Question

Speed can never be zero. Why?

Answer 1

 Speed is a scalar quantity meaning it does not have a direction assigned to it. The negative in front of a speed would imply that it was travelling at a negative speed which does not work. Velocity, on the other hand, is the same as speed only with direction. So if you were travelling at a -50km/h speed, it would be impossible, but if you had a velocity of -50km/h, then that would mean you were travelling at 50km/h south (or west, the opposite direction, etc.) 

Answer 2

An object that starts at a resting position before moving is still accelerating when the speed is zero. If objects with a speed of zero could not be accelerating, they would never be able to move.