Solve for elapsed time
05-30-2018, 09:13 PM
Post: #1
 DrD Senior Member Posts: 1,133 Joined: Feb 2014
Solve for elapsed time
With these provided values, (including the units), how could the prime be made to find the elapsed time?

[CAS]
vi:=18.51_(m/s);
d:=79.8_(m);
a:=11.17_(m/s^2);
t=?

d=vi*t + 1/2 * a*t^2;

(Of course, the units could be removed, using left(var), and a mathematic result obtained; however it's the physical result that's desired).

-Dale-
05-30-2018, 10:56 PM
Post: #2
 roadrunner Senior Member Posts: 420 Joined: Jun 2015
RE: Solve for elapsed time
Solve will do it if you use exact values. In CAS:

d:=(vi,t,a)->vi*t+(1/2)*a*t^2

then:

USIMPLIFY(solve((exact(d(18.51_(m/s),t,11.17_(m/s^2)))) = (exact(79.8_(m))),t))

gives:

{2.47014692829_(s),−5.78438148514_(s)}

05-31-2018, 11:43 AM (This post was last modified: 05-31-2018 11:48 AM by DrD.)
Post: #3
 DrD Senior Member Posts: 1,133 Joined: Feb 2014
RE: Solve for elapsed time (Solved)
Thanks, for the very helpful information, Road! Using exact(value) is the key to success, here. (Good catch on that ...!) In my case, even though [CAS] [Exact] checkbox was checked, I see, (now), that it says it only affects the "Returned" values.

(I wonder why it doesn't work, more generally?)

[CAS]
Code:
 vi:=exact(18.51_(m/s)); d:=exact(79.8_(m)); a:=exact(11.17_(m/s^2));

USIMPLIFY(solve(d = (vi*t+(1/2)*a*t^2),t));
t = {−5.78_(s), 2.47_(s)}

-Dale-
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