Dear Uncle Colin,

I keep getting tripped up in my mechanics questions because I can never figure out which way the frictional force has to go. Do you have any advice?

Mechanics Understanding

Hi, MU, and thanks for your message!

This is a situation where I have One Big Trick and not much else to say. The thing to do is to ask “what would happen if friction wasn’t there?” and reason that friction has to oppose that motion.

For example, if you have a ladder leaning against a wall, you can argue that the ladder would slide down a smooth wall, so the friction force at that end must act upwards. Similarly, the ladder would slide away from the wall on a smooth floor, so friction must act towards the wall.

It takes a little practice to get used to visualising this, but once you have the idea in place and it clicks, you find yourself wondering how you ever used to get it wrong!

Hope that helps,

- Uncle Colin