Definition of velocity

The definition of velocity is different than that of speed.

Velocity versus speed:

The velocity is speed in a given direction.

The speed is just how fast a car is moving

Thus, to have velocity, you must both have speed and direction 

If a car travels 40 miles per hour, we are talking about its speed.

If a car travels 40 miles per hour to the west, we are talking about its velocity .

You could use the words velocity and speed interchangeably as long as the direction is not taken into consideration or not included in the context of the problem that you are solving.

Definition of Constant velocity 

A velocity is constant if the following two conditions are met.

  • The speed is constant

  •  The direction is constant

Constant speed means that the object is moving at the same speed or does not go faster or slower.

Constant direction means that the path is a straight line.

It may make sense to you that why the object should not move faster or slower in order to have constant speed.

It might not make sense why the path should be a straight line.

Simply put, if the path is not a straight, the direction will change.

For example, because the path is not completely straight, the green dot below changes direction 2 times.

The first time the direction changes is on the blue line and the second time is on the black line.


The same thing will happen if the path is a circle. The direction will change constantly.

You can from the figure above that the green dot changed direction 3 times.

Any object moving on a circular path will never have a constant velocity because the direction of the object changes every instant. However, the object may have constant speed.

From the definition of velocity, you can see that a velocity is changing whenever the speed, the direction, or both is changing.

 Again, even if an object moves at constant speed around a curved path, it does not move at constant velocity 

When you are looking at your speedometer and you see 65 miles per hour and your car is in cruise control, you are seeing speed.

You are not seeing velocity since the speedometer does not know your direction.

Some cars are equipped with a compass. If you combine your speedometer with the reading on the compass, then you will have velocity.

This ends our definition of velocity.  

Then, take this velocity quiz

Go further with some velocity word problems

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