The post is part of the subtopic in Chapter 2 **Force and Motion** in **SPM Physics Form 4** syllabus. **Berry Berry Easy** collaborate with a young Scientist, See Chee Keong who will be graduated from **Universiti Sains Malaysia** (**USM**) in the end of 2012 would like to present you **Linear Motion** which is the basic subtopic for Physics students. Most of the students would be very curious towards the concept of **Speed and Velocity** in our everyday life. Berry Berry Easy will help to explain these concepts.

**SPM Physics Form 4 Notes – Force and Motion (Part 3)**

**Speed and velocity**

In our daily life, we may say “Ei, that car move very fast leh.”, but how fast is the car? In physics, the term “speed” and “velocity” is to describe the degree of quickness of a moving object. These terms are very important to traffic police officers, without the concept of speed, they cannot catch offenders who speed. In our daily life, most of us would usually emphases on speed and yet the direction is not important. As long as people know how fast the car move, then we are happy. But it may not be true in physics world, we are more concern on the velocity, because the direction of the object is very important when we want to calculate its velocity.

**Speed**can be defined as the**rate of change of distance**. It is a scalar quantity, since it is closely relate to distance.- Speed = distance / time
**Velocity**defined as**rate of change of displacement**. It is vector quantity.- Velocity = displacement / time

Rate of change of displacement tell you on the displacement change with respect to time. For example, if an object move with velocity of 2ms^{-1} and start from the rest. When t=0, d=0; t=1, d=2; t=2, d=4.

Figure 2.4 shows the graph of displacement against time. From the graph, we know that the gradient of the graph is d/t and it is exactly the velocity. Hence we can make a conclusion that the gradient of the graph of displacement against time is equal to **velocity**. This is a very important concept, but student seldom take note about it. In this graph, the gradient is constant (velocity constant), and is equal to 2ms^{-1}

Another important thing is that the direction is very important to velocity. When we mention about constant velocity, it means that the object travels with constant magnitude and in same direction. When the direction change, although the magnitude still the same, we cannot say it have a constant velocity.

Figure 2.5 shows a person move with 2ms^{-1 }along a circular path. The magnitude of the quickness is the same, but the direction keeps changing. Hence we say that the velocity of this person keep changing when he moves through the path.