# Newton's second law cosine and sine

1. Jun 21, 2009

### DmytriE

Newton's second law is taking my mind for a spin and for some reason had me contemplating how it works for several hours. This is all with respect to an incline and an object sitting on the incline with no friction. If the problem does not give you the mass of the object can you completely just ignore the mass and work with just F=gsin(theta)?

Also how do you determine what is mg*sin(theta) and what is mg*cos(theta)? Here is a link to the standard picture of an object on an incline with the arrows drawn in.

http://www.wellesley.edu/Physics/phyllisflemingphysics/107_s_workenergy_images/figure_for13.gif [Broken]

How you you know that the opposite of the normal force has a cosine rather than sine? I know this law is simplistic but I just can't wrap my head around it. The three letters don't give me much to work with.

Last edited by a moderator: May 4, 2017
2. Jun 22, 2009

### Staff: Mentor

Almost. The force parallel to the incline depends on the mass (F = mgsinθ), but since a = F/m, the acceleration does not: a = gsinθ.

As far as whether to use sinθ or cosθ, you need to review your trig and the definitions of sine and cosine. Review how trig is used to find the components of vectors. Read these links: http://www.physicsclassroom.com/Class/vectors/U3L3b.cfm" [Broken]

Last edited by a moderator: May 4, 2017