27 Feb, 2013
PRESENTATION: If the resultant force of the external forces acting on a system of particles is null, the linear momentum of the system remains constant in time.
- Jeff and the Raccoon, Susan Ramlo, Phys. Teach. 44, 160 (2006)
- Using a Force Plate to Correct Student Misconceptions, Edward P. Wyrembeck, Phys. Teach. 43, 384 (2005)
INTRODUCTION: This concept evolved historically between the 14th and 17th centuries, the latter being when these ideas reached Galileo, Descartes and other Physicists who finally defined impulse and quantity of motion accurately. The quantity of motion or linear momentum is a vectorial magnitude that is defined as the product of the mass of a body and its velocity at any given instant. Linear momentum can be considered as a measurement of the difficulty in changing the object’s state of movement (Newton’s first law). It is related to force.
OBJECTIVE: To verify that if momentum is conserved, the quotient of the angles formed by the movement of two balls is inversely proportional to the mas of each one.
MATERIALS: hoop, aluminum ball, iron ball, knife.
SETUP: The hoop is placed on a flat surface that is as uniform as possible. The two balls are placed inside it so that they are touching each other and the hoop. Finally, the knife is placed so that it is between the two balls and perpendicular to the plane surface on which the hoop lies.
EXPLANATION: When a system of forces with a resultant force that is not null acts on a particle, it makes the particle move with acceleration (F=macm). This resultant force is equal to the variation in the linear momentum with time.
However, if the net external force acting on the particle is null, then the variation of the linear momentum with time is 0, showing, therefore, that the linear momentum of the particle is conserved.
CONCEPTS: centre of mass, Newton’s 2nd Law, Newton’s 3rd Law, momentum, law of conservation, impulse.
- Tipler P.A. Física. Barcelona: Reverté, 2010.
- De Juana J.M., Física General, Pearson, 2009.
- Serway R.A y J.W.Jewett. Física, Thomson-Paraninfo, 2010.
- R. Ehrlich, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, Princeton University Press, 1997.
STUDENTS 2011-2012: Jairo Rúa, Sara Regueira, Antonio Larramendi
LINK pdf STUDENTS (in Spanish):