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Place three seemingly identical spheres on a horizontal track touching each other. Roll another sphere slowly into the spheres at rest. Wow! All of a sudden the last sphere takes off with tremendous velocity. The system seems to have gained energy!
Materials: Track Two wooden blocks Five metal spheres (2.0 cm) One magnetic metal sphere (2.0 cm) (Safety glasses recommended, but not provided.)
Note: The track can be used in several different orientations: straight; raised 4.0 cm on either or both ends; and raised 6.0 cm on either or both ends.
Procedure A. Student Investigation
Teacher Note: As with Newton's cradle (NEW-100) both the kinetic energy and the momentum of the initial moving spheres are transferred to the final moving spheres. In this case, in order for both the Law of Conservation of Momentum (mv) and the Law of Conservation of Kinetic Energy (1/2 mv2) to hold, the initial moving spheres must equal the final moving spheres 'What comes in is what goes out.' Deviations are due to friction, and the slight magnetism of the spheres, causing them to stick together slightly and not roll well.
Teacher Note: As the released sphere gets closer to the stationary magnetic sphere at the bottom of the track, it becomes more and more attracted to the magnetic sphere. This causes a great increase in velocity. As a result, the sphere on the other end shoots out quickly. Notice that the final moving sphere is initially separated by another sphere from magnetic sphere. Consequently, it is attracted less to the magnetic sphere.
Procedure B. Teacher Demo Repeat steps 1 thru 5 from Procedure A in front of the class. Before each step, ask students to predict the result.
Procedure C. Open Ended Provide students with all of the materials listed at the top. Ask them to experiment with the materials, carefully recording what they do and the results.
How does this activity demonstrate the Law of Conservation of energy?
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Students can use the Magnetic Accelerator to construct, use and present arguments or experiments to support the claim that when the motion energy of an object changes, energy is transferred to or from the object.
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Students can use the Magnetic Accelerator in an investigation and use mathematical representations to support the claim that the total momentum of a system of objects is conserved when there is no net force on the system.
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