Q.A body constrained to move along the $z$-axis of a coordinate system is subject to a constant force $\vec{F}$ given by

[!FORMULA] $\vec{F} = -\hat{i} + 2\hat{j} + 3\hat{k}\ \text{N}$

where $\hat{i}, \hat{j}, \hat{k}$ are unit vectors along the $x$-, $y$- and $z$-axis of the system respectively. What is the work done by this force in moving the body a distance of $4\ \text{m}$ along the $z$-axis?

The Dot Product and the Angle Between Vectors

Imagine you're pushing a heavy box across the floor. You push at an angle — not straight forward, but partly downward and partly forward. The part of your push that actually moves the box is only the forward component. The downward part just presses the box into the floor.

That's the core intuition behind the dot product: it measures how much one vector "goes in the direction of" another vector.

Step 1: What is a dot product?

Given two vectors $\vec{a}$ and $\vec{b}$ in 2D or 3D space, their dot product (also called the scalar product) is defined algebraically as:

You multiply corresponding components and add them up. The result is a single number (a scalar), not a vector.

For example, if $\vec{a} = (3, 4)$ and $\vec{b} = (2, -1)$, then:

Step 2: The geometric meaning — the angle connection

Here's the beautiful part. The dot product also has a completely different geometric definition:

where $|\vec{a}|$ and $|\vec{b}|$ are the magnitudes (lengths) of the vectors, and $\theta$ is the angle between them when they're placed tail-to-tail.

This is the dot product angle formula. It connects algebra (component multiplication) to geometry (angle and length).

Step 3: Why does this make sense?

Think about the extreme cases:

• Vectors point in the same direction ($\theta = 0^\circ$): $\cos 0 = 1$, so $\vec{a} \cdot \vec{b} = |\vec{a}| |\vec{b}|$ — the maximum possible value. All of one vector's "push" is in the other's direction.

• Vectors are perpendicular ($\theta = 90^\circ$): $\cos 90^\circ = 0$, so $\vec{a} \cdot \vec{b} = 0$. Neither vector has any component along the other. This is a crucial test for orthogonality.

• Vectors point opposite ($\theta = 180^\circ$): $\cos 180^\circ = -1$, so $\vec{a} \cdot \vec{b} = -|\vec{a}| |\vec{b}|$ — the most negative value. They're completely against each other.

• Any other angle: the dot product is somewhere between these extremes, proportional to how much one vector "projects" onto the other.

Step 4: Finding the angle from the dot product

If you know the components of two vectors, you can find the angle between them by rearranging the formula:

Then use $\theta = \cos^{-1}(\text{that value})$.

Example: Find the angle between $\vec{a} = (1, 2)$ and $\vec{b} = (3, 4)$.

1. Compute dot product: $1 \times 3 + 2 \times 4 = 3 + 8 = 11$
2. Compute magnitudes: $|\vec{a}| = \sqrt{1^2 + 2^2} = \sqrt{5}$, $|\vec{b}| = \sqrt{3^2 + 4^2} = 5$
3. $\cos \theta = \frac{11}{\sqrt{5} \times 5} = \frac{11}{5\sqrt{5}} \approx 0.9839$
4. $\theta = \cos^{-1}(0.9839) \approx 10.3^\circ$

The vectors are nearly aligned.

Step 5: The precise statement …