Areas related to circles - Perimeter and area of a circle for class 10 maths.
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Areas related to circles - Perimeter and area of a circle for class 10 maths.
1. Areas Related To Circles
Problems based on
Perimeter and area of a circle
Chapter : Areas Related To Circles Website: www.letstute.com
2. Problems based on
Perimeter and area of a circle
Q) A wheel rotates 25000 times to cover a distance of 90 km. Find its
radius.
Given: Rotation of the wheel = 25000 times
Distance covered = 90 km
by the wheel
To find: Radius of the wheel = ?
?
Chapter : Areas Related To Circles Website: www.letstute.com
3. Problems based on
Perimeter and area of a circle
Solution:
Let ‘r’ be the radius of the wheel.
?
Circumference
of the wheel
= Distance covered in
one rotation.
= 90 Km
2πr
∵ ퟗퟎ 퐊퐦= 90 x 1000 x 100 cm
= 9000000
25000
= 360 cm
2πr = 360 cm
Chapter : Areas Related To Circles Website: www.letstute.com
4. Problems based on
Perimeter and area of a circle
2 x 22 x r
= 360cm
r = 360x7 cm
2x22
= 180 x 7
22
= 90 x 7
11
= 630
11
r = 57.27 cm
7
Hence, the radius of the wheel is 57.27cm
?
Chapter : Areas Related To Circles Website: www.letstute.com
5. Problems based on
Perimeter and area of a circle
Q) The diameter of a cart wheel is 21 cm. How many revolutions
will it make in moving 1.32 km?
21 cm
Given: Diameter of the cart wheel = 21 cm
To Find: Number of revolutions = ?
made in 1.32 Km
Chapter : Areas Related To Circles Website: www.letstute.com
6. Problems based on
Perimeter and area of a circle
Solution: Let the radius of the cart wheel be ’r’.
Thus, r = Diameter = 21 cm
2 2
Circumference of the cart wheel = 2πr
= 2x22x21cm
7 2
= 462
7
= 66 cm
21 cm
Chapter : Areas Related To Circles Website: www.letstute.com
7. Problems based on
Perimeter and area of a circle
21 cm
Converting 1.32 Km into cm, we get,
1.32 Km = 1.32 x 1000 m [∵ 1 Km = 1000 m]
= 1.32 x 1000 x 100 cm [∵1m = 100 cm]
= 132000 cm
Chapter : Areas Related To Circles Website: www.letstute.com
8. Problems based on
Perimeter and area of a circle
Number of revolutions = Total distance covered
Circumference (Distance covered by
1 round of the cart wheel)
= 132000 cm
66 cm
= 12000
6
2000
21 cm
=
Hence, the cart wheel will make 2000 revolutions in moving
1.32 km.
Chapter : Areas Related To Circles Website: www.letstute.com
9. Problems based on
Perimeter and area of a circle
Q) A wheel of a bicycle makes 6 revolutions per second. If the
diameter of the wheel is 80 cm, find its speed.
Given: Number of revolutions per second = 6
Diameter = 80cm
To find: Speed = ?
Formula: Speed = Distance
Time
Chapter : Areas Related To Circles Website: www.letstute.com
10. Problems based on
Perimeter and area of a circle
Solution: Let the radius of the wheel be denoted as ‘r’.
Thus, r = Diameter = 80 = 40 cm
2 2
Circumference of the wheel = 2 πr
= 2x22x 40 cm
7
= 1760
7
= 251.42cm
Chapter : Areas Related To Circles Website: www.letstute.com
11. Problems based on
Perimeter and area of a circle
Distance covered in 1 revolution = circumference = 251.42cm
Distance covered in 6 revolutions
= 6 x Distance covered in 1 revolution
= 6 x 251.42 cm
= 1508.52 cm
Since, 1 m = 100 cm
?m = 1508.52 cm
= 1508.52 cm
100
= 15.08 m
∵ 15.08 m = 1508.52 cm
Chapter : Areas Related To Circles Website: www.letstute.com
12. Problems based on
Perimeter and area of a circle
Speed = Distance
Time
= 15.08 m
1 second
Result: Speed = 15.08 m/second
Hence, the speed of the wheel is 15.08m/second.
Chapter : Areas Related To Circles Website: www.letstute.com
13. Problems based on
Perimeter and area of a circle
Q) Find the radius of the circle whose perimeter and area are
numerically equal.
Given: Perimeter and the area of the circle are equal
To Find: Radius of the circle = ?
Solution: Let ‘r’ be the radius of the circle.
Then, its area = πr2 and
its perimeter = 2πr
It is given that the area of the circle is numerically equal to its
perimeter.
Chapter : Areas Related To Circles Website: www.letstute.com
14. Problems based on
Perimeter and area of a circle
Thus, πr2 = 2πr
πr2 - 2πr = 0
πr(r-2) = 0
Either πr = 0 or
r-2 = 0
r = 0 (rejected) or
r = 2
Hence, the radius of the circle is 2 units
Chapter : Areas Related To Circles Website: www.letstute.com
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Editor's Notes
Let us take a look at the first question, A tyre rotates 25000 times to cover a distance of 90 km. Find its
radius.
In this questn, we have been given that the tyre rotates 25000 times covering a distance of 90 km
What we need to find is the radius of the tyre.
Lets see the solution,
Firstly, let r be the radius,
We know that, circum of the wheel is = to distance cov in 1 rotan
We also know the formula for circum which is 2 pir r. the distance vovered in 1 rotatn is 360 cm.
We get 360 cm in the follow manner,
We nned to convert 90 km into cm,
= 9 x 1000 x 100 cm
= 9000000
25000 which is the no. of tyms the wheel rotates
= 360 cm
By substituting, pie byu 22/7, we get
The next question says that, A wheel of a bicycle makes 6 revolutions per second. If the diameter of the wheel is 80 cm, find its speed.
Lets see what is given to us,
We have been given, no. of revolutions per second which r 6
And the diameter which is 80 cm
We need to find out the speed.
Now, do you know the formula for speed???
The formula is speed = dist/ time
Let us see the solution,
Now, we need to find the distance covered in 4 revolutions….
Since distance covered by 1 revolution is 251.42 cm , the distance covered by 6 revolutions is 6x 251.42 cm
=1508.52 cm
= 15.08 m
Now finally………we can find the speed which is distance / time
=15.08 m
1 second
=15.08x18 km/h (18/5 is obtained by conerrting meter/sec intio km/hr= 60x60= 18/5
5 100
=54.28 km/h
Let us take a look at the next question,
The question says that Find the radius of the circle whose perimeter and area are numerically equal.
In this question we have been given, that the perimeter and the area of the circle are equal.
We need to find the radius of the circle.
Lets us come to the solution.
Let us first represent r to be the radius of the circle.
We know that the area of the circle is πr2 and
its perimeter = 2πr
We have be given that the area of the circle is equal to its perimeter.
Thus, by using the formulas we get,
πr2 = 2πr
By taking 2pie r on LHS, we get,
πr2 - 2πr = 0
By taking pie r common, we get,
πr(r-2) = 0
Now,
Either πr = 0 or
r-2 = 0
But pie r cannot be 0 as for it to be 0, r should b 0 since we know that pie is 22/7,
In a circle r can never be 0. thus r = 0 is rejected.
Thus, r = 2
Hence………………..