# Determining direction of friction in 3d problems

How to determine the direction of friction in complex problems? I can determine direction of friction in 2d problems which involve wedges and pulleys. But when it comes to 3d problems i always get wrong in taking a component of friction in some direction. For example i didnt realise that friction is also giving the ball in following situation an angular acceleration. Please share any ideas or tricks that can be used to avoid such mistakes involving friction.

• the friction force act toward the tangent of the path – Eli Dec 24 '18 at 22:27
• \begin{aligned}F=\dfrac {m\cdot g\cdot \mu }{\sqrt {\left( xp\right) ^{2}+\left( yp\right) ^{2}}}\cdot \begin{pmatrix} xp \\ yp \end{pmatrix}\\ xp=\dfrac {d}{dt}x\left( t\right) \\ yp=\dfrac {d}{dt}y\left( t\right) \end{aligned} – Eli Dec 24 '18 at 22:39