Chapter 8 Motion CLASS 9

If a body does not change its position with respect to time and the surroundings, it is said to be at rest and else it is said to be in motion. Motion of objects can take place along one direction, two directions or three directions at a time.

If an object moves along a straight path it is said to be linear or one-dimensional motion. If an object moves along two directions at a time like that of a ball hit for a sixer in a cricket, it is two-dimensional. The haphazard motion of a honey bee can be three-dimensional.

The change in position of an object is termed displacement. It requires both direction and magnitude for its complete description and hence such physical quantities are called a vectors. The length of the path covered by a moving body is its distance and is independent of direction. Thus, such physical quantities are called scalars.

The rate of distance covered by a body is its speed and is measured in metre per second in international units. If a body covers equal distances in equal intervals of time however small the intervals may be, it has uniform speed. If a body covers unequal distances in equal intervals or equal distances in unequal intervals then it is said be moving with non-uniform speed.

    •  Speed is a scalar quantity.

    •  Speed = distance travelledTime taken. .

    •  A body is said to be moving with uniform speed if it has equal intervals of time, however small these intervals may be.

    •  A body is said to be moving with non uniform speed  if it has unequal distances in equal intervals of time or equal distances in unequal intervals of time, however small these intervals may be.

Average speed:

    •  Average speed = total distance travelledtotal time taken

    •  Instantaneous speed = lim∆t→0∆s∆t= dsdt.

    •  If a particle covers the 1^st half of the total distance with a speed 'a' and the second half with a speed 'b'

                             Average speed =  2aba+b .

    •  If a particle covers 1^st 1/3^rd  of the total distance with a speed 'a', 2^nd  1/3^rd of the distance with a speed 'b' and 3^rd 1/3^rd of the distance with speed 'c'

                        Average speed =  3abcab+bc+ca .

    •  1 kmph = 518   ms^-1 ; 1mph = 2215   fts^-1

    •  For a body with uniform speed, distance travelled = speed x time.

The rate of displacement of a body is its velocity and is measured in metre per second in international units. If a body has equal displacements in equal intervals of time however smaller the intervals may be, it is said to be moving with uniform velocity. If the body is moving such that it has unequal displacements in equal intervals or equal displacements in unequal intervals of time, it is said to be moving with non-uniform velocity. The ratio of total displacement to total time taken by the body gives its average velocity. The velocity of a body at a given instant is its instantaneous velocity.

    •  Velocity is a vector quantity.

    •  For a body moving with uniform velocity , the displacement is directly proportional to the time interval.

    •  If the direction or magnitude or both of the velocity of a body change, then the body is said to be moving with non-uniform velocity .

    •  Average velocity = Net displacementTotal time taken

    •  For a body moving with uniform acceleration, the Average velocity = u+v2.

    •  The velocity of a particle at any instant of time or at any point of its path is called instantaneous velocity v⃗   =   lim∆t→0∆s⃗ ∆t=ds⃗ dt   .