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## Class 11 Physics (India)

### Unit 8: Lesson 5

Horizontally launched projectiles

# What are velocity components?

Learn how to simplify vectors by breaking them into parts.

## Why do we break up vectors into components?

Two-dimensional motion is more complex than one-dimensional motion since the velocities can point in diagonal directions. For example, a baseball could be moving both horizontally and vertically at the same time with a diagonal velocity v. We break up the velocity vector, v, of the baseball into two separate horizontal, v, start subscript, x, end subscript, and vertical, v, start subscript, y, end subscript, directions to simplify our calculations.
Trying to tackle both the horizontal and vertical directions of a baseball in one single equation is difficult; it’s better to take a divide-and-conquer approach.
Breaking up the diagonal velocity v into horizontal v, start subscript, x, end subscript and vertical v, start subscript, y, end subscript components allows us to deal with each direction separately. Essentially, we'll be able to turn one difficult two-dimensional problem into two easier one-dimensional problems. This trick of breaking up vectors into components works even when the vector is something other than velocity, for example, forces, momentum, or electric fields. In fact, you'll use this trick over and over in physics, so it's important to get really good at dealing with vector components as soon as possible.

## How do we break a vector into components?

Before we talk about breaking up vectors, we should note that trigonometry already gives us the ability to relate the side lengths of a right triangle—hypotenuse, opposite, adjacent—and one of the angles, theta, as seen below.
sine, start color #1fab54, theta, end color #1fab54, equals, start fraction, start color #e84d39, start text, o, p, p, o, s, i, t, e, end text, end color #e84d39, divided by, start text, h, y, p, o, t, e, n, u, s, e, end text, end fraction
cosine, start color #1fab54, theta, end color #1fab54, equals, start fraction, start color #6495ed, start text, a, d, j, a, c, e, n, t, end text, end color #6495ed, divided by, start text, h, y, p, o, t, e, n, u, s, e, end text, end fraction
tangent, start color #1fab54, theta, end color #1fab54, equals, start fraction, start color #e84d39, start text, o, p, p, o, s, i, t, e, end text, end color #e84d39, divided by, start color #6495ed, start text, a, d, j, a, c, e, n, t, end text, end color #6495ed, end fraction

When we break any diagonal vector into two perpendicular components, the total vector and its components—v, comma, v, start subscript, y, end subscript, comma, v, start subscript, x, end subscript—form a right triangle. Because of this, we can apply the same trigonometric rules to a velocity vector magnitude and its components, as seen below. Notice that v, start subscript, x, end subscript is treated as the adjacent side, v, start subscript, y, end subscript as the opposite, and v as the hypotenuse.
sine, start color #1fab54, theta, end color #1fab54, equals, start fraction, start color #e84d39, v, start subscript, y, end subscript, end color #e84d39, divided by, v, end fraction
cosine, start color #1fab54, theta, end color #1fab54, equals, start fraction, start color #6495ed, v, start subscript, x, end subscript, end color #6495ed, divided by, v, end fraction
tangent, start color #1fab54, theta, end color #1fab54, equals, start fraction, start color #e84d39, v, start subscript, y, end subscript, end color #e84d39, divided by, start color #6495ed, v, start subscript, x, end subscript, end color #6495ed, end fraction

Note that the vs in these formulas refer to the magnitudes of the total velocity vector, the total speed, and can therefore never be negative. The individual components v, start subscript, x, end subscript and v, start subscript, y, end subscript can be negative if they point in a negative direction. The convention is that left is negative for the horizontal direction, x, and down is negative for the vertical direction, y.

## How do you determine the magnitude and angle of the total vector?

We saw in the previous sections how a vector magnitude and angle can be broken up into vertical and horizontal components. But what if you start with some given velocity components: v, start subscript, y, end subscript and v, start subscript, x, end subscript? How could you use the components to find the magnitude v and angle theta of the total velocity vector?
Finding the magnitude of the total velocity vector isn't too hard since for any right triangle the side lengths and hypotenuse will be related by the Pythagorean theorem.
v, squared, equals, start color #6495ed, v, start subscript, x, end subscript, end color #6495ed, squared, plus, start color #e84d39, v, start subscript, y, end subscript, end color #e84d39, squared
By taking a square root, we get the magnitude of the total velocity vector in terms of the components.
v, equals, square root of, start color #6495ed, v, start subscript, x, end subscript, end color #6495ed, squared, plus, start color #e84d39, v, start subscript, y, end subscript, end color #e84d39, squared, end square root

Also, if we know both components of the total vector, we can find the angle of the total vector using start text, t, a, n, end text, theta.
tangent, start color #1fab54, theta, end color #1fab54, equals, start fraction, start color #e84d39, v, start subscript, y, end subscript, end color #e84d39, divided by, start color #6495ed, v, start subscript, x, end subscript, end color #6495ed, end fraction
By taking inverse tangent, we get the angle of the total velocity vector in terms of the components.
start color #1fab54, theta, end color #1fab54, equals, tangent, start superscript, minus, 1, end superscript, left parenthesis, start fraction, start color #e84d39, v, start subscript, y, end subscript, end color #e84d39, divided by, start color #6495ed, v, start subscript, x, end subscript, end color #6495ed, end fraction, right parenthesis

## What's confusing about vector components?

When using theta, equals, tangent, start superscript, minus, 1, end superscript, left parenthesis, start fraction, v, start subscript, y, end subscript, divided by, v, start subscript, x, end subscript, end fraction, right parenthesis, the fact that we put v, start subscript, y, end subscript on top as the opposite side and the v, start subscript, x, end subscript on the bottom as the adjacent side means that we are measuring the angle from the horizontal axis. It seems like figuring out how to draw the angle could be confusing, but here are two good tips:
Assuming we have selected right/up as the positive directions, if the horizontal component v, start subscript, x, end subscript is positive, the vector points rightward. If the horizontal component, v, start subscript, x, end subscript is negative, the vector points leftward.
Again, asumming we have selected right/up as the positive directions, if the vertical component v, start subscript, y, end subscript is positive, the vector points upward. If the vertical component v, start subscript, y, end subscript is negative, the vector points downward.
So, for example, if the components of a vector are v, start subscript, x, end subscript, equals, minus, 12, start text, space, m, slash, s, end text and v, start subscript, y, end subscript, equals, 10, start text, space, m, slash, s, end text, the vector must point leftward—because v, start subscript, x, end subscript is negative—and up—because v, start subscript, y, end subscript is positive.
Concept check: If a paper airplane has the velocity components v, start subscript, x, end subscript, equals, minus, 7, start text, space, m, slash, s, end text and v, start subscript, y, end subscript, equals, minus, 5, start text, space, m, slash, s, end text, which direction is the paper airplane moving—assuming we choose right and up as positive directions?

## What do solved examples involving vector components look like?

### Example 1: Bend it like Beckham

A soccer ball is kicked up and to the right at an angle of 30degrees with a speed of 24.3 m/s as seen below.
What is the vertical component of the velocity at the moment shown?
What is the horizontal component of the velocity at the moment shown?
To find the vertical component of the velocity, we'll use s, i, n, theta, equals, start fraction, start text, o, p, p, o, s, i, t, e, end text, divided by, start text, h, y, p, o, t, e, n, u, s, e, end text, end fraction, equals, start fraction, v, start subscript, y, end subscript, divided by, v, end fraction. The hypotenuse is the magnitude of the velocity 24.3 m/s, v, and the opposite side to the angle 30degrees is v, start subscript, y, end subscript.
sine, theta, equals, start fraction, v, start subscript, y, end subscript, divided by, v, end fraction, start text, left parenthesis, U, s, e, space, t, h, e, space, d, e, f, i, n, i, t, i, o, n, space, o, f, space, s, i, n, e, point, right parenthesis, end text
v, start subscript, y, end subscript, equals, v, sine, theta, start text, left parenthesis, S, o, l, v, e, space, f, o, r, space, v, e, r, t, i, c, a, l, space, c, o, m, p, o, n, e, n, t, point, right parenthesis, end text
v, start subscript, y, end subscript, equals, left parenthesis, 24, point, 3, start text, space, m, slash, s, end text, right parenthesis, sine, left parenthesis, 30, degrees, right parenthesis, start text, left parenthesis, P, l, u, g, space, i, n, space, v, a, l, u, e, s, point, right parenthesis, end text
v, start subscript, y, end subscript, equals, 12, point, 2, start text, space, m, slash, s, end text, start text, left parenthesis, C, a, l, c, u, l, a, t, e, space, a, n, d, space, c, e, l, e, b, r, a, t, e, !, right parenthesis, end text
To find the horizontal component, we'll use cosine, theta, equals, start fraction, start text, a, d, j, a, c, e, n, t, end text, divided by, start text, h, y, p, o, t, e, n, u, s, e, end text, end fraction, equals, start fraction, v, start subscript, x, end subscript, divided by, v, end fraction.
cosine, theta, equals, start fraction, v, start subscript, x, end subscript, divided by, v, end fraction, start text, left parenthesis, U, s, e, space, t, h, e, space, d, e, f, i, n, i, t, i, o, n, space, o, f, space, c, o, s, i, n, e, point, right parenthesis, end text
v, start subscript, x, end subscript, equals, v, cosine, theta, start text, left parenthesis, S, o, l, v, e, space, f, o, r, space, h, o, r, i, z, o, n, t, a, l, space, c, o, m, p, o, n, e, n, t, point, right parenthesis, end text
v, start subscript, x, end subscript, equals, left parenthesis, 24, point, 3, start text, space, m, slash, s, end text, right parenthesis, cosine, left parenthesis, 30, degrees, right parenthesis, start text, left parenthesis, P, l, u, g, space, i, n, space, v, a, l, u, e, s, point, right parenthesis, end text
v, start subscript, x, end subscript, equals, 21, point, 0, start text, space, m, slash, s, end text, start text, left parenthesis, C, a, l, c, u, l, a, t, e, space, a, n, d, space, c, e, l, e, b, r, a, t, e, !, right parenthesis, end text

### Example 2: Angry seagull

An angry seagull is flying over Seattle with a horizontal component of velocity v, start subscript, x, end subscript, equals, 14, point, 6, start text, space, m, slash, s, end text and a vertical component of velocity v, start subscript, y, end subscript, equals, minus, 8, point, 62, start text, space, m, slash, s, end text.
What is the magnitude of the total velocity of the seagull?
What is the angle of the total velocity?
Assume right/up are positive, and assume all angles will be measured counterclockwise from the positive x axis.
We'll use the Pythagorean theorem to find the magnitude of the total velocity vector.
v, squared, equals, v, start subscript, x, end subscript, squared, plus, v, start subscript, y, end subscript, squared, start text, left parenthesis, T, h, e, space, P, y, t, h, a, g, o, r, e, a, n, space, t, h, e, o, r, e, m, point, right parenthesis, end text
v, equals, square root of, v, start subscript, x, end subscript, squared, plus, v, start subscript, y, end subscript, squared, end square root, start text, left parenthesis, T, a, k, e, space, s, q, u, a, r, e, space, r, o, o, t, space, o, f, space, b, o, t, h, space, s, i, d, e, s, point, right parenthesis, end text
v, equals, square root of, left parenthesis, 14, point, 6, start text, space, m, slash, s, end text, right parenthesis, squared, plus, left parenthesis, minus, 8, point, 62, start text, space, m, slash, s, end text, right parenthesis, squared, end square root, start text, left parenthesis, P, l, u, g, space, i, n, point, right parenthesis, end text
v, equals, 17, point, 0, start text, space, m, slash, s, end text, start text, left parenthesis, C, a, l, c, u, l, a, t, e, space, a, n, d, space, c, e, l, e, b, r, a, t, e, !, right parenthesis, end text
To find the angle, we'll use the definition of start text, t, a, n, g, e, n, t, end text, but since we now know v, we could have used start text, s, i, n, e, end text or start text, c, o, s, i, n, e, end text.
tangent, theta, equals, start fraction, v, start subscript, y, end subscript, divided by, v, start subscript, x, end subscript, end fraction, start text, left parenthesis, U, s, e, space, t, h, e, space, d, e, f, i, n, i, t, i, o, n, space, o, f, space, t, a, n, g, e, n, t, point, right parenthesis, end text
theta, equals, tangent, start superscript, minus, 1, end superscript, left parenthesis, start fraction, v, start subscript, y, end subscript, divided by, v, start subscript, x, end subscript, end fraction, right parenthesis, start text, left parenthesis, I, n, v, e, r, s, e, space, t, a, n, g, e, n, t, space, o, f, space, b, o, t, h, space, s, i, d, e, s, point, right parenthesis, end text
theta, equals, tangent, start superscript, minus, 1, end superscript, left parenthesis, start fraction, 8, point, 62, start text, space, m, slash, s, end text, divided by, 14, point, 6, start text, space, m, slash, s, end text, end fraction, right parenthesis, start text, left parenthesis, P, l, u, g, space, i, n, space, m, a, g, n, i, t, u, d, e, s, point, right parenthesis, end text
theta, equals, 30, point, 6, degrees, start text, left parenthesis, C, a, l, c, u, l, a, t, e, space, a, n, d, space, c, e, l, e, b, r, a, t, e, !, right parenthesis, end text
Since the vertical component is v, start subscript, y, end subscript, equals, minus, 8, point, 62, start text, space, m, slash, s, end text, we know the vector is directed down, and since v, start subscript, x, end subscript, equals, 14, point, 6, start text, space, m, slash, s, end text, we know the vector is directed right. So, we will draw the vector in the fourth quadrant.
The seagull is moving 17, point, 0, start text, space, m, slash, s, end text at an angle of 30, point, 6, degrees below the horizontal.

## Want to join the conversation?

• What is the meaning of terminal velocity?
• How do we calculate the air resistance and in turn terminal velocity?
• Although the same question has been asked, i didn't quite get your conclusion. When finding the angle of the vector, you didn't keep the y-component negative. i did and my angle was negative. Does it matter? My answer's still correct?
• Hi Ama. Just to add in to some of the replies, when Sal says "30.6 degrees BELOW the horizontal", it's conceptually the same as saying "-30 degrees ABOVE the horizontal", which sounds crazy, but essentially you're talking about the same angle.

This is why it's important to say 30 degrees BELOW the horizontal. If you just said "30 degrees", then that would be an angle in Quadrant 1, which is an upwards and to the right direction of the vector, which is incorrect.

So specifying its direction in relation to a horizontal axis will nullify the use for the sign of the angle. By drawing the Vector out first, it will be simpler to see if you should be using ABOVE or BELOW a horizontal. Calculating angles of a vector this way allows you to not have to worry about using the negative values of the components when using the inverse trig functions.

Hope this helps!
• I notice in the presentations, you use double lines on each side of the components. What do the double lines mean?
• Hello Bob,

When we work with vectors double lines such as this ||X|| or single lines such as |X| are referring to the length of vector X.

Regards,

APD
• But how would we know if our triangle is right angled? would we have to make a scale diagram first or?? im so confused
• Assuming you draw a horizontal and a vertical vector, as they showed, you will always end up with a right triangle. This is because the horizontal and vertical lines are perpendicular to each other, creating a right angle. Of course, you can instead draw the vectors as non-horizontal and vertical lines (as they also showed), but I think it would make the triangle much harder to use. Just remember that as long as your triangle has two lines perpendicular to each other, it is a right triangle with a right angle.
Hope this helps! Let me know if you still don't understand =)
• so this is kind of like finding the degrees of a point on the unit circle using:
sin( )
cos( )
tan( )
csc( )
sec( )
cot( )
arcsin( )
arccos( )
arctan( )
arccsc( )
arcsec( )
arccot( )

Right?
Or no?
• Sorta? But you will not need to know the list after tangent from what I have read up on the MCAT.
• Can I have more than two vectors that affects my velocity?
• Yes. There is a 3rd axis of movement known as z used in 3-D diagrams. Movement on this axis implies going in or out of the page. Also you can have an infinite quantity of vectors affecting velocity as long as you account for each individual vector's velocity.
• Can any one give me the all formulas of Projectile motion chapter
(1 vote)
• Is arcsin the same as sin^-1 ? And for the rest too?