Pardon me if this is a duplicate.
Say I have two objects that are heading towards each other (one going to the right and the other heading left). Both have a constant mass (let's say 2kg and 4kg respectively) and velocity (5m/s and 3m/s) before the collision. Using this information alone, is there any way to know if the collision will be elastic or inelastic. Do I need more information on the objects such as the shape or materials?
Yes, you do need more information.
If this is a basic physics problem you need to be told something about the final state, or that the objects are ideally rigid and in a vacuum. Then they cannot deform or make a sound. In other words energy is conserved.
For real materials this will never be satisfied, there is always some energy loss. So a truly elastic collision is an idealization, a limit that may never be reached in the classical world. Perhaps it is for elementary particles.
Comparing the measured results of a collision experiment to the prediction for an elastic collision can be used to determine how close to "elastic" the collisions really is, i.e. how well the ideally rigid assumption was.
Due to Gert's most wonderful comment here are some additions.
My reference to energy conservation was specifically to mechanical energy as we learn it in introductory physics, Kinetic + Potential. Collision processes are usually assumed to be all kinetic but in fact one can analyze collisions in the presence of a conservative force like gravity.
We are taught that inelastic collisions do not conserve energy but in fact, as Gert points out, total energy is always conserved. There are conservative forces for which the work done can be described by a potential function and non-conservative forces, e.g. drag or friction, which are not described in the same manner. When conservative forces are at work in the collision the energy stored in the system during the collision will be converted back to kinetic energy. You can describe the "in state" of such a process by $(K_1 + K_2)_i = (K_1 + K_2)_f$, where subscripts 1, and 2 refer to the particles involved in the collision and subscripts $i$ and $f$ refer to the initial and final states. If kinetic energy alone is conserved (again, assuming no conservative forces are working on the system) then we say that the collision is elastic. Otherwise it is inelastic. Knowing only the initial states and nothing more you cannot know if a collision will be elastic.
For real materials this will never be satisfied, there is always some energy loss. Energy is ALWAYS conserved and NEVER lost. What you mean to say is that in an ELASTIC collision, KINETIC energy is always conserved. W/o that qualifier your answer lacks meaning.
– Gert Jun 06 '21 at 21:47