The Coefficient of restitution is defined as $$e=\frac{v_2-v_1}{u_1-u_2}$$ $$v_2\to \text{final velocity body 2}$$ $$v_1\to \text{final vel of body 1}$$ $$u_1\to\text{initial vel of body 1}$$ $$u_2\to\text{initial vel of body 2}$$ It was a question in an interview. The question is why is this equation considered in kinetics and not kinematics.

this equation does not contain any mass term but still it is considered in kinetics.Why?

  • $\begingroup$ Kinetics as to kinetic theory of gases, I hope? $\endgroup$ Nov 26, 2015 at 13:18
  • $\begingroup$ kinetics of rigid bodies $\endgroup$ Nov 26, 2015 at 13:21
  • $\begingroup$ It was in the college the professor asked $\endgroup$ Nov 26, 2015 at 14:38

1 Answer 1



In physics and engineering, kinetics is a term for the branch of classical mechanics that is concerned with the relationship between the motion of bodies and its causes, namely forces and torques.


Kinematics is the branch of classical mechanics which describes the motion of points, bodies (objects), and systems of bodies (groups of objects) without consideration of the causes of motion.

So apparently kinetics deal with motion of bodies and its cause while kinematics just its motion. Obviously the coefficient of restitution would result in a change between initial and final motion so it is considered cause of change of motion and therefor part of kinetics and not kinematics.

  • $\begingroup$ The above definition is Wikipedia Ctrl C V. But the question is how to show, even if the mass is not considered why is it in kinetics. In your above argument Both the branches deals with cause in motion. then? $\endgroup$ Nov 26, 2015 at 14:37
  • $\begingroup$ @Vinay5forPrime - Correct, but it is 'important' to understand the difference between the two definitions. Your question does not contain the word 'how', it instead asks 'why' and my answer answers that question. It has nothing to do with masses but with the causes of motion. $\endgroup$
    – nluigi
    Nov 26, 2015 at 14:50

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