Class 7 Science

# Motion and Speed

Learning Goals

• Types of motion
• Speed: Uniform speed and Average Speed

When an object changes its position with elapse of time in respect to a stationary object, such as a building, electric pole, etc., the object is said to be in motion.

When the position of a car changes from one pole to another, we say that the car is in motion. But when the position of a car does not change with change in time, we say that the car is in rest.

Slow and Fast Motion: If an object A covers a distance in less time and another object B covers the same distance in more time, the object A is called a faster moving object and object B is called a slower moving object. In other words, object A has faster motion and object B has slower motion.

## Types of Motion

### Rectilinear Motion

Motion along a straight line is known as RECTILINEAR MOTION OR MOTION ALONG A STRAIGHT LINE. For example: Motion of a car along a straight line. A band of army marching during a parade is an example of rectilinear motion.

### Curvilinear Motion

Motion along a curve line is known as CURVILINEAR MOTION. For example: Motion of car or any moving object along a curve line.

Rectilinear motion is rarely seen in real life. Most of the objects show curvilinear motion. Let us take the example of a car. Roads keep on bending at frequent distances. So, when a car moves on such a road, it cannot maintain rectilinear motion. So, the car generally shows curvilinear motion.

### Circular Motion

Motion along a circle is known as CIRCULAR MOTION. For example: Motion of cyclist along a circular park.

## Speed

Distance covered by an object in unit time is called speed.

Speed = Distance ÷ Time

Speed may be divided into three types: Uniform speed, Non-Uniform Speed and Average Speed

### Uniform Speed

If an object covers a fixed distance in each unit of time the motion is known as UNIFORM MOTION or UNIFORM SPEED. Uniform speed is rarely seen in real life because objects keep on changing their speed.

### Non-uniform Speed

If an object does not cover a fixed distance in each unit of time the motion is known as NON-UNIFORM MOTION or NON-UNIFORM SPEED. Non-uniform speed is the norm in real life.

If a car starts from a point, it takes some time to gather certain speed. After some time, the car may have to slow down due to traffic load. The car also needs to slow down when it has to take turn to keep up with bends on the road. So, the speed of a car keeps on changing and thus it shows non-uniform speed.

### Average Speed

Total distance covered divided by total time taken is known as AVERAGE SPEED.

Average Speed = Total Distance Covered ÷ Total time taken

Speed is also known as VELOCITY but there is a difference between the two. Velocity is a vector while speed is scalar quantity. A quantity with both measurement and direction is known as vector. While talking about velocity, we also talk about direction of motion.

## Review Question:

Question 1: A car covers a distance of 100 km in 5 hour, calculate its speed.

Answer: Given, Total distance = 100 km

Total time taken = 5 hour

We know that, Average Speed = Distance ÷ time

= 100 km ÷ 5 hour = 20 km per hour

Question 2: A car covers 20 km in the first hour and cover 30 km in the last 4 hours. Find its average speed.

Answer: Given, Total distance covered = 20 km + 30 km = 50 km

Total time taken = 1 hour + 4 hour = 5 hour

We know that, Average Speed = Distance ÷ time

= 50 ÷ 5 = 10 km per hour