous, question, quick, velocity 
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