When a body changes its position, it covers some distance within a certain time. To find the speed of a body we divide the distance covered by time taken.
Speed = distance covered / time taken
When an athlete covers a distance of 200 meters in 20 seconds, his speed is
200 meters / 20 meters = 10 m/s
The standard unit of distance is meter and the standard unit of time is second. Therefore, the standard unit of speed is meter/second or m/s (read as meter per second). For convenience, the speeds of some objects may be expressed in m/min (read as meter per minute or km/h read as kilometer per hour.
By knowing the speed, we can see whether the body is having slow or fast motion.
Suppose in a race, an athlete. A covers a distance of 200 meters in 20 seconds whereas another athlete B covers the same distance in 25 seconds.
Speed of athlete A = 200 m / 20 s or = 10 m/s
Speed of athlete B = 200 m / 25s = 8 m/s
Speed of A is greater than that of B. It means that athlete A moves faster than athlete B.
Uniform and Non-Uniform Motion:
A car travels at a constant speed of 40 km/h. In the first hour it covers 40 km, in the second hour 40 km, and so on.
We will say that this car has uniform motion.
A body is said to have uniform motion in it covers equal distances in equal intervals of time.
An athlete starts out with a speed of 5 m/s. after one hour, he is tired and his speed slows down to 3 m/s.
His speed is not constant. A body is said to have non-uniform motion if it moves unequal distances in equal intervals of time.
Variation in the Speed of a Moving Object:
We can change the speed of a moving object. Suppose a cyclist is moving at a speed of 10 km/h. If he is given a push from behind, his speed increases. If he is pulled from behind, his speed decreases. This is because the push is applied in the direction of motion of the bicycle, and pull is applied opposite to its direction of motion. Push or pull is a kind of force which can change the speed of a moving object. Similarly, cyclist stops the bicycle by applying force on the brakes.
It is thus clear that forces can change the speed of a moving object.
Velocity: The Velocity of an object is its speed in a particular direction.
We can say,
Velocity = Distance traveled in a given direction / time taken
If an object travels a distance ‘s’ in time ‘t’ in a given direction, then its velocity ‘v’ is given by:
V = s / t
The unit of velocity is the same as that of speed, namely meters per second (m/s).