02 Jan, 2013
In the rhythm
PRESENTATION: A number of identical metallic balls are hanging suspended by strings from a horizontal bar, and they are in contact arranged in a row. Different types of elastic collisions can be observed with this setup. The introduction of any soft material between two adjacent balls produces inelastic collisions. The mass effect may be studied by changing the size of one of the balls. A similar study may be performed using a row of coins placed on a glass surface.
- Edme Mariotte and Newton’s Cradle, Rod Cross, Phys. Teach. 50, 206 (2012)
- Happy Balls, Unhappy Balls, and Newton’s Cradle, David Kagan, Phys. Teach. 48, 152 (2010)
INTRODUCTION: In an isolated (closed) mechanical system containing many particles, in the absence of external forces the total linear moment is conserved if the forces exerted by the particles are parallel to the line linking them. In addition, the energy conservation law states that energy cannot be either created or destroyed, only changed from one type to another, as for instance when electric energy is converted to calorific energy by an electric heater.
OBJECTIVE: Verify the conservation of energy and linear momentum.
MATERIALS: small metallic spheres, string, a bar to act as a stand.
SETUP: Five spheres are suspended, each of them by two strings so that the balls are aligned horizontally and in mutual contact.
EXPLANATION: The movement is started by displacing one of the balls placed at one end of the row, elevating it to a certain height. When it falls the stored potential energy is converted to kinetic energy, and when it collides with the neighbouring ball this energy is transferred through the balls until the last one at the opposite end of the row, which ascends due to the effect of this transmitted kinetic energy, until all of it is transformed into potential energy, and the process starts again in the opposite sense. Eventually, the motion stops due to the effect of friction.
CONCEPTS: Conservation, kinetic energy, gravitational potential energy, linear momentum, elastic collisions, deformation, friction, Newton’s cradle.
- I. Newton, Principios Matemáticos, Editorial Altaya, 1997.
- W. Rankin, Newton para principiantes, Editorial Era Naciente, 2001.
STUDENTS 2012-2013: Cristina Rivas, Víctor Rodríguez, Antón Rodríguez, Miguel Rodríguez
LINK pdf STUDENTS (in Spanish):