Change of position is called motion. It can be best described in terms of distance moved or displacement. Change in position is taken with respect to an object at rest which is called reference point.
Uniform Motion: Body covers equal distance in equal time interval.
Non-Uniform Motion: Body covers unequal distance in equal time interval.
Distance travelled per unit time is called speed. SI unit for speed is m/s.
Speed = Distance travelled/Time taken
It is a scalar quantity as it has magnitude only.
Average Speed: The ratio of total distance travelled to the total time taken by the body gives its average speed.
A body is said to have constant or uniform speed if it travels equal distance in equal interval of time.
Displacement per unit time is known as velocity. It is nothing but speed of an object in a definite direction.
Velocity = Displacement/Time taken
It is a vector quantity as it has both magnitude and direction.
Average Velocity: It is the arithmetic mean of initial velocity and final velocity for a given period of time.
Rate of change of velocity with time is known as acceleration. SI unit of acceleration is m/s^2.
Acceleration = Change in velocity/Time taken
A body has uniform acceleration if it travels in a straight line and its velocity increases by equal amounts in equal intervals of time.
A body has non uniform acceleration if it travels in a straight line and its velocity increases or decreases by unequal amounts in equal intervals of time.
Actual length of path covered by a moving body is called distance. Distance is independent of direction, hence it is a scalar quantity.
When a body moves from one position to another the shortest distance between the initial and final position of the body along with its direction is known as displacement. It is a vector quantity.
EQUATIONS OF MOTION
First Equation of Motion: Velocity - time relationship
The first equation of motion is v = u + at, where v is the final velocity and u is the initial velocity of the body and a is uniform acceleration.
First equation of motion gives velocity acquired by body at any time t.
Second Equation of Motion: Position - time relationship
Second equation of motion is s = ut + 1/2 at^2, where u is initial velocity, a is uniform acceleration and s is the distance travelled by body in time t.
Second equation of motion gives distance travelled by a moving body in time t
Third Equation of Motion: Position - Velocity relationship
Third equation of motion is v^2 = u^2 + 2as, where u is initial velocity, v is the final velocity, a is uniform acceleration and s is the distance travelled by the body.
This equation gives the velocity acquired by the body in travelling a distance s.
GRAPHICAL REPRESENTATION OF MOTION
To describe motion of an object, we can use line graphs.
Distance Time Graphs
Slope of distance time graph can be used to calculate speed of a body.
Distance on y axis, time on x axis.
For uniform speed: Straight line.
For non-uniform speed: Curved line.
For Increasing speed: Straight line sloping upwards.
For decreasing speed: Straight line sloping downwards.
For objects at rest: Straight line parallel to x axis.
Velocity Time Graphs
Slope of velocity time graph can be used to calculate acceleration.
Area enclosed by velocity time graph and time axis gives magnitude of displacement.
Velocity on y axis, time on x axis.
Velocity time graph when speed remains constant (no acceleration): Straight line parallel to x axis.
Velocity time graph showing acceleration: Straight line.
UNIFORM CIRCULAR MOTION
When an object moves in a circular path at a constant speed then motion of the object is called uniform circular motion.
Examples of uniform circular motion are:
(a) Motion of artificial satellites around the earth.
(b) Moon moves in uniform circular motion round the earth.
(c) Cyclist moving on a circular track with a constant speed
IMPORTANT POINTS TO REMEMBER