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Vectors and geometry · Vectors

Magnitude and Direction of a Vector. Applications

Before you start, make sure you know: Pythagorean theorem (Secondary Year 3)

A drone over the Zócalo in Mexico City climbs and moves along a displacement vector. How far did it travel in a straight line? And which way is it pointing? Knowing the magnitude of a vector tells you how much; knowing its direction tells you which line it moves along. With these two ideas you will solve problems about distances, forces, and velocities.

Magnitude of a vector and distance between points

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Calculate the magnitude of the vector $\vec{v} = (3, -4)$.

Calculate the magnitude of the vector $\vec{v} = (3, -4)$.

Solve
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Find the exact distance between the points $A(-2, 5)$ and $B(4, -3)$.

Find the exact distance between the points $A(-2, 5)$ and $B(4, -3)$.

Solve
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A chess player moves their pawn from the square represented by point $(1, 2)$ to square $(1, 4)$, and then moves a knight from $(1, 4)$ to $(3, 5)$. Calculate the…

A chess player moves their pawn from the square represented by point $(1, 2)$ to square $(1, 4)$, and then moves a knight from $(1, 4)$ to $(3, 5)$. Calculate the magnitude of the total displacement vector of the piece from its initial to its final position.

Solve
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Calculate the exact perimeter of the triangle whose vertices are at the coordinates $A(-1, 2)$, $B(3, 5)$ and $C(5, -1)$.

Calculate the exact perimeter of the triangle whose vertices are at the coordinates $A(-1, 2)$, $B(3, 5)$ and $C(5, -1)$.

Solve

Unit vector

Process for obtaining the unit vector of v = (-5, 12): divide it by its magnitude 13, which gives u = (-5/13, 12/13). v = (−5, 12)|v| = 13 divide by |v| u = (1/13)·(−5, 12)= (−5/13, 12/13) u is a unit vector: |u| = 1
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Obtain a unit vector that has the same direction and sense as the vector $\vec{w} = (-5, 12)$.

Obtain a unit vector that has the same direction and sense as the vector $\vec{w} = (-5, 12)$.

Solve

Direction of a vector

The direction of a vector is the line it lies on. Two vectors have the same direction if they are parallel, that is, if one is a multiple of the other: $\vec v = k\,\vec u$. If $k>0$ they have the same sense; if $k<0$, opposite senses.

Another way to describe it is the slope $m = \dfrac{v_2}{v_1}$ (if $v_1\neq 0$): vectors with the same slope have the same direction.

Special cases: a vector $(0, b)$ lies on the $y$-axis (vertical) and a vector $(a, 0)$ lies on the $x$-axis (horizontal).

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Determine the gradient (which indicates the direction) of the vector joining the points $P(1, -4)$ and $Q(3, 8)$.

Determine the gradient (which indicates the direction) of the vector joining the points $P(1, -4)$ and $Q(3, 8)$.

Solve
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Check analytically whether the vectors $\vec{u} = (2, -5)$ and $\vec{v} = (-4, 10)$ have the same direction. If so, indicate whether they have the same sense or opposite…

Check analytically whether the vectors $\vec{u} = (2, -5)$ and $\vec{v} = (-4, 10)$ have the same direction. If so, indicate whether they have the same sense or opposite senses.

Solve
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Find the components of a vector of magnitude $7$ that has the direction of the y-axis and a negative sense.

Find the components of a vector of magnitude $7$ that has the direction of the y-axis and a negative sense.

Solve

Finding a vector with a given magnitude and direction

From the origin, the vector u = (1, -2) points to the right and downward; the longer vector v (figure not to scale) has the same direction and the opposite sense, pointing up and to the left. u (magnitude √5) v (magnitude 10)
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A drone travels in a straight line according to the vector $\vec{d} = (k, 6)$. If the total distance traveled is $10$ meters, calculate the possible values of the…

A drone travels in a straight line according to the vector $\vec{d} = (k, 6)$. If the total distance traveled is $10$ meters, calculate the possible values of the parameter $k$.

Solve
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Find the coordinates of a vector $\vec{v}$ knowing that its magnitude is $5\sqrt{2}$ and that it has the same direction and sense as the vector $\vec{u} = (-1, 1)$.

Find the coordinates of a vector $\vec{v}$ knowing that its magnitude is $5\sqrt{2}$ and that it has the same direction and sense as the vector $\vec{u} = (-1, 1)$.

Solve
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A boat is at the point $A(2, -1)$ and travels in a straight line for a distance of $13$ nautical miles. If the direction vector of its trajectory is proportional to…

A boat is at the point $A(2, -1)$ and travels in a straight line for a distance of $13$ nautical miles. If the direction vector of its trajectory is proportional to $\vec{d} = (5, 12)$ and it moves in the same sense as $\vec{d}$, determine the exact coordinates of the arrival point.

Solve
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An architect is designing a sloping roof. The profile of the roof follows the direction of the vector $\vec{u} = (4, 3)$. If due to structural constraints the total…

An architect is designing a sloping roof. The profile of the roof follows the direction of the vector $\vec{u} = (4, 3)$. If due to structural constraints the total length of the profile must be $8.5$ meters, find the components of the vector representing this profile.

Solve
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Two cables pull on the top of a pole. The first cable exerts a force represented by the vector $\vec{T}1 = (120, 160)$ N. The second cable exerts a force $\vec{T}2$ of…

Two cables pull on the top of a pole. The first cable exerts a force represented by the vector $\vec{T}_1 = (120, 160)$ N. The second cable exerts a force $\vec{T}_2$ of magnitude $150$ N, which has the same direction but opposite sense to $\vec{T}_1$. Find the exact components of the vector $\vec{T}_2$.

Solve
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An exploration robot is initially at point $A(-3, 2)$. It is programmed to move in a straight line in the direction and sense of vector $\vec{u} = (2, -1)$ until…

An exploration robot is initially at point $A(-3, 2)$. It is programmed to move in a straight line in the direction and sense of vector $\vec{u} = (2, -1)$ until reaching a point $B$, such that the distance $AB$ is equal to $4\sqrt{5}$. Next, it turns and advances in the direction and sense of vector $\vec{w} = (1, 2)$ for a distance of $3\sqrt{5}$ until stopping at point $C$. Calculate the exact coordinates of points $B$ and $C$.

Solve

Resultant of several vectors

The resultant of several forces (or velocities, or displacements) is their vector sum. You add component by component and then calculate the magnitude.

Forces F1 = (30, -40), which goes down to the right, and F2 = (-15, 20), which goes up to the left, with the resultant R = (15, -20), which goes down to the right. All three lie on the same line and are drawn offset so they can be seen. F1 = (30, −40) F2 = (−15, 20) R = (15, −20)

If the object is in equilibrium, the resultant is the zero vector $(0,0)$. Then the missing force is the opposite of the sum of the others.

With velocities it works the same way: the velocity of a plane relative to the ground is the sum of its velocity relative to the air and the wind's velocity. And with successive movements at constant velocity, the displacement is $\vec v\cdot t$ in each leg.

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Two coplanar forces represented by the vectors $\vec{F}1 = (30, -40)$ N and $\vec{F}2 = (-15, 20)$ N act on a body. Calculate the magnitude of the resultant force…

Two coplanar forces represented by the vectors $\vec{F}_1 = (30, -40)$ N and $\vec{F}_2 = (-15, 20)$ N act on a body. Calculate the magnitude of the resultant force $\vec{R} = \vec{F}_1 + \vec{F}_2$ and determine a unit vector in the direction of this resultant.

Solve
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In a physics simulation, a particle starts from the origin of coordinates and moves with a constant velocity vector $\vec{v} = (3, 4)$ m/s for $2$ seconds. Immediately…

In a physics simulation, a particle starts from the origin of coordinates and moves with a constant velocity vector $\vec{v} = (3, 4)$ m/s for $2$ seconds. Immediately afterwards, it changes its velocity to $\vec{w} = (-1, 2)$ m/s and maintains this motion for a further $3$ seconds. Calculate how far from the origin the particle is at the end of its journey.

Solve
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Three coplanar forces maintain an object in static equilibrium at the origin of coordinates (that is, the resultant force is the zero vector). The components of two of…

Three coplanar forces maintain an object in static equilibrium at the origin of coordinates (that is, the resultant force is the zero vector). The components of two of them are known: $\vec{F}_1 = (40, 30)$ N and $\vec{F}_2 = (-20, 50)$ N. Calculate the components of the third force $\vec{F}_3$ and its exact magnitude.

X Y F₁ F₂ F₃ = ?
Solve

Problems with parameters

When a distance or magnitude condition includes an unknown, set up $|\vec v| = $ value, square both sides, and solve the equation (almost always a quadratic).

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Determine the coordinates of the points located on the x-axis that are at an exact distance of $5$ units from the point $P(-2, 3)$.

Determine the coordinates of the points located on the x-axis that are at an exact distance of $5$ units from the point $P(-2, 3)$.

Solve
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An aircraft flies with a velocity relative to the air given by the vector $\vec{v}a = (250, 150)$ km/h. In that area, the wind blows with a velocity represented by…

An aircraft flies with a velocity relative to the air given by the vector $\vec{v}_a = (250, 150)$ km/h. In that area, the wind blows with a velocity represented by $\vec{v}_v = (k, -50)$ km/h. If we know that the actual velocity of the aircraft relative to the ground, calculated as $\vec{v}_r = \vec{v}_a + \vec{v}_v$, has an exact magnitude of $300$ km/h, determine the possible values of the parameter $k$.

Solve
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Given the points $A(1, 2)$ and $B(m, m-1)$, find the real value or values of $m$ such that the straight-line distance between $A$ and $B$ is exactly $5$ units.

Given the points $A(1, 2)$ and $B(m, m-1)$, find the real value or values of $m$ such that the straight-line distance between $A$ and $B$ is exactly $5$ units.

Solve
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Find analytically all vectors $\vec{v} = (x, y)$ of magnitude $\sqrt{10}$ that satisfy the condition that the sum of their two components is exactly $4$.

Find analytically all vectors $\vec{v} = (x, y)$ of magnitude $\sqrt{10}$ that satisfy the condition that the sum of their two components is exactly $4$.

Solve

Points on a line and trajectories

A straight-line trajectory that leaves a point $P$ in the direction of a vector $\vec d$ (its direction vector) is made up of the points

$$X = P + t\,\vec d,\qquad t\in\mathbb{R}$$

If $t>0$ you move in the sense of $\vec d$; if $t<0$, in the opposite sense. In components: $x = p_1 + t\,d_1$, $y = p_2 + t\,d_2$.

If the line is given by two points $R$ and $S$, a direction vector is $\overrightarrow{RS} = S - R$.

To find where it crosses an axis or another line, substitute $x$ and $y$ into the condition (for example $y=0$ for the $x$-axis, or $x=12$ for a vertical line) and solve for $t$.

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On a video game coordinate plane, the main character is at position $P(-4, 1)$. They must collect a coin located at a point $Q$ belonging to the line passing through…

On a video game coordinate plane, the main character is at position $P(-4, 1)$. They must collect a coin located at a point $Q$ belonging to the line passing through points $R(0, 5)$ and $S(2, 7)$. If the character moves exactly $3\sqrt{2}$ units in a straight line to reach $Q$, find the possible coordinates where the coin is located.

Solve
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A light signal leaves point $A(0, 4)$ with a direction vector $\vec{d} = (2, -1)$. Upon reaching the x-axis at a point $P$, it reflects, changing its direction and sense…

A light signal leaves point $A(0, 4)$ with a direction vector $\vec{d} = (2, -1)$. Upon reaching the x-axis at a point $P$, it reflects, changing its direction and sense according to the new direction vector $\vec{d}' = (2, 1)$. If the signal continues its trajectory until hitting the vertical line $x = 12$ at a point $Q$, calculate the total distance travelled by the light from $A$ to $Q$.

Solve

Geometric applications

The magnitude is useful for measuring sides, perimeters, and diagonals of figures given by coordinates. In a parallelogram $ABCD$ we have $\overrightarrow{AB}=\overrightarrow{DC}$, and from this you get the missing vertex: $C = D + \overrightarrow{AB}$.

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Consider a rhomboid $ABCD$ for which three of its vertices are known: $A(0, 0)$, $B(6, 2)$ and $D(2, 4)$. Calculate the coordinates of vertex $C$, obtain the magnitude…

Consider a rhomboid $ABCD$ for which three of its vertices are known: $A(0, 0)$, $B(6, 2)$ and $D(2, 4)$. Calculate the coordinates of vertex $C$, obtain the magnitude of its two diagonals and justify analytically whether or not it is a rhombus (that is, whether all its sides are equal).

X Y A B C D
Solve

Back to the beginning: the drone that moves according to $\vec d=(k,6)$ and travels $10$ m satisfies $k=\pm8$; the magnitude tells you how far it travels and the vector tells you which way.

Summary

See all the exercises in this section