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Learning Introductory Physics with Activities

Section 22.2 Area Vectors

Exercises Warm-up Activities

1.

You want to find the dot product between two vectors, \(\vec{A}\) and \(\vec{B}\text{.}\) What do you know about the directions of vectors if the dot product is positive? What if the dot product is negative? What if the dot product is zero?
The dot product was introduced in The Dot Product.
To describe surfaces, we will need to define a new quantity.

Definition 22.2.1. Area Vector.

The area vector \(\vec{A}\) for a surface points perpendicular to the surface, with a magnitude equal to the area of the surface. For an infinitesimal surface, the area vector is typically written \(d\vec{A}\text{.}\) An area vectors has units of \(\mathrm{m}^2\)
Figure 22.2.2. An example of the area vector.
Area vectors will help us define different surfaces, and will become particularly useful in the next few sections.

Exercises Activities

Below is a picture of a closed surface (the outside cover of a book).
Figure 22.2.3. A simple representation of a book.

1. Area Vectors for a Book.

Which direction is the area vector for the front cover of the book? For the spine? How many different area vectors do you need for the entire book?

2.

Write a differential area vector \(d\vec{A}\) for a small part of the front cover of the book in terms of differential elements like \(dx\) and \(dy\text{.}\) Check that your expression has the correct units!