# 2: Vectors

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- 3977

Vectors are a component part of physics in much the same way as sentences are a component part of literature. In introductory physics, vectors are Euclidean quantities that have geometric representations as arrows in one dimension (in a line), in two dimensions (in a plane), or in three dimensions (in space). They can be added, subtracted, or multiplied. In this chapter, we explore elements of vector algebra for applications in mechanics and in electricity and magnetism. Vector operations also have numerous generalizations in other branches of physics.

- 2.1: Prelude to Vectors
- Vectors are essential to physics and engineering. Many fundamental physical quantities are vectors, including displacement, velocity, force, and electric and magnetic vector fields. Scalar products of vectors define other fundamental scalar physical quantities, such as energy. Vector products of vectors define still other fundamental vector physical quantities, such as torque and angular momentum.

- 2.2: Scalars and Vectors (Part 1)
- Vectors are geometrically represented by arrows, with the end marked by an arrowhead. The length of the vector is its magnitude, which is a positive scalar. On a plane, the direction of a vector is given by the angle the vector makes with a reference direction, often an angle with the horizontal. When a vector is multiplied by a scalar, the result is another vector of a different length than the length of the original vector.

- 2.3: Scalars and Vectors (Part 2)
- Two or more vectors can be added to form another vector. The vector sum is called the resultant vector. Vectors can be added to other vectors or scalars to other scalars, but scalars cannot be added to vectors and vice versa. Vector addition is commutative and associative. For constructing a resultant vector, the parallelogram rule is useful for two vectors while the tail-to-head method is useful for more than two vectors.

- 2.4: Coordinate Systems and Components of a Vector (Part 1)
- The vector component is the product of the unit vector of an axis with its scalar component along that axis. A vector is the resultant of its vector components. The scalar x-component of a vector can be expressed as the product of its magnitude with the cosine of its direction angle, and the scalar y-component can be expressed as the product of its magnitude with the sine of its direction angle.

- 2.5: Coordinate Systems and Components of a Vector (Part 2)
- In a plane, there are two equivalent coordinate systems. The Cartesian coordinate system is defined by unit vectors i^ and j^ along the x-axis and the y-axis, respectively. The polar coordinate system is defined by the radial unit vector r^, which gives the direction from the origin, and a unit vector t^, which is perpendicular (orthogonal) to the radial direction.

- 2.6: Algebra of Vectors
- Analytical methods of vector algebra are important mathematical tools of physics as they are used routinely in mechanics, electricity, and magnetism. These methods allow us to find resultants of vector addition exactly, contrary to graphical methods, which are approximate and require drawing out the individual vectors.

- 2.8: Products of Vectors (Part 1)
- One kind of vector multiplication is the scalar product, also known as the dot product, which results in a number (scalar). The scalar product has the distributive property and the commutative property, and is obtained by multiplying the magnitudes of the two vectors with the cosine of the angle between them. This type of vector multiplication is used to find angles between vectors and in the definitions of derived scalar physical quantities such as work or energy.

- 2.9: Products of Vectors (Part 2)
- Another kind of vector multiplication is the vector product, also known as the cross product, which results in a vector perpendicular to both of the factors. The vector product has the distributive property and the anticommutative property, and is obtained by multiplying the magnitudes of the two vectors by the sine of the angle between them. The direction of the vector product can be determined by the corkscrew right-hand rule.

Thumbnail: A signpost gives information about distances and directions to towns or to other locations relative to the location of the signpost. Distance is a scalar quantity. Knowing the distance alone is not enough to get to the town; we must also know the direction from the signpost to the town. The direction, together with the distance, is a vector quantity commonly called the displacement vector. A signpost, therefore, gives information about displacement vectors from the signpost to towns. (credit: modification of work by “studio tdes”/Flickr).