Physics Help Forum Is this correct 2 (Different topic)

 Mar 10th 2017, 04:48 AM #1 Junior Member   Join Date: Feb 2017 Posts: 14 Is this correct 2 (Different topic) While waiting for a responce in my earlier post, I looked theough my project one more time and realized that it had anoter big problem. Again I found a formula I'd like to verify, this time with velocity. Velocity^2 = 2 x acceleration x distance Then take the square root of v^2 to get velocity. The equation above is to get velocity from acceleration and distance. Is it correct?
 Mar 10th 2017, 05:42 AM #2 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,280 Yes, although to be entirely accurate the formula is: $\displaystyle v_f ^2 - v_i^2 = 2ad$ Where v_f is final velcity and v_i is initial velocity. Your formula is correct if v_i = 0.
 Mar 10th 2017, 11:47 AM #3 Junior Member   Join Date: Feb 2017 Posts: 14 Thanks, although what unit would my final velocity be in, Mph, kelometer per hour, meters per sec? Last edited by Zman; Mar 10th 2017 at 11:51 AM.
 Mar 10th 2017, 02:23 PM #4 Senior Member   Join Date: Aug 2010 Posts: 308 That depends upon what units the known quantities are! If acceleration is in "kilometers per hour per hour" and the distance is in "kilometers", then the velocity is in "kilometers per hour". If acceleration is in "meters per second per second" and the distance is in "meters", then the velocity is in "meters per second". If the units are mixed- that is if acceleration is in "kilometers per hour per hour" and distance is in "meters" then you have to change one to match the other.
 Mar 12th 2017, 07:43 PM #5 Junior Member   Join Date: Feb 2017 Posts: 14 Thanks
 Mar 13th 2017, 03:32 PM #6 Junior Member   Join Date: Feb 2017 Posts: 14 I plan to take the distance / speed(velocity without direction) to get time. I know this is true but something's been bothering me. If I have been traveling 600 miles at 40 mph then if we plug it into our formula we get 600/40 = 15 hours. But is I convert our 600 miles to KM than we get 965.6064 kilometers. This at 40 mph still should be the same right? 965.6064/40 = 24.14016???
Mar 13th 2017, 04:08 PM   #7
Physics Team

Join Date: Jun 2010
Location: Morristown, NJ USA
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 Originally Posted by Zman This at 40 mph still should be the same right? 965.6064/40 = 24.14016???
Be careful with units! Your original formula was:

$\displaystyle \frac { 600 \ miles }{40 \frac {miles}{hour} } = 15 \ hours$

When you converted 600 miles to Km, but did not convert 40 miles per hour to kilometers per hour, what you get is this:

$\displaystyle \frac { 965 \ Km} {40 \ miles/Hr } = 24.1 \frac {Km-Hr}{mile}$

Don't know about you, but this is not a unit of time I'm used to! You need to convert 40 MPH to kilometers per hour, which is 64.4 KPH:

$\displaystyle \frac {965 \ Km}{64.4 \ Km/Hr} = 15 \ Hr$

Hope this helps.

Last edited by ChipB; Mar 13th 2017 at 08:16 PM.

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