When applying to a physics course the interview process can be a bit daunting and unlike anything you’ve experienced before, however they aren’t all that different to interviews for other subjects involving critical thinking.
Every physics interview has one fundamental goal, which is to evaluate the aptitude of the student’s way of thinking for dealing with physics problems. Interviewers will also be looking at secondary aspects of the student such as passion for the subject and a desire to learn but without sufficient aptitude for critical thinking these will not be enough.
A physics interview will typically consist of a student being asked to solve a few physics or maths problems, while discussing their method of solution with the interviewers. A common mistake is where students think that solving a problem quickly in their head or without discussion with their interviewers will somehow be the best way to demonstrate their abilities, in this scenario it is far more likely that the interviewers will continue to ask sufficiently more difficult or abstract questions until the student is unable to answer them. In a situation where you are able to work out the answer quickly it is far better to explain what you think the answer and then explain the thought process that lead to that answer. Talking over the thought process with the interviewers after providing an answer lets them know that you were able to solve the problem quickly but that you have also used a robust method as opposed to having simply remembered the answer from prior practice, something that interviewers tend to be very good at identifying.
It is almost preferable to be asked a problem which you are unable to answer as the interviewers will get to see you working at your limits and at a difficult problem, which allows them more insight into your thinking processes than question which you can immediately answer. If you are given a problem you don’t know how to answer draw on the knowledge of the interviewers as they might be expecting you too, for instance if you are asked to evaluate a potential energy and know that you need to take the expression for force and integrate over distance, but cannot perform the integral, explain this to them, they most likely care far more about the fact that you know what process would be used to solve the problem, than a specific mathematical technique. With that said however do not limit yourself to only applying drawn out standard techniques when short cuts exist. For instance often simply relations like conservation of energy or conservation of momentum allow you to answer a question with a simple sentence rather than some drawn out proof, the ability to spot shortcuts in solving problems will be viewed very favourably.