Register to reply 
V^2 = vi^2 + 2ad, is v velocity or speed? 
Share this thread: 
#1
Jan1511, 04:19 PM

P: 2,568

1. The problem statement, all variables and given/known data
A student throws a set of keys vertically upward to her sorority sister, who is in a window 2.00 m above. The second student catches the keys 2.30 s later. (a) What was the velocity of the keys just before they were caught? I did y(t) = 2  4.9t² + v₀t 2 = 0  4.9(2.3)² + v₀(2.3) v₀ = 12.1m/s Now I have two formulas, v² = v₀² + 2ad and v = v₀ + at, but they give me different answers. v² = v₀² + 2ad v² = (12.1)² + 2(9.8)(2) v² = 107.21 v = 10.4m/s v = v₀ + at v = 12.1  9.8(2.3) v = 10.4m/s I understand the square root will give us a negative as well, but the question is, HOW DO WE KNOW which to take? I hadn't use the v = v₀ + at, I wouldn't know whether to take positive or negative. 


#2
Jan1511, 08:53 PM

Sci Advisor
HW Helper
P: 6,654

Use:
v= v_{0} + at to start. To find the distance, integrate this with respect to time: [tex]d = \int vdt = \int v_0dt + \int atdt[/tex] What is a? (be careful about the sign). That will leave you with a quadratic equation with two possible solutions, only one of which is real. Since the question asks for velocity, you have to give magnitude and direction of v. AM 


#3
Jan1511, 09:05 PM

PF Gold
P: 7,120

If you don't want to go the calculus route, you can keep the [tex]\pm[/tex] on the answer to [tex]v[/tex]. One answer is simply going to be unphysical, you don't know right away which to use, you just have to figure out which is consistent with the other equations and the situation.



#4
Jan1511, 09:07 PM

P: 2,568

V^2 = vi^2 + 2ad, is v velocity or speed?
Does that mean the v in v² = v₀² + 2ad is actually speed and not velocity?



#5
Jan1511, 09:18 PM

PF Gold
P: 7,120




#6
Jan1611, 01:24 AM

P: 4,513

I had the same thoughts. I haven't looked at Newtonian mechanics in decades. So where does the bogus solution come from?
v² = v₀² + 2ad (1/2)mv²  (1/2)mv₀²= adm Delta K + Delta P = 0 The change in kinetic energy is equal to the negative change in gravitational potential energy. But, then I see that both solutions are valid. We can throw a ball upward and it hits the ceiling elastically, so its velocity is sign inverted but the energy balance is still conserved. So either solution is validnot bogus at all, but only one solution is valid for freely falling objects. The OP has given us a challenge: Can you determine which solution is the right one without knowledge of differential or integral calculus? This is a precalc question. 


#7
Jan1611, 01:35 AM

PF Gold
P: 7,120

Newton's Laws are timereversal symmetric. You could run time backwards and nothing would seem out of the ordinary in terms of physics (a person dropping a ball off a cliff could be seen as someone having a ball thrown up at them). So you have to go based on what you know physically what must happen.



#8
Jan1611, 01:41 AM

P: 4,513

OK, can you use your time reversal symmetry to recover the initial conditions...
An object is tossed vertically upward to you. Did you catch it as it was coming up at you at 10.4 m/sec, or did you catch it after it passed you and comes back down at 10.4 m/sec? This is the dilemma of flyingpig. You don't get to use calculus. You are just given a canned set of equations. 


#9
Jan1611, 02:52 AM

PF Gold
P: 7,120




#10
Jan2211, 01:07 AM

Emeritus
Sci Advisor
HW Helper
PF Gold
P: 7,801

The solution to the equation v^{2} = 107.21 is v = ±√(107.21).
That is, there are TWO possible answers. This can be shown in a number of ways. Two are: Yes, it's true that [tex]\sqrt{x^2}=\leftx\right\,.{[/tex] 


#11
Jan2211, 04:19 PM

P: 2,568




#12
Jan2911, 01:03 AM

P: 2,568

Oh my god, I just realized something.
The v in [tex]v^2 = v_{0}^2 + 2a\Delta_{x}[/tex] is speed. And I have a cheap proof Let F = ma and therefore F/m = a Then we have [tex]v^2  v_{0}^2 = 2\frac{F}{m}\Deltax[/tex] [tex]\frac{1}{2}m(v^2  v_{0}^2) = F\Delta x[/tex] [tex]\Delta KE = F\Delta x[/tex] clearly, we know that v in KE must be speed and this is why [tex]v^2 = v_{0}^2 + 2a\Delta x[/tex] will never give me the answer. Am I right or what? Cmon! 


#13
Jan2911, 01:28 AM

PF Gold
P: 7,120

No. The v in [tex]{{mv^2}/2}[/tex] is the velocity as well. V is velocity, that's it. Stop trying to convince yourself that it is speed. Use the other kinematics you know.
[tex]v(t) = v_0 + at[/tex] The acceleration can be positive or negative. Speeds are strictly positive. If v(t) were indeed a speed, how could you ever account for what will eventually happen with all negative accelerations? 


#14
Jan2911, 01:41 AM

P: 2,568




#15
Jan2911, 04:15 AM

Sci Advisor
HW Helper
Thanks
P: 26,157

hi flyingpig!
if the lower student throws the keys upward with speed v_{o}, the upper student can catch them on the way up or on the way down … the speed v will be the same, won't it?! if you only use v^{2} = v_{o}^{2} + 2ad, you have no input as to t, so both solutions are correct 


#16
Feb1511, 05:49 PM

P: 2,568

the [tex]\frac{mv^2}{2}[/tex] is KE, it must be speed, velocity is a vector and multiplying two vectors must mean you must take either a dot or cross product, and I don't think KE involves either, so it must be speed. A breakthrough! 


#17
Feb1511, 05:50 PM

P: 2,568




Register to reply 
Related Discussions  
Constant velocity questions (average speed vs. average velocity)  Introductory Physics Homework  2  
Average speed and instantaneous, displacement and distance, speed and velocity  Introductory Physics Homework  1  
Speed or velocity?  General Physics  5  
Difference between average speed, instantaneous speed, velocity, acceleration? help!  Introductory Physics Homework  5 