Question

Express the arc length of a curve in terms of the speed of an object moving along the curve.

Short answer

Answer: The arc length, S, of a curve can be expressed in terms of the speed, v(t), of an object moving along the curve using the integral: S = ∫v(t) dt for t in the range [t1, t2].

Step-by-step solution

Define Curve and Arc Length Integral

Let's consider a curve, C, described by the vector-valued function r(t) = (x(t), y(t), z(t)), where x(t), y(t), and z(t) represent the x, y, and z coordinates of the curve, respectively. The arc length, S, of a curve C between t1 and t2 is given by the integral: S = ∫||r'(t)|| dt for t in the range [t1, t2], where ||r'(t)|| denotes the magnitude of the derivative of the vector-valued function r(t) with respect to t.

Define Speed of Moving Object

The speed, v, of an object moving along the curve C is the derivative of its position on the curve with respect to time, t. It can be found as: v(t) = ||r'(t)||.

Express Arc Length in Terms of Speed

Now, we can replace the expression of ||r'(t)|| in the arc length integral with v(t): S = ∫v(t) dt for t in the range [t1, t2]. In this form, the arc length, S, of a curve is expressed in terms of the speed, v(t), of an object moving along the curve.

Key concepts

Vector-Valued Function

A vector-valued function is a mathematical function that associates a vector with each point in its domain, often involving time or another parameter, to represent a curve in space. Imagine it as a tool that describes how an object moves through 3D space over time.
For instance, if we take a vector-valued function \( r(t) = (x(t), y(t), z(t)) \), it provides a position at each time \( t \), with \( x(t) \), \( y(t) \), and \( z(t) \) conveying how the position changes along each axis in space.
This function forms the basis for describing curves and paths in physics and engineering. It enables us to work with complex motions by breaking them down into manageable components, which we can analyze individually.

Speed of an Object

The speed of an object can be thought of as how fast an object is moving along a path, disregarding its direction. When we talk about a curve defined by a vector-valued function, the speed is derived by finding the derivative of this function, \( r'(t) \).

• The derivative, \( r'(t) \), gives us a new function that highlights the rate of change of position.
• By taking the magnitude, we can find the speed: \( v(t) = ||r'(t)|| \).

If you imagine walking along a winding path, the speed you walk at any point reflects how fast your position changes, emphasizing only the pace without caring which way you are headed.
This concept of speed is vital because it allows us to connect how quickly something moves with other physical quantities, especially when dealing with the arc length of a curve.

Arc Length Integral

The arc length integral offers a powerful method for calculating the distance along a curve described by a vector-valued function. Given a function \( r(t) = (x(t), y(t), z(t)) \), the goal is to determine how long the curve is from one point to another as \( t \) varies:

• The arc length, represented as \( S \), is found by integrating the speed over a set interval \([t_1, t_2]\).
• The expression \( S = \int_{t_1}^{t_2} ||r'(t)|| \ dt \) reflects this process, spelling out that we sum up all the instant speeds across the path.

This integral can be thought of as summing up tiny sections of the path, each calculated at different times, then adding them all together to get the full arc length.
Expressing the arc length in terms of speed offers a neat way of translating between how fast an object is moving and the path distance it covers.