Basic Knowledge of Uniform and Non-Uniform Motion and Average Speed

Everyday we move from place to place.

Either it's on the road or by train or by air, we are moving.

While moving, we are said to be in motion.

When an object is in motion, it is moving from one position to another from one instant to another.

We can rephrase it as "the body is changing its position with respect to time".

Let's understand the motion of this car. It covered $4 m e t e r s$ in $2 s e c o n d s$ from O to A same as A to B and B to C. It covered $4 m$ in all $2 s$ of time.

The rate of change of position of the car is $4 m e t e r s / 2 s e c o n d s$ = $2$ $m / s$ . This $2 m / s$ is said to be the $S P E E D$ of the car.

Speed

$S P E E D$ of a body is defined as the rate of change of position of the body. It is equal to the ratio of the distance travelled in a time interval and the length of the time interval.

Depending on the value of speed, motion is of two types.

They are uniform and non-uniform motion.

Uniform motion

When a body is covering equal distances in equal intervals of time, its speed will be uniform throughout the journey. This is called uniform motion.

One example of uniform motion is the motion of the second's hand in the clock. It always covers one circle for every minute.

The motion of fan a while after switching on becomes uniform. This is also an example of uniform motion.

The revolution of the earth around the sun is uniform motion. As is the motion of the moon revolving around the earth.

Now let's learn about non-uniform motion.

Non-uniform motion

When a body is covering $u n e q u a l$ $d i s t a n c e s$ in equal intervals of time, then that motion is called $n o n - u n i f o r m$ $m o t i o n$ .

In this type of motion, the speed of the body is not constant.

There are many examples of Non-Uniform Motion.

When we travel in a car, we push the accelerator to increase the speed and brake to decrease the speed. This is an example of non-uniform motion.

A train coming to halt is an example of non-uniform motion as it decreases its speed.

Running in a race is also an example of non-uniform motion, as you start with high speed. But eventually, you slow down.

In non-uniform motion, it's difficult to know the speed at each instant. So we study this kind of motion using Average speed.

Average speed

For example in a car race, a car travelled a distance of 500km in non-uniform motion in 5 hrs.

The instantaneous speed is always shown in the speedometer.

In this example, If the car had travelled uniformly then that speed would be S = $\frac{t o t a l d i s t a n c e}{t o t a l t i m e}$ . S = $\frac{5 0 0 k m}{5 h r s}$ S = $100 k m p h$ . This is nothing but average speed.

Average speed $(S_{A v g})$ is the ratio of the total distance travelled by the object and the total time taken to cover this distance.

Let’s take another situation to understand average speed.

Let’s consider a car $A$ is travelling with a uniform speed of $50 k m p h$ for $2 h r s$ . So the total distance travelled by the car is $50$ x $2 = 100 k m$

So the average speed is $S_{a v g}$ = $\frac{t o t a l d i s t a n c e}{t o t a l t i m e t a k e n}$ $S_{a v g}$ = $\frac{1 0 0}{2}$ $=$ $50 k m p h$ . So the average speed is same the speed in case of uniform motion.

Now let's take another car B which moved with a speed of $50 k m p h$ for $1 h r$ and $60 k m p h$ for $1 h r$ . Then this is non-uniform motion. Now let's try to calculate the average speed.

The total distance travelled is $D = A B + B C$ . $A B$ = $50 K m p h$ X $1 h r$ = $50 k m$ $B C$ = $60 K m p h$ X $1 h r$ = $60 k m$ $D = 50 + 60 = 110 k m$ $T o t . t i m e = 1 + 1 = 2 h r s$

Average speed = $\frac{T o t . D i s t a n c e}{T o t . T i m e}$ Average speed = $\frac{1 1 0}{2} = 55 k m p h$

Now we have studied Average speed.

Let's take a small recap

Bodies are said to be in uniform motion when they cover an equal distance in equal intervals of time.

The speed of the body remains constant throughout the journey in uniform motion.

Bodies are in non-uniform motion when they don't cover equal distances for equal intervals of time.

Bodies will have varying or changing speeds throughout the journey. ⇒ Velocity does not remain constant.

Average Velocity is the ratio of total distance covered to total time taken

Average speed doesn’t depend on the varying speeds. And it depends only on the distance covered in a given time. $S_{A v g} = \frac{T o t a l d i s t a n c e t r a v e l l e d}{T o t a l t i m e t a k e n}$

The End