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Scalar and Vector

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Scalar and Vector

In everyday life, we interchangeably use distance, displacement, speed, velocity, acceleration, etc. To physicists, all quantities, whether static or in motion, can be differentiated by classifying them as either scalars or vectors.

A quantity with a magnitude (size) only is referred to as a scalar quantity. Mass, energy, power, distance, and time are some examples of scalar quantities because they have no direction associated with them.

A quantity that has a magnitude and a direction associated with it is a vector quantity. Acceleration, force, gravity, and weight are some vector quantities. All vector quantities are associated with a specific direction.

Scalars and vectors: meaning and examples

As we’ve already stated, a quantity with a magnitude and a direction is known as a vector quantity.

Weight is an example of a vector quantity because it is a product of mass and acceleration due to gravity. The acceleration of gravity has a direction that is vertically downwards, which makes weight a vector quantity.

Let's look at some examples of scalars and vectors.

Suppose you have a box and you move it by a distance of 5 meters.

Scalar and Vector Vector example StudySmarterFigure 1. A movement of an object from point A to point B in a specified direction is a vector. Oğulcan Tezcan – StudySmarter Originals

If you tell somebody that the distance between points A and B is 5 meters, you are talking about a scalar quantity because you are not specifying any direction. Five meters is just a magnitude (distance), and the direction could be any. So, distance is a scalar quantity.

However, if you tell someone you moved the box 5 meters to the right (east), as depicted in figure 1, you are now talking about a vector quantity. Why? Because you have now specified a direction associated with the movement. And in physics, this is referred to as displacement. Hence, displacement is a vector quantity.

Now let’s say it took you 2 seconds to move the box to the right.

Scalar and Vector Vector quantity relative to time StudySmarterFigure 2. Diagram showing a displacement vector relative to time. Oğulcan Tezcan – StudySmarter Originals

If you were to calculate how quickly you moved the box, you are calculating the speed of the movement. In the above example, the speed is:

The speed is a scalar quantity as it does not have any direction.

However, if you say the box moved with a speed of 2.5m/s to the right, this becomes a vector quantity. The speed with a direction is velocity, and a change in velocity is, in turn, known as acceleration (m/s2), which is also a vector quantity.

ScalarVector
distance displacement
speedvelocity and acceleration

Mass and weight: which one is a scalar and a vector quantity?

The mass and weight of a body may seem the same, but they are not.

Mass: The quantitative measure of inertia of a body, which is the tendency of a body to resist the force that can cause a change in its speed or position. Mass has an SI unit of kilograms.

Weight: The gravitational pull acting on a mass. It has an SI unit of Newtons.

Scalar

Mass does not have any direction, and it will be the same no matter where you are in the universe! So we can categorise mass as a scalar quantity.

Vector

Weight, on the other hand, is the force acting on an object, and since force has a direction, weight is a vector quantity.

Another way to look at this is if you place one object on Earth and another object with the same mass on the Moon. Both objects will have the same mass but a different weight due to the gravitational pull on the Moon (1.62 m/s2), which is smaller compared to Earth.

How can we represent vectors?

We can represent vectors with an arrow, as shown below.

Scalar and Vector Vector representation StudySmarterFigure 3. Representation of a vector. Wikimedia Commons

The length depicts the magnitude, the tail is the initial point of a vector, the sense of a vector is given by the order of two points on a line parallel to the vector, and the orientation tells you at which angle the vector is pointing. The combination of orientation and sense specify the direction of the vector.

Vector examples: how can we perform vector addition?

Let’s look at some examples of how to perform vector addition.

Say you have two vectors of 10N and 15N, and both are pointing towards the east. The sum of these vectors becomes 25N towards the east.

Scalar and Vector Vector addition example StudySmarter Figure 4. Vectors in the same direction are added. Usama Adeel – StudySmarter Originals .

Now, if we change the direction of the 15N towards the west (-15 N), the resultant vector becomes -5 N (pointing towards the west). A vector quantity can have positive and negative signs. The sign of a vector shows that the direction of the vector is the opposite of the reference direction (which is arbitrary).

Scalar and Vector Vector subtraction example StudySmarterFigure 5. Vectors in the opposite direction are subtracted. Usama Adeel – StudySmarter Originals

Now, of course, all vector additions are not as straightforward as shown above. What would you do if the two vectors were perpendicular to each other? This is where we need to improvise a little bit.

Head-to-tail rule

With this rule, we can calculate the resultant vector by joining the tail of the first vector with the head of the second vector. Take a look at the figures below.

Scalar and Vector Head-to-tail rule vector example StudySmarterFigure 6. Perpendicular vectors are joined via the head-to-tail rule. Usama Adeel – StudySmarter Originals

A vector force of 30N acts in the east direction, while a vector force of 40N acts in the north direction. We can calculate the resultant vector by joining the tail of the 30N vector with the head of the 40N vector. The vectors are perpendicular, so we can use the Pythagorean theorem to solve the resultant vector as shown in figure 7.

Scalar and Vector Resultant vector example StudySmarterFigure 7. Vector perpendicular addition. Usama Adeel – StudySmarter Originals

With a bit of trigonometry and applying the Pythagorean theorem, the resultant vector becomes 50N. Now, as we discussed, a vector quantity has a magnitude as well as a direction, so we can calculate the angle of the 50N vector by using an inverse tangent of 40/30 (perpendicular/base). The angle is then 53.1° from the horizontal for the above example.

Resolving a vector into its components

Using the same example from above, what if we only had the 50N vector force with an angle from the horizontal and were asked to find its horizontal and vertical components?

Splitting a single vector into two or more vectors that produce a similar effect to the original vector is called the resolution of vectors.

Let’s have a look at an example to explain this concept further.

Suppose a vector force F of 150N is applied at an angle of 30 degrees from the surface.

Scalar and Vector Resolving vectors example StudySmarterFigure 8. Vector at an angle. Usama Adeel – StudySmarter Originals

We can split the vector F into a horizontal component (Fx) and a vertical (Fy) component as depicted below:

Scalar and vector Vector decomposition example StudySmarterFigure 9. Resolution of vectors. Usama Adeel – StudySmarter Originals

Calculating Fx and Fy by using trigonometry gives us:

Resolving components of a force on an inclined plane

As you might have figured out by now, calculations in physics are never this straightforward! Not every surface is horizontal – sometimes surfaces may be at an incline, and you have to calculate and resolve components along an inclined plane.

Scalar and Vector Inclined plane vector decomposition example StudySmarterFigure 10. The direction of weight on an inclined plane. Usama Adeel – StudySmarter Originals

Figure 10 shows a box on a surface at an angle θ from the horizontal. The weight of the box, mg, is acting downwards with a mass m and the gravitational pull g.

If we split the mg vector into the horizontal and vertical components,

  • the vertical component will be perpendicular to the inclined surface, and
  • the horizontal component of mg will be parallel to the inclined surface.

Scalar and Vector Vector lines of forces example StudySmarterFigure 11. Resolution of mg vector on an inclined surface. Usama Adeel – StudySmarter Originals

The θ angle between the mg and mgcosθ will be the same as the inclined surface angle from the horizontal. The force that will accelerate the box down the slope will be mgsinθ (Fg), and the reaction force Fn (from Newton’s third law) will be equal to mgcosθ. Hence,

Scalars and Vectors Vectors on an inclined plane example StudySmarterFigure 12. Resolution of vectors and direction of motion on an inclined plane. Usama Adeel – StudySmarter Originals

Equilibrium of coplanar force systems

If forces are acting on a body and the body is stationary or moving with a constant velocity (not accelerating), such an instance is called equilibrium. The lines of forces must pass through the same point for an object to be in equilibrium.

In the diagram below, a uniform ladder is leaning against a smooth wall (no friction). The ladder’s weight acts downwards, and the normal reaction force acts at an angle of 90° from the wall.

Scalar and Vector Equilibrium of forces example StudySmarterFigure 13. A ladder leaning against a wall is an example of a body in equilibrium. Usama Adeel – StudySmarter Originals

If you extend these forces, you will see that they cross at a certain point. Because the object is in equilibrium, the force from the ground must also pass through the same point as the other forces do.

Scalar and Vector Lines of forces in equilibrium example StudySmarterFigure 14. Lines of forces intersect at a common point if a body is in equilibrium. Usama Adeel – StudySmarter Originals

By resolving the force from the ground into its vertical and horizontal components, the normal reaction force from the ground acts upwards, and the friction force from the ground acts along the surface.

Scalar and Vector Lines of forces example StudySmarterFigure 15. Resultant of the friction and ground vectors. Usama Adeel – StudySmarter Originals

In essence, what happens is that all the forces cancel each other.

  • The normal force from the wall (right force) = frictional force acting along the ground (left force).
  • Weight from the ladder (downward force) = reaction force from the ground (upward force).

Scalar and Vector - Key takeaways

  • A scalar quantity has a magnitude only, whereas a vector quantity has a magnitude and a direction.
  • A vector can be represented with an arrow.
  • To find the resultant vector, vectors in the same direction are added, whereas vectors in the opposite direction are subtracted.
  • The resultant vector of two vectors can be calculated with the head-to-tail rule, and the resultant vector of perpendicular vectors can be calculated with the Pythagorean theorem.
  • If a vector is at an angle to the horizontal (or vertical), it can be resolved into its x and y components.
  • The line of forces must intersect at a common point and cancel each other out for an object to be in equilibrium.

Frequently Asked Questions about Scalar and Vector

The difference between a scalar and a vector is that scalar quantities have a magnitude only, whereas vector quantities have a magnitude as well as a direction. 

A scalar quantity is a quantity with a magnitude (size) only. A vector quantity is a quantity that has both a magnitude and a direction associated with it. 

Force is a vector quantity.

No, power is not a vector quantity. It is a scalar quantity. 

Speed is a scalar quantity. Velocity is a vector quantity. 

Final Scalar and Vector Quiz

Question

Which of the following has a magnitude but no direction?

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Answer

Scalar.

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Question

How do you calculate the resultant vector if the vectors are in the same direction?

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Answer

Vectors in the same direction are added.

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Question

What rule can you follow if two vectors are in a perpendicular direction and you have to calculate the resultant force?


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Answer

If two vectors are in a perpendicular direction and you have to calculate the resultant force, you can use the head-to-tail rule. 

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Question

When do we need to resolve a vector into its components?


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Answer

We resolve a vector into its components if a vector at an angle to the horizontal (or the vertical) can be resolved into its components.

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Question

Does the resolution of vectors happen only in two dimensions (x and y)?


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Answer

No, because a vector may have three components as well (x, y, z).

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Question

Define a normal force.


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Answer

A normal force is the contact force that two objects experience when they come in contact with one another.

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Question

Is a normal force equal and opposite in direction?


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Answer

Yes

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Question

What happens to the lines of forces if an object is in equilibrium? Choose the correct answer.

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Answer

All the lines of forces when extended intersect at one common point.

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Question

Does the sum of all forces acting on a body in an equilibrium add up to zero?


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Answer

Yes.

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Question

Are mass and weight the same?


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Answer

No. Mass and weight are not the same. Mass is a scalar quantity, whereas weight is a vector quantity. 

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Question

Is momentum a scalar quantity or a vector quantity?


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Answer

Momentum is a vector quantity. 

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Question

 What does the length of a vector represent? Choose the correct answer. 


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Answer

Magnitude.

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Question

What is the magnitude of a unit vector? Choose the correct answer. 


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Answer

Unity.

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Question

An object is accelerating downwards on an inclined plane. What is the component of the vector acting in the direction of motion? Choose the correct answer. 

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Answer

mgsinθ

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