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IB DP Maths AA SL Study Notes

3.1.2 Arc Length

Understanding Arc Length

Arc length is the distance between two points along a curve. It's not the straight-line distance but the distance travelled if one were to move along the curve. In the context of circles, it's a segment of the circle's circumference, influenced by the circle's radius and the angle (in radians) subtended by the arc at the circle's centre. For those new to the concept, understanding the basics of radians is crucial as it forms the foundation for calculating arc lengths.

The Formula

For circles, the formula for arc length is: Arc Length (s) = r x θ Where:

  • s is the arc length.
  • r is the circle's radius.
  • θ is the angle in radians subtended by the arc.

If the angle is in degrees, it needs to be converted to radians first. This conversion is essential in understanding how arc length relates to graphs of sine and cosine, where angles and their trigonometric functions play a significant role.

Beyond Circles

While circles are a common context for arc length, the concept applies to any curve. For curves described by functions, calculus might be needed to determine the arc length. Here, the arc length is found by integrating the length of tiny straight-line segments that approximate the curve over a specific interval. This approach to calculus can be further explored through first-order differential equations, which are vital in calculating the lengths of more complex curves.


Arc length has numerous applications:

1. Geometry: It's vital for understanding ellipses, parabolas, and hyperbolas.

2. Trigonometry: It's crucial for deriving trigonometric function values based on the unit circle.

3. Physics: It's used in understanding planetary motion and object rotation.

4. Engineering: It's essential for road design and machinery part creation.

5. Astronomy: It's used to measure vast space distances.

The concept of arc length is also significant in the study of parametric equations, where the length of a curve defined parametrically can be calculated.


Example 1: A circular track has a 5-metre radius. If an athlete runs from one point to another on the track, covering an angle of pi/6 radians at the centre, how long did they run?

Solution: Using the formula: Arc Length (s) = r x θ s = 5 metres x pi/6 s = 5pi/6 metres The athlete ran approximately 2.62 metres.

This example illustrates the practical application of arc length in measuring distances in real-life scenarios, such as the sector area of a circle which is closely related to the concept of arc length.

Example 2: A roller coaster track forms a parabolic curve described by the function y = x2 from x = -2 to x = 2. What's the track's length for this section?

Solution: This requires calculus. The arc length of a function over an interval [a, b] is given by the integral from a to b of the square root of (1 + (dy/dx)2) dx. For y = x2, dy/dx = 2x. Using this formula and integrating from -2 to 2 gives the arc length of the track for that section.

The calculation involved in this example is an application of calculus principles that are foundational for understanding the dynamics of roller coaster design, among other engineering applications.


Arc length can be thought of as a segment of the perimeter of a shape. In the context of a circle, the entire circumference is the perimeter, and any section of this circumference is an arc length. For other shapes, if you were to focus on a curved section of the boundary, the length of that curved section would be its arc length. Thus, while the perimeter represents the total boundary length of a shape, the arc length pertains to a specific curved segment of that boundary.

Absolutely! While circles are the most common context for discussing arc length, the concept is applicable to any curve, whether it's part of an ellipse, parabola, hyperbola, or any other curve described by a function. For more complex curves, determining the arc length might require the use of calculus, especially when the curve is described by a function. In such scenarios, the arc length is found by integrating the length of infinitesimally small straight-line segments that approximate the curve over a specific interval.

Yes, arc length has numerous real-world applications. In engineering, it's used in the design of roads, bridges, and tunnels, especially when these structures need to curve around natural obstacles. In astronomy, arc length helps in determining the distance between stars and other celestial objects when viewed from Earth. In sports, athletes running on tracks or cycling on velodromes often cover specific arc lengths. Additionally, in various manufacturing processes, understanding arc length is essential when producing curved components. The concept is pervasive and plays a vital role in many practical scenarios.

The preference for radians in arc length calculations stems from the definition of a radian itself. One radian is defined as the angle subtended at the centre of a circle by an arc whose length is equal to the radius of the circle. This direct relationship between the angle in radians and the arc length makes calculations more straightforward and eliminates the need for a proportionality constant. When using degrees, a conversion factor is required, making the formula less direct and intuitive.

Arc length and chord length are both measures associated with circles, but they represent different concepts. The arc length is the distance between two points along the curve or the boundary of the circle. It's the length of the segment of the circle's circumference between these two points. On the other hand, the chord length is the straight-line distance between the same two points, cutting through the interior of the circle. In essence, while the arc length follows the curvature of the circle, the chord length provides a direct route between the two points.

Practice Questions

A sector of a circle has an area of 50 square metres and a radius of 10 metres. Determine the arc length of the sector.

The area of a sector is given by the formula: Area = 0.5 x r2 x θ. Using the given values, we can set up the equation: 50 = 0.5 x 102 x θ. Solving for θ, we get: θ = 50/50 = 1 radian. Now, to find the arc length (s) of the sector, we use the formula: s = r x θ. s = 10 x 1 = 10 metres. Therefore, the arc length of the sector is 10 metres.

A circular pond has a diameter of 10 metres. A bridge is built over the pond, subtending an angle of 60° at the centre of the pond. Calculate the length of the bridge.

To determine the length of the bridge, which is essentially the arc length, we can use the formula: Arc Length (s) = r x θ. Given the diameter is 10 metres, the radius (r) is half of that, which is 5 metres. The angle θ is given in degrees, so we first convert it to radians: θ = 60° x (π/180) = π/3 radians. Plugging these values into the formula, we get: s = 5 x π/3 = 5π/3 metres. Thus, the length of the bridge is approximately 5.24 metres.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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