## Mass on slope formula

### Steph-x

Posted 27 January 2010 - 04:57 PM Ok, see in a free body diagram like this (only pretend the F arrow is the pointing the opposite way, I stole it off scholar)? I know the formula is F = W sin20. But I can't figure out why, whenever I try to work it out for myself I get the F and W the other way round.

This is what I'm doing at the minute, where am I going wrong?

sinx = opposite/hypotenuse
sin20 = W/F
W = F sin20.

(PS, if you could explain it by tomorrow it'd be useful. I kind of have my prelim...)

### Marcus

Posted 27 January 2010 - 05:32 PM

the mistake you're making is you are assuming that slope is the triangle you use. the triangle you have to use the red coloured one, in this diagram,

W is labeled mg, on this, and F would be the bottom most line on this triangle.

### Steph-x

Posted 27 January 2010 - 05:45 PM

Thank you. I vaguely remember that now...
Why is physics the opposite of common sense? ### JasMcG

Posted 09 April 2010 - 03:39 PM

An easy way to remember if it's cos or sin is: perpendiCular = Cos.
That is, there's a "c" in perpendicular and cos.

There is no "s" for sin in parallel, so this wee daft "c" rule is foolproof! 