Describing turning events - close that door!

The door through forces spectacles

Let's think about closing a door. Put on your forces spectacles and think about the forces acting. The door is anchored along one edge, so there is an obvious pivot. As you push on the door, you provide a driving force to make the door move. The hinges (especially if not well oiled) provide a frictional counter force in the opposite direction. This counter force is added to by the drag as you push the door through the air. Remember the useful pattern, from the SPT: Motion topic:

• If the driving force is greater than the counter forces, an object will speed up.
• If the driving force equals the counter forces the object continues at a steady speed.

Here, and in our other examples, we'll try and keep the analysis simple by dealing with situations where the door spins at a constant rate, not speeding up or slowing down. So we'll keep to the same patterns as much as possible. But, for thinking about the door or anything else that spins, it is not enough just to check that the driving and counter forces balance each other. We must also take note of how far the forces are from the pivot to see that the door spins steadily.

The skills in identifying forces will be drawn on again. You can draw an abstract forces picture for the door, showing the pivot, and the forces correctly located with respect to that pivot.

Drawing diagrams such as this is a first step in analysing the world of levers, where forces act around pivots. If you apply your force very close to the pivot the turning effect (more on this shortly) produced is rather small. If you push farther away from the pivot the turning effect is bigger.