1.Distinguish between speed and average speed:
The speed of a body is the distance travelled by the body in unit time interval. Speed is a scalar quantity. It is represented by the letter u or v.
The SI unit of  speed is ms^-1
The ratio of the total distance travelled by the body to the total time of journey is called the average speed.

2.What is mean by velocity?
The velocity of a body is the distance travelled by the body in a specified direction in unit time interval.

                      
Velocity is a vector quantity as it has both magnitude and direction. For velocity, both its magnitude and  direction must be known. Two bodies are said to be moving with same velocities, if they move with same speed in same direction. If two bodies move same speeds, but in different directions, their velocities are not to be the same.
       Unit of velocity is same as that of speed  ie, SI unit of velocity is ms^-1
3.Distinguish between uniform and non - uniform velocity:
If a body travels equal distances in equal intervals of time along a particular direction, the body is said to be moving with uniform velocity.
Eg: Rain drops reach on earth’s surface falling with uniform velocity.
If a body moves unequal distances in equal intervals of time or if it moves equal distances in equal intervals of time, but its direction of motion is not the same, then the velocity of the body is said to be non - uniform.
Eg: If a body moves in circular motion, the direction of motion changes continuously with time. So it has non  uniform velocity.
4.Distinguish between acceleration and retardation.
Acceleration is the change of velocity in unit time. 
                                    or
 Acceleration is the rate of change of velocity with time.

           
Acceleration is a vector quantity. SI unit  of acceleration is ms^-2
Negative acceleration is called retardation and its unit is same as acceleration.
5.What are timers? Mention any three timers:
The devices which are useful for analyzing motion in the laboratory are called timers. Following are three types of timers.
(i) Motion sensors.
(ii)Ticker tape timers or tape charts.
(iii)Photo gate timers.
6.How do motion sensors work?
Motion sensors use the ultrasonic echo technique to determine the distance of an object from the sensor. Connection of a data logger and computer to the motion sensor enables  a distance- time graph to be plotted directly as time in second along X-axis and distance in metre along Y- axis. Further data analysis by the computer allows a velocity - time graph to be obtained as time in second along  X – axis and velocity in ms^-1 along Y-axis.
7.Explain a ticker tape timer:
A ticker tape timer enables us to measure speeds and hence accelerations. A ticker tape timer has a marker which vibrates 50 times a second and makes dots at 1/50 second intervals as the paper tape being pulled through it. 1/50 s is called a ‘tick’ and 1/5 s is called a ten tick . The distance between successive dots equals the average speed of what ever is pulling the tape, in cm per 1/50 s, ie, cm per tick. The ten tick ( 1/5 s) is also used as a unit of time. Since ticks and ten ticks  are small we drop the average and refer to the speed. Then we can plot graph with time / ten ticks along X-axis and distance / cm along Y – axis. This graph represents a body moving with uniform speed since equal distances have been moved in each ten tick interval.
8.Explain velocity - time graphs:
If the velocity of the body is plotted against the time, the graph obtained is a velocity – time graph. It provides a way of solving motion programs.

 
The slope or gradient of a velocity - time graph represents the acceleration of the body.

9.Write the first equation for uniform acceleration:
If a body is moving with uniform acceleration ‘a ‘ and its velocity increases from  u  to  v and time taken is  t.
Change in velocity  = final velocity – Initial velocity  = v – u

10.Write the second equation for uniform acceleration:
The velocity of a  body moving with uniform acceleration increases steadily . Its average velocity therefore equals half the sum of its initial and final velocities.

11. Write the third equation for uniform acceleration:
From first equation of uniform acceleration,
                              V  = u + at

                 
12. Write the fourth  equation for uniform acceleration:

        
13. State Newton’s  first law and explain inertia:
According to Newton ‘s first law, every   body continues its state of rest or uniform velocity unless an external force makes it behave differently.
Inertia: Newton ‘s first law tells the definition of inertia. Every matter has a built - in opposition to changes its present state if it is at rest or if it is moving. This property  of matter is called inertia. The larger the mass of a body greater is its inertia,  ie,  the mass of a body measures its inertia.