5.13

## UNSW Sydney

Skip to 0 minutes and 9 secondsLike any other force, friction is not revealed until the body it acts upon is abstracted by a free-body diagram. As forces go, friction is particularly mysterious. But for practical purposes, we can represent its behaviour pretty well by the simple model we developed this week. As you develop engineer's eyes, you'll see friction acting all around you. And you will see the characteristics we have explored in the model. As your car breaks to a halt with a jerk at the last moment, you'll think of the transition from kinetic friction to static friction. And as you try to push a fridge along the floor without it toppling, you'll think of the analysis that we have undertaken in this week.

Skip to 0 minutes and 59 secondsYou'll notice how many turns the deckhand takes with the mooring rope around the bollard to secure a ferry to the wharf. And you might wonder if that squeal from the engine bay of your car is telling you that there's not enough tension on the belt drive to the air conditioner. Yes, there are engineer's ears as well. Next week, it's rolling resistance and wind loading.

# Through Engineers' Eyes

This quick video highlights your progress towards Engineers Eyes. The concept wheel shows how it all fits together.

Week 5 concept wheel (Click to expand)

Friction is tricky. Sometimes you want more; sometimes you want less.

First you had to learn how to describe friction. Then you learnt how to represent it on FBDs (They have their own conventional interactions).

Next you had to work out how best to attack a friction problem using laws of friction and equilibrium. There were basic concepts and two special cases (tip/slide problems and rope-around-a-bollard problems).

I find that friction problems are mind stretching. It’s satisfying when you get them out; irritating when you don’t!

Perhaps you found that practice was good.

### Talking points

These are to get you started. Share any other thoughts you’ve had.

• Do you see beauty and mystery in the equation for the ratio of tensions in a rope around a bollard?
• We have said that ‘the Engineers’ basic model of friction is rough and ready but nevertheless useful’. What do you think about this statement?