Science corner

6th February 2004 at 00:00
Sophie Duncan introduces Newton's law of the conservation of momentum

Science Corner this week explores one of Newton's laws of motion: the conservation of momentum. In any collision between two objects, momentum equal to the mass of the object multiplied by its speed is conserved.

This is the principle behind the toy usually called the Newton's cradle. To make your own, make a box-shaped frame using a construction toy. It needs to be quite sturdy, and open so the beads can swing freely. Dimensions should be about 25cm by 20cm by 12cm.

Take five similar balls at least 3cm in diameter made of a dense material such as wood. Beads are ideal. Wooden balls can be used, but would need to have a nail in the top to suspend them. The positioning of these beads is important. They must rest side by side, equidistant from either side of the frame, and be level with one another. Take a piece of sturdy cardboard 24cm by 10cm. Mark the midpoint, the position of the middle bead. Mark the other bead positions at distances equal to the diameter of the beads. Draw two parallel lines about the midpoint of the length of the cardboard, 4 cm apart. Cut from the edge of the cardboard to the nearest of these lines along each of the five bead positions. Repeat on the other edge of the cardboard. Attach the beads to some fishing line and then pull the thread through the slits in the cardboard, until all five beads are suspended.

Place the cardboard across the top of the frame and adjust the fishing line until all five are in a line. Tape into position.

Ask your students to predict what will happen if you pull one bead out to the side and release it. Try two or three balls. Momentum is conserved.

Sophie Duncan is project manager for science at the BBC

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