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Wrap a rope once around a rough tree branch or post. Attach a heavy bucket to one end and try to hold it by the other end (Fig. 6.43). Now, add one more turn of the rope and repeat. You will find that each extra turn increases the ‘grip’ between the rope and the branch, increasing the friction and reducing the force required, making it much easier to hold the same load. The reduction in effort is much larger than you might expect from just adding one turn. This shows that friction does not increase in a simple linear way, small changes in contact can lead to large changes in force. In the same way, friction between a rope and a post allows large ships to be held safely at a pier.
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Sample answer
Why does one more turn of the rope help so much?
Rope wrapped once
The rope grips the branch a little
↓
One more turn added
The grip between rope and branch increases
↓
Friction rises sharply
The force you need to hold the bucket falls
↓
A few turns of rope can hold a heavy load, and even a ship at a pier
- Each extra turn increases the grip between the rope and the branch, so friction increases.
- The force you need to hold the bucket drops much more than one extra turn would suggest.
- So friction does not increase in a simple linear way. A small change in contact gives a large change in force.
- This is how a few turns of rope around a post hold a large ship safely at a pier.
