Motion
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3. Speed
 Speed is defined as the total distance travelled by the object in the time interval during which the motion takes place. SI unit of speed is meter per second. So,
where s is the distance travelled by the body and t is the time taken by the body to travel distance s.
 Speed of a body gives us the idea how slow or fast that body is moving.
 The ratio of total distance to total time taken by the body gives its average speed. The speed of a body at a given instant is its instantaneous speed.
 A body is said to have constant or uniform speed if it travels equal distance in equal intervals of time.
4. Velocity
 The rate of change of displacement of a body with the passage of time is known as velocity of the body. Velocity of an object is measured in meter per second in SI units. So,
 Velocity is nothing but the speed of an object moving in a definite direction.
 The velocity of an object can be uniform or variable. It can be changed by changing the object’s speed, direction of motion or both.
 So velocity of a body is a vector quantity involving both distance and displacement whereas speed of a body is a scalar quantity and it only has magnitude and does not have specific direction.
 Thus a body is said to be moving with uniform velocity if it covers equal distances in equal intervals of time in a specified direction.
 A body is said to be moving with non uniform velocity if it covers unequal distances in equal intervals of time and viceversa in a specified direction or if it changes the direction of motion.
 The velocity of a body can be changed in two ways first by changing the speed of the body and second by changing the direction of motion of the body by keeping the speed constant. Also both speed and direction of the body can be varied in order to change the velocity of the body.
 When velocity of the object changes at a uniform rate, then average velocity is given by the arithmetic mean of initial velocity and final velocity for a given period of time. That is,
Where u is the initial velocity of the object and v is the final velocity of the object.
5. acceleration

Acceleration is a measure of the change in the velocity of an object per unit time and mathematically it is given as
 If the velocity of an object changes from an initial value u to the final value v in time t, the acceleration a is given by,
\(a = \frac{{v  u}}{t}\)
and this kind of motion is called accelerated motion.
 A body has uniform acceleration if it travels in a straight line and its velocity increases by equal amount in equal intervals of time for example freely falling bodies, motion of ball rolling down the inclined plane etc.
 A body has non uniform acceleration if its velocity increases or decreases by unequal amount in equal intervals of time.
 If acceleration is in the direction of the velocity then it is positive acceleration and if it is in the direction opposite to the direction of velocity then it is negative and the negative acceleration is termed retardation.
 SI unit of acceleration is ms^{2}.
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