Translation:The concept of velocity in physics is really complicated.
In this sentence, is there a way to know that "khái niệm" is singular and not plural, or is that just based on context?
các or những explicitly states plurality, but their absence does not necessarily mean singularity. for instance, "tôi thích ăn táo" could mean "I like to eat apples". VNmese tend to cut short unnecessary words when it does not compromise the comprehension or the flow.
Speed for vận tốc was not accepted. Besides I think the concept is pretty straightforward.