Equation Showing as "False" 3

[Buggle]

When I open a blank instance of MathCAD Prime 6.0.0.0, and I evaluate the following equation, it evaluates as false. When I remove the units it evaluates as true. What is happening and why do the units impact this?

12.75 in + 6*1.5 in = 21.75 in ---- This evaluates as false.
12.75 + 6*1.5 = 21.75 ------------- This evaluates as true.

12.75 + 6*1.5 does equal 21.75.


Note that I am using the Comparison version of the equals sign (CTRL =).

Any help is appreciated! Thanks!

 

[jhardy1]

Zero metres per second is most assuredly NOT equal to zero metres!

If you're doing the maths to rendezvous the SpaceX Crew Dragon capsule with the ISS, you want to calculate something like:

v[sub]Dragon[/sub] - v[sub]ISS[/sub]

and try to arrange for this to be close to zero m/s when the separation closes to zero metres.

Things could get pretty ugly if your software decides that zero metres (separation) is the same thing as zero m/s (differential velocity) and allows for a rendezvous with an indeterminate closing velocity.

To put it another way - Mathcad might test for:

Successful docking = Zero separation AND Zero closing velocity

If you're not careful with your test algorithms, SMath might deem a successful docking to occur when:

Successful docking = Zero separation OR Zero closing velocity

Units matter!

http://julianh72.blogspot.com