Lakhmir Singh Solutions Class 9 Physics Solutions for Exercise in Chapter 1 - Motion
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A motorcyclist drives from place A to B with a uniform speed of 30 km/h and returns from place B to A with a uniform speed of 20 km/h. Find his average speed.
Answer:
Speed from A to B = 30 km/h
Let D be the distance from A to B
Time taken, T1 to travel from A to B = distance travelled/speed
T1 = D/30
Speed from B to A = 20 km/h
Time taken, T2 from B to a = distance travelled/speed
T2 = D/20
Total time taken, T = T1 + T2 = D/12
Total distance from A to B and B to A = 2D
Therefore, average speed = total distance travelled/total time taken = 24 km/h
Video transcript
"hey kids welcome to lido q a video
i am vineet your lido tutor and i am
excited to bring you yet another
question on your screen a motorcyclist
drives from place a to b
okay so a to b
at a speed of
30 kilometer per hour and from b to a
again
at a speed of 20 kilometer
per hour right
find the average speed to find the
average speed we need the distance total
distance and the total time
now we don't know the total distance or
the time or the distance one way even
so what we are going to do is we are
going to find the total distance in time
in terms of one variable so
let distance
from a to b
be d
then
speed is equal to d by
t 1 and t 1
be the time taker
this implies 30 equals to d by t 1
implies t 1 is equal to d by
30 now we are getting t1 in terms of t
then we will get t2 in terms of t and
add them in terms of d
distance is already in terms of t then
we can find the average speed
let's see how from
b to a b
t 2
therefore t 2 will be
distance now since the distance f
from a to b is d
the same distance is covered from b2
by speed that is 20
right
therefore total
distance is equal to
d plus d that is equal to 2t
right total
time taken is equal to
t1 plus t2 that is equal to
d by 30 plus d by 20
that will be equal to
into 3 sorry into 2
into 3 because 60 is the lcm
so 2d plus 3 d by 16
this is equal to 5 t by 60 which is
equal to
d by 12 right
now we are going to find
the average speed right
so let's draw a box for the average
speed
because right
therefore
average speed
is equal to total distance that is
by time that is t by 12 this is equal to
2 d
into 12 by t d
d cancels out 24
kilometers per hour
right so 24 kilometer per hour is the
speed
so i am going to make it a little bit
clearer
so this is equal to 24
kilometer per hour
therefore average speed
is equal to 24 kilometer
per hour that is our answer right guys
isn't it easy okay
if you have a doubt please leave a
comment below do like the video
and subscribe to our channel i'll see
you in our next video
until then bye kids keep learning"
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