I'm learning about how the energy of matter is quantized like how the energy of light is. My textbook illustrates the concept of quantization with the particle in a box:
"A particle of mass $m$ moves in one dimension as it bounces back and forth with speed $v$ between the ends of the box of length $L$. We'll assume that the collisions at the ends are perfectly elastic, so the particle's energy--entirely kinetic--never changes."
I don't understand how the particle's energy can be "entirely kinetic." When the particle hits the end of the the box and reverses its direction, isn't there an instance in time where the particle's speed is 0 which would imply that all of its energy is now potential energy and that it's kinetic energy is zero?