Boxcar accelerating up incline, pendulum suspended within

[lizzyb]

This seems like an easy problem but my answer is incorrect.

Imagine a boxcar traveling up a 9.85 degree slope with constant acceleration a. In the boxcar is a pendulum that hangs 24.05 degree from the perpendicular to the boxcar's ceiling and floor. Find the acceleration.

It seems to me that the pendulum is off 14.2 degrees from vertical, and so we have:

T sin(24.05 - 9.85) = m a : Fx
T cos(24.05 - 9.85) - mg = 0 : Fy

So solving for a we have a = tan(14.2) g

Yet this is not the correct solution. Any ideas? thanx.

 

[andrevdh]

You assumed that the acceleration is in the x-direction (horizontal). Maybe the given acceleration is along the direction of the incline.