CBSE Notes Class 7 Science Chapter 9 Motion And Time

Motion is a key concept in science that explains how objects change their position over time. We encounter various types of motion in our daily lives, whether it’s a car driving along the road or a child playing on a swing. By studying motion, we can gain insights into how and why objects move, as well as the various factors that influence their movement.

1.0Motion

• Motion refers to the change in an object's position over time. When an object's position remains constant with respect to time, it is considered to be at rest, stationary, or immobile. Conversely, any object that is changing its position is said to be in motion.
• Motion is determined by the frame of reference. For example, when a person standing outside a bus is used as a reference point, the bus is seen as moving. However, if a person sitting inside the bus is the reference, the bus appears to be at rest.

1. Oscillatory Motion:

• Oscillatory motion refers to the movement of a body back and forth between two extreme positions. The midpoint between these extremes is known as the mean position. The time taken to complete one full oscillation is called the time period. When an oscillatory motion repeats at regular intervals, it is classified as periodic motion. While not all periodic motions are oscillatory, all oscillatory motions are inherently periodic. 
• Examples of oscillatory motion include pendulums, swings, and objects attached to springs. In contrast, examples of periodic motion encompass the swinging of a pendulum, the rotation of the Earth on its axis, and the Earth's orbit around the Sun.

2. Uniform And Non-Uniform Motion: For uniform motion if it covers equal distances in equal time intervals, while an object in non-uniform motion travels varying distances in the same time intervals.

2.0Speed

Speed is the rate at which an object's position changes over time. The average speed of an object during a specific time interval is calculated by dividing the distance traveled by the duration of that interval.

Speed= $\frac{TotalDistanceCovered}{TotalTimeTaken}$

• The unit of distance is measured in meters or kilometers, while time is measured in seconds or hours. Consequently, common units of speed are expressed as meters per second (m/s) or kilometers per hour (km/h). It's important to note that units are always written in singular form; for instance, it should be km/h, not kms/hrs.
• Fast and Slow Motion/Speed-Speed can be classified as fast or slow based on the distance covered within a specific time interval. For example, a slower-moving object will cover less distance compared to a motorbike, which travels at a faster speed.

3.0Graphs

A graph is a visual representation that illustrates the relationship between two or more variable quantities. A simple graph typically displays the connection between two measurements or values in a grid format. Graphs and charts can take various forms depending on the number of quantities involved and the method of representation used.

4.0Distance-Time Graph

This is typically represented as a line graph, as it involves two variable quantities: distance and time. In a distance-time graph, distance is plotted on the Y-axis (vertical), while time is represented on the X-axis (horizontal).

When selecting the most suitable scale for drawing a graph, consider the following points:  

1.  Identify the maximum quantity for each axis.
2.  Divide this maximum quantity into smaller, equal segments to facilitate easy point marking. The number of segments should be based on the available data
3.  Aim to create a graph that utilizes as much of the paper space as possible, ensuring it appears clear and organized.

5.0Measurement Devices In Vehicles And Other Instruments

• Speedometer: Measures and displays the vehicle's instantaneous speed.
• Odometer: Measures and shows the distance traveled by the vehicle.
• Tachometer: Measures and displays the revolutions per minute (RPM) or the rotational speed of a shaft or disc.