Can velocity and displacement be negative in physics?

[idontspeakmath]

1) can velocity starts at not zero? but if all objects have to start moving from somewhere, then wouldn't initial velocity always be zero?
2) can velocity starts at negative?
3) can displacement be negative, or we're always heading in the positive direction?
4) if it's up to us to determine in what direction we're going, then wouldn't our answers be unsynchronized?
5) if a ball is thrown upward and then falls, would the acceleration be constant all the time? (even when it's at the very top when v is zero?)
6) if a ball is thrown downward and another is just dropped casually, would they reach the ground at the same time? why?

um, maybe to justify the obviousness of the answers to the questions that only i don't see . . . this is my first time dealing with physics theories.

--> this just reminds me of calvin&hobbes

 

[mezarashi]

I think you can answer all of these questions by thinking carefully. First of all, how do we define "start"? Is it defined by some divine power? Of course not. When you are timing lap times for formula one cars, you start timing each lap just as they pass through the start-finish line. Is their velocity zero at time zero? Obvious isn't it.

Drive down the road. You say that you are going forward. What does the guy driving down the other side of the road think about you?

The laws of physics work in every reference frame. You must start solving problems to really see the beauty of this.

(5) Acceleration is the change of velocity. If you originally have positive velocity and constant negative acceleration, then you would expect your positive velocity to decrease, reach zero and even become negative, is that right?

(6) This is not true. If a ball is thrown down. It will reach the ground first. Try it ;)