Demonstrate how to set up the pulley, and ask (older) students to make their own: Bridge a rod between two tables, then tape down to hold in place. Hang the string over the rod. Cut the string so that one end just reaches the floor and the other end just hangs over the rod. Show students how to make a handle for the plastic pot using masking tape (see photo): tape up one side of the pot, twist the tape to make a (non-sticky) handle, then tape down the other side of the pot; reinforce with a strip of tape around the pot. Either tie each end of the string directly to the pot handles, or attach S-hooks to the string for hanging the pots.
Give each student group a handful of marbles, a container for water and more pots to pour water from. Lay a towel under their pulley.
Give students their challenge: raise a marble in one pot, only by adding water to the other pot. Although this task is relatively easy, students will become familiar with their pulley system by achieving it. Discuss how the pulley changes the direction of the force: when the weight of the water moves one side down, an equal and opposite force pulls the marble on the other side up.
Allow students to experiment with number of marbles and amount of water. Given the chance they will will naturally experiment with these and other variables to explore the characteristics and uses of a pulley.
If students need focused challenges to guide their experimentation here are some ideas:
How much water will raise more marbles?
How can you balance the pots half way up from the floor?
What can you change to make the marble move upwards slower or faster?
Gather as a group to discuss what students found. Use their discoveries to guide concluding remarks about the forces, balanced and unbalanced, in a pulley system.
A pulley is a device that can change the direction of a force - when the weight of the water moves one side down an equal and opposite force pulls the other side up.
By increasing the weight on one side (with water or marbles) the speed that this side moves down is increased, therefore the speed that the other side moves up is also increased.
When the weight on each side is equal, the pulley is balanced and does not move.
The string over the rod has a little friction, so may affect the movement of the pulley a little. By using a pulley with a wheel attached to the rod, friction will be much reduced and make the system more sensitive.