Short notes of chapter-2(motion) of class-7

 ☆MOTION

●REST-

- A body is said to be at rest if it does not change its position with time with respect to a reference point or a fixed point in its surrounding.

-Eg. Table, books, chair, table lamp, sofa set etc.,

●MOTION-

-A body is said to be in motion if it changes its position with time with respect to a reference point or a fixed point in its surroundings.

-Eg. Moving car, flying birds, moving train, running fan etc.,

● Rest and motion are relative terms-

-An object can be in motion relative to one set of objects while at rest relative to some other set of objects.

-Absolute rest or absolute motion is not possible.

-A body may be at rest with respect to one observer and at the same time it can be in the state of motion for another observer.

☆Different types of motion-

1.Translatory motion-

-A motion in which every particle or point of the object moves through the same distance in the same time is called translatory motion.

-Eg. Motion of a bicycle, a falling Apple, a moving box, walking man etc.,

- translatory motion can be of two types-

  ●Rectilinear motion/ linear motion-

- the the motion of of a a body along a straight line.

- example ball falling in vertically downward direction, car moving on a straight path, coin moving on a carrom board etc.,

● curvilinear motion-

-The motion of a body along a curved path.

- example cyclist taking a turn on the road, a car moving along a curved path, a ball thrown by a boy in the horizontal direction etc.,

2.Rotatory motion-

-A body D is said to be in a rotatory motion if it moves about a fixed axis. 

Or

The the motion in which all particles of a body move along circular path of different radii having centres in the same straight line is called rotatory motion the straight line containing centres of all circular part is called the axis of rotation.

- example spinning top, spinning wheel, ceiling fan or table fan ,motion of Potter's wheel, motion of a grinder etc.,

Note- different parts of the object move through different distances during the same time. The part of the body near the axis of of rotation travels a smaller distance then the distant Parts of the body.

3. Circular motion-

-The motion of a body along a circular path.

- special type of curvilinear motion in which the distance of a moving object from a fixed point does not change.

- example car around a circular path, satellite it around the earth, a girl is whirling a stone tied at the end of a string in a circular path, the the motion of the hands of a clock etc.,

4. Oscillatory motion-

-The to and fro motion of a body from its rest position or mean position is called the oscillatory motion.

- example motion of a simple pendulum, a swing, piston of a heat engine, sewing machine etc.,

5. Vibratory motion-

-The to and fro motion of a body from its rest position and a part of the body always remains fixed.

Or

A motion in which a part of the body always remains fixed and the rest part moves to and fro about its mean position.

- the shape and size of the body changes.

- example a wire stretched between two fixed rods A and B on a table, vocal cords , tabla, Dholak, musical instruments like guitar, violin, sitar, vibratory motion of a metallic strip with its one end fixed etc.,

6. Periodic motion-

-A motion which get repeated after a regular interval of time is called a periodic motion.

- body repeat its motion along the same path.

- example Earths  rotation in 24 hours, Earth revolution in 365 & 1/4 days, moon Revolution around the earth in 27.3 days, heart beats every 0.8 seconds, pendulum of a wall clock, needle of a sewing machine etc.,

7.Non-periodic motion-

- The motion which does not repeat itself after a regular interval of time is called Non-periodic motion .

- example footballer running on field, rolling of a ball,Tides in the sea, stopping of a vehicle, motion of a Potter's wheel, wheel of a bicycle while braking applied, spinning top ,etc.,

8. Random motion-

-The Zig -Zag motion of a body without any specific path and specific direction is called random or Zig-Zag motion.( at a molecular level this motion is known as Brownian motion)

- example butterfly , mosquitoes etc.,

9. Mixed motion/ Complex motion-

-A  body can have more than one type of motion simultaneously known as mixed motion.

- example The Wheels of a moving vehicle, drill machine, carpenters saw, a rolling ball, Earths motion etc.,

☆Speed-

- The distance travelled by the body in unit time.

- Speed is scalar quantity.

-speed=disance travelled/ time taken

- s=d/t

-S.I. unit= unit of distance/ unit of time

                =m/s or ms-¹

- other units 

Km/h

- relationship-

      1ms-¹=3.6 kmh-¹

       18 kmh-¹= 5ms-¹

      1kmh-¹=0.277 ms-¹

      1ms-¹= 100 cms-¹

      1cms-¹=0.01 ms-¹

      1ms-¹=18/5 kmh-¹

1kmh-¹=5/18 ms-¹

☆ uniform motion-

- A body covers equal distances in equal intervals of time, its motion is called uniform motion.

- example motion of a car with a constant speed, train running at a constant speed etc.,

☆ Non-uniform motion-

- if a body covers an unequal distances in equal intervals of time.

Or

Body covers equal distances in an equal intervals of time.

- example usual motion of a train between its two stoppages, motion of a bike on a busy road, athlete running on a track etc.,

☆ average speed-

-The ratio of the total distance travelled by the body to the total time taken in the journey.

-Average speed= Total distance/Total time taken

-S.I. unit is m/s or ms-¹

-a scalar quantity.

☆Mass-

- The quantity of matter contained in a body is called its mass.

- man is constant everywhere.

-S.I. unit is kg

- scalar quantity.

- Measures by a beam balance .

- mass of a body cannot be zero.

☆weight-

- The weight of a body is the the force with which Earth attracts the body.

- Varies  from place to place because it depends on the acceleration due to gravity (g).

-S.I. unit is newton(N).

-Vector quantity.

-Measured  by a spring balance.

- weight of a body is zero at the centre of earth and In an artificial satellite.

-  w= m×g   ,m= mass of a body

                      g= acceleration due to gravity.

     g=9.8 m/s² 

    g=9.8 Nkg-¹

Note- the the man of of a body is same on earth surface as well as on moon's surface but the weight of the body on moon's surface is (1/6)th of the weight of the body on earth surface because the force of attraction of moon on that body is 1/6 th the force of attraction exerted by the earth.

-  1 Newton is nearly the force of attraction of Earth on a mass of 0.1 kg.

- 1kgf=9.8 N