Best Answer: You need a reference point because all motion is defined relative to a point which is taken to be fixed. For example, when you say that a car is moving at 20 m/s, you mean that it's moving at 20 m/s relative to the ground, which you assume to be a fixed point. However, you also know that the ground (and the Earth) are really zipping through space at high speed as we orbit the Sun. If you want to define the car's speed relative to the Sun, you have to take the Earth's motion into account as well. Whenever you define the motion of an object on or above Earth, it's usually understood that the ground if your reference point. If it's not, you should specify another point.
Speed is a scalar quantity, which means that it only tells you how fast an object is moving. Velocity is a vector quantity: it tells you how fast an object is moving AND in what direction. For example, I might say that a car is moving at 20 m/s. That's the car's speed. But if I say that the car is moving at 20 m/s NORTH, then I've given you its velocity.
In velocities, direction can be given in several different ways:
(A) As a positive or negative sign. Example: The rocket is moving at +250 m/s. In this case, we've defined "up" as the positive direction and "down" as the negative direction.
(B) As an angle or compass heading. Example: The plane took off at 100 m/s 20º above the horizontal OR the car is driving at 30 m/s due west.
