Solving inclined plane in different axis.

[tousif1995]

I can work out the acceleration of a block that will take along the plane when the axes are the ones along and perpendicular to the inclined plane. But, when I choose a horizontal and vertical axis I end up with the same mistake as this guy in this thread did:

https://www.physicsforums.com/showthread.php?t=238180

In that thread, it is said that the vertical forces don't add up to zero. I don't get it and hence I cannot solve the problem when the axis is horizontal and vertical. Can anyone help?

 

[Doc Al]

In that thread, it is said that the vertical forces don't add up to zero. I don't get it and hence I cannot solve the problem when the axis is horizontal and vertical. Can anyone help?

The acceleration of the block is parallel to the incline, so it will have a vertical component. So the vertical forces on the block cannot sum to zero.

Obviously, it's much easier to solve the problem using coordinates aligned parallel and perpendicular to the surface. But if you insist, you can solve it using vertical and horizontal axes. It just takes a bit more work, since you must add the constraint that the acceleration is parallel to the surface.

 

[tousif1995]

Thank you!

masinθ = -(Rcosθ-mg)
Rsin = macosθ

Just solved it! feels good