Motion and Time
Class 7 Physics - Comprehensive NCERT Guide
Motion
Straight Line Motion
This is when an object moves in a straight line.
Examples:
A car moving on a straight road.
A stone dropped vertically down.
A train moving on a straight track.
Circular Motion
This is when an object moves along a circular path, around a fixed point or axis.
Examples:
A fan blade rotating.
The moon revolving around the Earth.
A child on a merry-go-round.
Periodic Motion
This is when the motion repeats itself after a fixed interval of time.
Examples:
A pendulum swinging back and forth.
The vibration of a guitar string.
Earth's revolution around the Sun.
1. Rotation - Motion in which an object spins around its own axis. The axis is inside the object.
Examples:
Earth rotating on its own axis (causes day and night).
A spinning top.
A fan blade spinning.
2. Revolution - Motion in which an object moves in a circular path around another object or external point. The axis is outside the object.
Examples:
Earth revolving around the Sun (causes seasons).
Moon revolving around the Earth.
A student walking around a circular park.
Soldier in march past - motion in straight line (Rectilinear Motion)
Motion of a train on a straight bridge - motion in straight line (Rectilinear Motion)
Motion of a horse pulling a cart on a straight road - motion in straight line (Rectilinear Motion)
Bullock cart on straight - motion in straight line (Rectilinear Motion)
Hand of athlete in race - Periodic
Pedal of bicycle - Circular Motion
Motion of a child in a merry-go-round - circular motion
Earth around the Sun - Circular (Revolution) + Periodic
Motion of a child on a see-saw - oscillatory + periodic
Motion of the hammer of an electric bell - oscillatory
Motion of Swing - Periodic Motion + Oscillatory
Motion of Pendulum - Periodic Motion + Oscillatory
Motion of your hands while running - periodic + oscillatory
Speed
Speed is the distance covered by an object in a unit time. It is the measure of how fast an object is moving.
It is defined as total distance covered by total time taken.
Speed - Tells how fast something is moving right now
Average Speed - Tells how fast something moved on average over a trip
Speed - Can change from moment to moment
Average Speed - Takes the whole journey into account
Example:
Speedometer of a car shows current speed
Average speed: Total distance covered in journey ÷ total time taken
If an object moving along a straight line with a constant speed, it is said to be in uniform motion. Or, if an object covers equal distance in equal interval of time, it is said to be in uniform motion.
If the speed of an object moving along a straight line keeps changing, its motion is said to be non-uniform. Or, if an object doesn't cover equal distance in equal interval of time.
Measure of Time
They looked at the position of the sun or the length and direction of shadows to estimate the time of day.
A day is the time between one sunrise and the next sunrise.
A month was measured from one new moon to the next new moon.
A year is the time taken by the Earth to complete one revolution around the Sun.
We use clocks and watches to measure time intervals shorter than a day.
The motion which repeats itself at regular interval of time is known as periodic motion. For example, motion of the Earth around the Sun.
All clocks and watches work using some kind of periodic motion, which repeats regularly.
Simple Pendulum
A simple pendulum is a device that consists of a small metallic ball (called a bob) suspended from a rigid stand using a thread or string. The bob can swing freely back and forth.
The metallic ball of the pendulum is called bob.
The to and fro (back and forth) motion of simple pendulum is called oscillatory motion.
One complete oscillation of a pendulum occurs when its bob starts from the mean position, moves to one extreme position, then to the other extreme position, and finally comes back to the mean position.
The time taken to complete one oscillation is called the time period of the pendulum.
Given:
Total time = 32 seconds
Number of oscillations = 20
He found that a pendulum of a given length takes always the same time to complete one oscillation.
It means that: If we do not change the length of the pendulum, then it will always take the same amount of time for one complete oscillation, no matter how many times we swing it.
Example:
Suppose a pendulum is 1 metre long, it will always take, say 2 seconds for one complete oscillation. Then it will always take 2 seconds, no matter how many time we swing it.
This observation led to the development of pendulum clocks. Winding clocks and wristwatches were refinements of the pendulum clocks.
The time period of a pendulum depends on the length of the thread.
It does NOT depend on:
i. The mass of the bob
ii. The material of the bob
iii. The size of the bob
A quartz clock is a clock that uses a tiny piece of quartz crystal to keep time very accurately. They are more accurate than old mechanical clocks.
Quartz is a special mineral. When electricity passes through it, it vibrates (oscillates) at a very constant rate. These vibrations help the clock count time accurately, second by second.
Units of Time and Speed
The basic unit of time is second (s).
The basic unit of speed is metre per second (m/s).
Some of the time measuring devices that were used before pendulum clocks were Sundials, water clocks and sand clocks etc.
Speedometer is the device which is used to measure speed. It records the speed in km/hr.
Odometer is a device which is used to measure distance traveled by a vehicle.
Do You Know?
The National Physical Laboratory, New Delhi provides time-keeping services in India.
It is located in New Delhi, India.
The clock they use can measure time intervals with an accuracy of one-millionth of a second.
The most accurate clock in the world has been developed in the U.S.A.
The National Institute of Standards and Technology (NIST) in the U.S.A. developed it.
Measuring time accurately is important for scientific research, space missions, communication, and navigation systems.
Atomic Clock
They are used in GPS, space research, atomic experiments, and to keep global time standards.
Explore Graphical Representations of Motion
For detailed explanations and visualizations of distance-time graphs and motion diagrams
View Graphs Q&A Blog
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