Length of an Arc
What is an arc?
- An arc is a part of the circumference of a circle
- It is easiest to think of it as the crust of a single slice of pizza
- The length of an arc depends of the size of the angle at the centre of the circle
- If the angle at the centre is less than 180° then the arc is known as a minor arc
- This could be considered as the crust of a single slice of pizza
- If the angle at the centre is more than 180° then the arc is known as a major arc
- This could be considered as the crust of the remaining pizza after a slice has been taken away
How do I find the length of an arc?
- The length of an arc is simply a fraction of the circumference of a circle
- The fraction can be found by dividing the angle at the centre by 360°
- The formula for the length, , of an arc is
-
- Where is the angle measured in degrees
- is the radius
How do I use radians to find the length of an arc?
- As the radian measure for a full turn is , the fraction of the circle becomes
- Working in radians, the formula for the length of an arc will become
- Simplifying, the formula for the length, , of an arc is
-
- is the angle measured in radians
- is the radius
Worked example
A circular pizza has had a slice cut from it, the angle of the slice that was cut was rad.
The radius of the pizza is 12 cm. Find
The formula for the length of an arc, where the angle is in radians is
Use the formula for the length of an arc, , to find the curved length of the perimeter, the major arc
As we are finding the perimeter of the whole shape, we need to add on the two straight lengths formed by the slice which has been cut out
Unless asked to otherwise, it is best to give answers in an exact form