What about velocity?

Distinguishing between speed and velocity

In the world of the sciences a clear distinction is made between the terms speed and velocity. In the 11–14 curriculum this distinction is not needed and our advice would be to stick with the more familiar term speed. Should you want to find out more, read on.

Speed and velocity are often used as if they were the same quantity and indeed they are measured in the same units (metre/ second or kilometre / hour). The key difference is that the speed of an object tells us about how quickly it is moving, while the velocity specifies how fast it is moving in a certain direction.

This being the case:

• Speeds are scalar quantities which have magnitude only (specified with a single number),

while

• Velocities are vector quantities for which both magnitude and direction must be given (specified with two or more numbers).

The upshot is that scalars can be described by a single quantity (number plus unit), whereas vectors need more than one such quantity, and these form an ordered set (so the position of the numbers is important – don't mix up the direction with the magnitude). These kinds of ordered sets of quantities are very useful in the sciences, which is why many quantities are best described using vectors. But these really begin to be useful when children study physics beyond the current level – starting with that normally studied at 14–16 years old.

For example, when talking about a car's velocity, we might say that it is travelling at 30 mph due north.

A trolley may be described as having a velocity of 10 metre / second to the right. This may seem to be a trivial difference, but it becomes important in situations where direction matters.

A pupil walking backwards and forwards across a room may have a constant speed of 2 metre / second. However, speed gives no indication of the direction in which the pupil is walking. Using velocity enables us to state that the pupil has a velocity of +2 metre / second when walking to the right and a velocity of -2 metre / second when walking to the left. This gives a more precise account of the motion of the pupil.

The designation of one direction as positive and the other direction as negative is arbitrary. Often up is chosen as positive (down is negative) and right is chosen as positive (left is negative). It's a good idea to be very clear about what you've chosen, as assuming that the direction in which something is initially moving is positive, without noticing your assumption, can lead to difficulties later.