Hg
Name: Class 9
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The Hg Classes (8
th
to 12
th
) By: Er Hershit Goyal (B.Tech. IIT BHU), 134-SF, Woodstock Floors, Nirvana Country, Sector 50, GURUGRAM +91 9599697178.
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Motion
• Motion is change of position; it can be described in terms of the distance moved or the displacement.
• Distance – The total length covered by an object to go from point A to point B is the distance. Distance
is a scalar quantity. Unit – m or km
• Displacement – The shortest distance between point A and point B is the displacement between A and
B. Displacement is a vector quantity.
• Uniform Motion – If an object covers equal distances in equal intervals of time, then the object is said
to be in uniform motion; otherwise, it is non-uniform.
• Speed and velocity – Distance covered per unit time is speed; and velocity is displacement per unit
time. Speed is a scalar and velocity is a vector quantity. Unit – m/s or km/hr
v = s/t
• Acceleration – It is the change in velocity per unit time. Unit – m/s
2
or km/hr
2
a = (v – u) / t
• Retardation – If speed decreases with time (v < u), acceleration is negative, which is called retardation.
• Average Speed = v
av
= Total distance travelled/Total time taken
In-case of velocity is changing at a uniform rate; v
av
= (u + v)/2
• Average Velocity = Total displacement/Total time taken
• Uniform and non-uniform motion of objects can be described well using graphs.
o Distance time or s-t graphs
o Velocity time or v-t graph
o Acceleration time or a-t graphs
• Graphs for uniform motion:
o s-t graph is a straight line, its slope (constant) gives us the speed.
o v-t graph is a horizontal line; area under it gives us the distance.
• Graphs for non-uniform motion:
o s-t graph is a curve, its slope is variable.
o v-t graph is a straight line, its slope (constant) gives us the acceleration.
o area under v-t graph gives us the distance travelled.
• 3 equations of motion:
o v = u + at
o s = ut + ½ at
2
o v
2
– u
2
= 2as ,where u is initial velocity, v is final velocity, s is distance travelled in time t, under
uniform acceleration a
• Uniform Circular Motion: If an object moves in a circular path with uniform speed, its said to be in
uniform circular motion.
• To convert speed in km/hr to m/s multiply by 5/18; and to convert speed in m/s to km/hr multiply by
18/5.
