Motion and Time Class 7 Science Notes - Chapter 13

An Introduction to Motion

Distance

  • Distance is the total path covered by the object in the given interval of time.
  • Displacement is the shortest path covered by the moving object in the given interval of time.
  • Distance=Speed×Time.
  • Unit of distance is generally measured in units of length like metres, kilometres etc.

Introduction to motion

  • An object is said to be in motion if it changes its position with respect to time. Eg: A car moving on a road.
  • An object is said to be at rest if the object does not change its position with time. Eg: A person standing on the ground.

Basics of Standard Units

Units and their standardization

  • The standard unit of distance is in metres.
  • The standard unit of time is in seconds (s).
  • The standard unit of speed is in metre per second (m/s).

Basics of Motion and Its Types

Types of motion

Types of motion are generally divided into 3. They are:

1. Rectilinear / translatory motion: When a body is moving in a straight line, without changing its direction, then the body is said to possess translatory/rectilinear motion.
Eg: A car moving on a straight road.

2. Circular motion: When a body moves in the shape of a circle about a fixed point and a fixed radius, then the body is said to be in circular motion.
Eg: Motion of planets around the sun.

3. Period / Oscillatory motion: When the motion of a body repeats after fixed intervals of time, then the body is said to in the periodic/oscillatory motion.
Eg: To and fro motion of a simple pendulum.

 

Periodic or oscillatory motion

  • Periodic or oscillatory motion is the motion in which a body repeats its motion after fixed intervals of time.
  • Eg: To and fro motion of a  simple pendulum, Motion of a car in a  circular path, Motion of planets around the sun.

Oscillations of a simple pendulum

  • When the bob of a simple pendulum moves from its mean position B to A and back to B again, then from B to C and back to B again, the pendulum is said to complete 1 oscillation.
  • In the case of the simple pendulum, the time period is the total time taken by the pendulum to complete one oscillation.

Time and Speed

Speed (Average Speed)

  • Speed is the total distance travelled by the object in a given interval of time.
  • Speed = Distance travelled/time taken
  • Unit of speed is generally measured in metre per second(m/s), kilometre per hour(km/h).

Uniform and Non-Uniform Motion

1. Uniform motion: When an object moving along a straight line moves with a constant speed, then the object is said to be in uniform motion.
Eg: A car moving in a straight line with a constant speed.

2. Non-uniform motion: When an object moving along a straight line changes its speed with respect to time, then the object is said to be in uniform motion.
Eg: The motion of a train.

Measurement of Time

  • Unit of time is generally measured in seconds, minutes, hours.
  • The time period is the total time taken by an object to complete one oscillation.

Units of Time

Unit of time is generally measured in seconds, minutes, hours.

Units of Speed

  • Speed = Distance travelled in the total time interval.
  • Unit of speed is generally measured in metre per second (m/s) and km/h( kilometre per hour).

Speedometer and Odometer

  • Speedometer records the speed of the vehicle directly in kilometre per hour (km/h).
  • An odometer measures the distance moved by the vehicle directly in kilometres(km).

Visualizing Motion

Distance-time graph

  • Distance-time graph is the graph plotted between the distance( in y-axis) and time ( in x-axis).
  • The slope of a distance-time graph gives the speed of an object.

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