Inclined plane and a box

fish

a box on a smooth inclined plane with an angle of 20 degrees to the horrizontal. If the inclined plane is 5 m long, how long does it take for the mass to reach the bottom of the inclined plane after the mass is released?

I did sin 20 degrees x 5.0 m = 1.71 m
how do i get the mass from this measurement?
1.71m can't equal mg=w
w=9.8m/s^2*mass
If I could find find (a)accel., then I could use one of the
kinematic equations.

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StephenPrivitera

The mass is irrelevant. The only part of the weight that affects the motion of the box is the component of the weight in the direction parallel to the inclined plane. That is, you have to resolve the force due to the weight into two components: one parallel to the plane and one perpendicular. The one perpendicular is canceled by the normal force exerted by the surface since there is no acceleration in that direction. The one parallel gives your acceleration. That is,
mgsin20=ma

fish

thanks Stephen, I got it now.
Once you solve for a, the masses
cancel each other out.

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