## The Screw

The crew's final task at this location is to construct a device to regulate the flow of water through the irrigation canals.

The canals have been constructed so that a rectangular slab of stone can be lowered into the canal to block the flow and the water can be released by lifting the slab again. This method is pretty simple but the slab of stone is heavy and difficult to lift.

We could leave Harry and a few of his family behind to lift the gates when needed but Harry doesn't want to stay and it would be better to leave something the inhabitants could use themselves.

To accomplish this, the crew has inserted a screw into the slab. As the screw is turned, the slab is raised or lowered.

If you look closely at a screw you can imagine it to be an inclined plane wrapped around a pole.

Do you recall in the last lesson we moved our inclined plane beneath a stone and lifted the stone? If we turn this screw we are moving an inclined plane which will exert a lifting force along its thread.

Our mini inclined plane moves as we turn the screw. If this inclined plane moves 3cm to the left, how high will it lift the object?

Talk Now

If we are applying our force through a distance of 3 cm and our resistance is only moving 1 cm, how do you predict the force applied to the resistance compares to the force you must apply to the screw?

Discuss this with your partner, then each of you should record your prediction in your lab packet. As soon as your predictions have been recorded go on to the next screen and we can test your predictions.