Class 7 Science Chapter 9 Notes Motion and Time
→ Bar graph : A graph in which statistical data represented in the form of bars of different heights.
→ Graphs : A graph is a mathematical relationship between two interdependent physical quantities.
→ Non-uniform motion : When a body covers unequal distances in equal interval of time or equal distances in unequal interval of time, then it is said to be non-uniform motion.
→ Oscillatory motion : The to and fro motion of a simple pendulum about its mean position is called oscillatory motion.
→ Speed : The distance travelled by an object per unit time (either in one hour, in one minute or in one second) is known as speed
\(\text { Speed }=\frac{\text { Distance travelled }}{\text { Time taken }} \)
→ Time period : The time taken by a pendulum for one oscillation is called it’s time period.
→ Uniform motion : It can be defined as the motion in which a body travels equal distance in equal intervals of time.
→ Unit of time : A unit of time is any particular time interval, used as a standard way of measuring or expressing duration.
→ Oscillation : It is defined ar the process of repeating variation of any quantity or medure about its equilibrium value in time.
→ Simple pendulum : It is another mechanical system that moves in an oscillatory motion.
→ Motion is defined as the change of position of an object with respect to time. Motion and rest are relative terms.
→ An object is said to be is at rest if the distance between the object and some reference point is not changing.
Types of motion :
- Translatory motion: If an object moves as a whole from one place to another, so that all its body parts move the same distance in a given time, it is said to be in a translatory motion. It is of two types :
- Rectilinear motion : If an object moves in a straight line, it is said to be in a rectilinear motion.
- Curvilinear motion : If an object moves along a Curved path, its motion is called curvilinear motion.
→ Circular motion: Motion of a body in a circular path around a fixed point is known as circular motion. Circular motion is of two types:
- Revolutionary motion : When an-object as a whole moves around a fixed centre, its motion is called r evolutionary motion.
- Rotatory motion : When an object moves in a circular path about its own axis, its motion is called rotatory motion.
→ Oscillatory motion: When an object moves to and fro about a fixed point, it is said to be in an oscillatory motion.
→ Periodic motion: The motion which repeats itself after a fixed interval of time is called a periodic motion.
→ Speed is defined as the distance travelled by an object in a unit time.
- Speed of objects helps us to determine which one is moving faster than the other.
- The basic unit of speed is metre per second (m/s).
→ A speedometer shows the speed of a vehicle with which it is moving and an odometer measures the distance travelled by it.
→ The change in the position of a moving body in a particular direction is called its displacement.
- If a body moves between two points along different paths, its displacement remains same for all the paths, though the distance covered by it may be different for the different paths.
- If a body starts moving from a point and returns to the same point, its displacement will be zero.
→ Velocity is defined as displacement per unit time. The SI unit of velocity is m/s.
- Speed of a moving body can never be zero but velocity of a moving object will be zero if it returns to its original position.
→ If an object travels equal distances in equal intervals of time, then it is said to be in uniform motion.
→ If an object travels unequal distances in equal intervals of time, then it is said to be in non-uniform motion.
→ Time refers to the interval between two events taking place.
- The SI unit of time is second.
Units of time in decreasing order :
- 1 millennium — 10 centuries
- 1 century — 10 decades
- 1 decade — 10 years
- 1 year — 365 days
- 1 day — 24 hours
- 1 hour — 60 minutes
- 1 minute — 60 seconds
Old devices that were used for measuring time are :
1) A candle clock is a thin candle with consistently, spaced marking, that when burned, indicates the passage of periods of time.
2) Sand clock is also known as the hour glass. A sand clock works on the principle that all the sand from the upper chamber fall into the lower chamber in a fixed amount of time.
3) The sundial is based on the principle of shadow formation due to sunlight, the shadow of pointer varies with the position of the Sun in the sky and tells the hours of the day.
4) A water clock is a time keeping device that works on the principle of inward or outward regulated flow of water.
→ Though earlier clocks were not very accurate, modern watches were discovered, e.g., mechanical, digital, quartz and stop watch.
→ A simple pendulum consists of a small metal ball (or a small mass), known as bob, suspended by a light, flexible and inextensible string.
- The bob of a pendulum exerts oscillatory motion.
- The frequency of a pendulum is the number of oscillations completed by it in one second.
- The time taken by the bob to complete one oscillation is called the time period of the pendulum.
- The time period of a pendulum depends upon its length.
→ A graph is a diagrammatic representation of two interrelated quantities. It has two axes.
→The distance-time graph of a body moving in a uniform motion is represented by a straight line.
→The distance-time graph of a body moving in a non-uniform motion is represented by a curved line.
→ The distance-time graph of a stationary body is represented by a straight line, parallel to the time-axis.