CBSE Class 12 Physics Centre of Mass Assignment

CBSE Class 12 Physics Centre of Mass Assignment. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Comprehension-1

In physics we can across many examples of collisions. The molecules of a gas collide with one another and with the container. The collision of a neutron with an atom is well known. In a nuclear reactor fast neutrons produced in the fission of uranium atom have to be slowed down. They are, therefore, made to collide with hydrogen atoms. The term collision does not necessarily mean that a particle or a body must actually strike another. In fact, two particles may not even touch each other and yet they are said to collide if one particle influences the motion of the other. When two bodies collide, each body exerts an equal and opposite force on the other. The fundamental conservation law of physics are used to determine the velocities of the bodies after the collision. Collision may be elastic or inelastic. Thus a collision may be defined as an event in which two or more bodies exert relatively strong forces on each other for a relatively short time. The forces that the bodies exert on each other are internal to the system. Almost all the knowledge about the sub-atomic particles such as electrons, protons, neutrons, muons, quarks, etc. is obtained from the experiments involving collisions. There are certain collisions called nuclear reactions in which new particles are formed. For example, when a slow neutron collides with a uranium-235 nucleus, new nuclei baruim-141 and krypton-92 are formed. This collision is called nuclear fission. In nuclear fusion, tow nuclei deuterium and trituim collide (or fuse) to form a helium nucleus with the emission of a neutron.

1. Which one of the following collisions is not elastic?
(a) A hard steel ball dropped on a hard concrete floor and rebounding to its original height.
(b) Two balls moving in the same direction collide and strick to each other
(c) Collision between molecules of an ideal gas
(d) Collisions of fast neutrons with hydrogen atoms in a fission reactor.

2. Which one of the following statements is true about inelastic collisions?

(a) The total kinetic energy of the particles after collision is equal to that before collision
(b) The total kinetic energy of the particles after collision is less than that before collision.
(c) The total momentum of the particles after collision is less than that before collision.
(d) Kinetic energy and momentum are both conserved in the collision.

3. In elastic collisions
(a) only energy is conserved (b) only momentum is conserved
(c) neither energy nor momentum is conserved (d) both energy and momentum are conserved.

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