How do I restrict this to 3 decimal places?

Question

I use the following code to calculate the force of wind-pressure on a kite - it's simplistic, but gives me what I need. The problem I have is that I can't produce the result to 3 decimal places: N[(1/2)*\[Rho]*v^2*a, 3]] doesn't work. How do I fix this?

Manipulate[
 Grid[{
  {Style[StringForm["\n`` = `` Newtons\n", TraditionalForm[f], 
   N[(1/2)*\[Rho]*v^2*a, 3]], Large]},
  {}, 
  {StringForm["Wind velocity = `` m/s", v]}, 
  {StringForm["Air density = `` m/s", p]}, 
  {StringForm["Kite surface area = `` \\!\(\*SuperscriptBox[\(m\), \(2\)]\)", a]},
  {}
 }], 
{{v, 15, "WIND VELOCITY"}, 0, 25}, 
{{\[Rho], 1.225, "AIR DENSITY"}, 0.9, 1.5}, 
{{a, 0.35, "KITE SURFACE AREA"}, 0.2, 0.5}
]

Try Round[value,0.001] where the value is your number that will be rounded — Rom38, Mar 09 '21 at 11:26
Thank you @Alexei. I had already marked Daniel Huber’s response as the answer, but your suggestion is really useful too, and gets rid of that ` mark into the bargain - better for presentation. — Richard Burke-Ward, Mar 10 '21 at 12:40

score 1 · Accepted Answer · answered Mar 09 '21 at 11:28

1

From the help of N: "With machine-precision input, the results are always machine precision:". But, instead of N[..,3] you can multiply e.g. by 10, then round and finally divide by 10:

ToString[Round[10 N[(1/2)*\[Rho]*v^2*a, 3]]/10.]]

This produces:

answered Mar 09 '21 at 11:28

Daniel Huber

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How do I restrict this to 3 decimal places?

1 Answers1