Define the following terms : (a) Speed, (b) Uniform speed, © Variable speed, (d) Average speed, (e) Instantaneous speed, (f) Velocity, (g) Uniform velocity, (h) Variable velocity, (i) Uniform motion, (j) Average velocity in uniform motion, (k) Relative velocity, (1) Instantaneous velocity.

Speed: It is the ratio of total path length & corresponding time take by an object.

(a) Speed is a scalar quantity.

(b) The speed of a body can be zero or positive but never negative.

© The speed of a body can increase or decrease with time. .

(d) The C.G.S. unit of speed is cm/s and S.I. unit is m/s.

(e) If the speed of a body is zero, the body is at rest.

(f) The distance time graph of a body at rest, is a straight line parallel to time axis.

Types of speed

(i) Uniform speed. (Dimensions are [M°LT-1]):

(a) A body is said to be moving with a uniform speed, if it covers equal distances in equal intervals of time, howsoever

small these intervals may be

If a body moves with a constant speed, distance travelled by it in each second is the same.

The distance travelled at the end of 1st, 2nd, 3rd, …second will be in the ratio 1:2:3:

(ii) Variable speed. A body is said to be moving with a variable speed if it covers equal distance in unequal intervals of time or unequal distances in equal intervals of time, howsoever small these intervals may be.

(iii) Average or Mean speed. If a body is moving with a variable speed, then the average speed of the body is defined as the ratio of total distance travelled by the body to the total time taken, i.e.,

(iv) Instantaneous speed. When a body is moving with a variable speed, then the speed of the body at a given instant of time is known as its instantaneous speed. The body possesses different speed at different instant.

Velocity : It is as the ratio of displacement & the corresponding time interval taken by object.

(a) It is vector quantity

(b) It can be positive, negative or zero.

© Unit—cm/s in cgs system &m/s in SI.

(d) Dimensions—[M°L’ tr1]

(i) Uniform velocity. A body is said to be moving with a uniform velocity, if it undergoes equal displacements in equal intervals of time, howsoever small these intervals may be.

(ii) Variable velocity.

(a) The body is said to be moving with a variable velocity if it covers equal displacements in unequal intervals of time or unequal displacements in equal intervals of time or changes direction of motion while moving with a constant speed.

(b) The position-time graph of a body moving with variable velocity is a curve.

(iii) Average or Mean velocity.

(a) When a body is moving with a variable velocity, then the average velocity of the body for a given time is defined as the ratio of the total displacement of the body to the total time taken

When a body is moving with a uniform velocity, its average velocity is equal to its uniform velocity.

(iv) Instantaneous velocity. If a body is moving with a variable velocity, then the velocity of the body at a given instant of time is called its instantaneous velocity.