Class 7 Foundation

About Lesson

Motion and Time

Types of Motion

In Class VI, you learned about different types of motions, including motion along a straight line, circular motion, and periodic motion. Table 9.1 lists some common examples of motions. Identify the type of motion in each case.

Table 9.1 Examples of Different Types of Motion

Example of Motion	Type of Motion
Soldiers in a march past	Along a straight line
Bullock cart moving on a straight road	Along a straight line
Hands of an athlete in a race	Along a straight line
Pedal of a bicycle in motion	Circular
Motion of the Earth around the Sun	Circular
Motion of a swing	Periodic
Motion of a pendulum	Periodic

9.1 Slow or

Fast

Vehicles and other objects move at different speeds. To decide which object is moving slow and which one is moving fast, compare the distances covered by different objects in the same amount of time. The object that covers a larger distance in a given time is moving faster.

Activity 9.1: Look at Fig. 9.1 and Fig. 9.2 showing vehicles moving on a road. By comparing the distances they cover over time, you can determine which vehicle is moving fastest and which is moving slowest.

9.2 Speed

Speed is the distance covered by an object in a unit of time. If we know the distance covered by two objects in one hour, we can tell which one is faster. The formula for speed is: $Speed = Total time taken Total distance covered$

For example, if a car moves with a speed of 50 km/h, it implies that it will cover 50 kilometers in one hour.

9.3 Measurement of Time

Various natural events repeat themselves at regular intervals, helping our ancestors to measure time. A day was measured from one sunrise to the next, a month from one new moon to the next, and a year from the time the Earth takes to complete one revolution around the Sun. Shorter time intervals are measured using clocks and watches, which use periodic motions like that of a simple pendulum.

Activity 9.2: Set up a simple pendulum to measure time intervals. The time taken by the pendulum to complete one oscillation is called its time period.

Table 9.2 Time Period of a Simple Pendulum

S.No.	Time taken for 20 oscillations (s)	Time period (s)
1	42	2.1
2
3

By dividing the time taken for 20 oscillations by 20, you get the time period of the pendulum.

9.4 Measuring Speed

To measure speed, you need to know the distance covered and the time taken. The basic unit of speed is meters per second (m/s), but it can also be expressed in kilometers per hour (km/h).

Activity 9.3: Measure the speed of a ball rolling on the ground. Note the time it takes to come to rest and the distance it covers. Use the formula: $Speed = Time Distance$

Table 9.3 Distance Moved and Time Taken by a Moving Ball

Name of the Group	Distance moved by the ball (m)	Time taken (s)	Speed (m/s) = Distance / Time

9.5 Distance-Time Graph

Graphical representations like bar graphs, pie charts, and line graphs make information easier to understand. A distance-time graph helps to visualize the motion of an object.

Steps to Create a Distance-Time Graph:

Draw two perpendicular lines to represent the x-axis (time) and y-axis (distance).
Choose appropriate scales for time and distance.
Mark the values on the respective axes.
Plot the points corresponding to different sets of values.
Join the points to form the graph.

If the distance-time graph is a straight line, the object is moving with a constant speed.

Example: Motion of a Car

S.No.	Time (min)	Distance (km)
1	0	0
2	1	1
3	2	2
4	3	3
5	4	4
6	5	5

Graphical Representation:

Choose a scale (e.g., Time: 1 min = 1 cm, Distance: 1 km = 1 cm).
Plot the points on the graph paper.
Join the points to form a straight line.

Distance-time graphs provide a visual representation of motion, making it easier to interpret data and compare speeds.

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