Unit 1 Breakdown
You are on Lesson 4 of 5
- Unit 1.1 | Understanding vectors and the Standard Units used in Physics
- Unit 1.2 | The Kinematic (motion) variables: Displacement, Velocity, and Acceleration
- Unit 1.3 | Graphing motion
- Unit 1.4 | Using Kinematic Equations in 1 Dimension [Current Lesson]
- Unit 1.5 | Projectile Motion: Using Kinematic Equations in 2 Dimensions
In this lesson you will learn:
- The “Big 5” Kinematic equations
- How to use the equations
- A simple framework to solve 1-D kinematic questions
- Applying the framework to solve interesting real-world problems
Introduction
After touching down, how long does it take a plane to stop?
You drop a penny from the two story building. At what speed does it hit the ground?
These are kinematic problems, also known as motion problems. Your job is to find the one of missing kinematic variables: displacement, initial velocity, final velocity, acceleration, or time.
If you like numbers, this is the lesson for you.
And if you hate math, don’t worry… it’s incredibly simple!
Understanding kinematic equations
The “big” 5 kinematic equations, listed below, will be used to solve all kinematic problems.
- [katex] v_f = v_0 +at [/katex]
- [katex] v_f^2 = v_0^2 + 2a\Delta x [/katex]
- [katex]\Delta x = v_0t + \frac{1}{2}at^2 [/katex]
- [katex]\Delta x = v_ft – \frac{1}{2}at^2 [/katex]
- [katex] \Delta x = \frac{1}{2} t (v_f + v_0) [/katex]
These equations involve the kinematic variables, which was covered in Lesson 1.2 | The kinematic variables.
Variables and their symbols
Let’s quickly recap what each variable means.
- Displacement, it’s variable is ‘[katex]\Delta x [/katex]’ (pronounced “delta x”), measured in meters.
- You might see this written as ‘[katex]s[/katex]’ in some textbooks.
- Velocity, it variable is ‘[katex]v[/katex]’, measured in meters/second.
Notice that velocity is split into two variables:
- initial velocity ‘[katex]v_0[/katex]’ (pronounced “v knot”)
- NOTE: Sometimes written as ‘[katex]v_i[/katex]’ or ‘[katex]u[/katex]’ in some textbooks
- final velocity ‘[katex]v_f[/katex]’ (pronounced “v final”)
- Acceleration, its variable is ‘[katex]a[/katex]’ and has units of meters/second2.
- Time is the last variable. Its symbol is ‘[katex]t[/katex]’, and it’s measured in seconds.
To recap, we have 5 kinematic variables in total listed in the chart below.
| Variable name | Variable Symbol | Units |
|---|---|---|
| Displacement | [katex]\Delta x [/katex] | [katex]m[/katex] |
| Initial velocity | [katex]v_0 [/katex] | [katex]\frac{m}{s}[/katex] |
| Final velocity | [katex]v_f[/katex] | [katex]\frac{m}{s}[/katex] |
| Acceleration | [katex]a[/katex] | [katex]\frac{m}{s^2}[/katex] |
| Time | [katex]t[/katex] | [katex]s[/katex] |
The “Big 5” equations
Although we have 5 kinematic variables, each equation only uses 4.
This means, that each equation is doesn’t use exactly 1 variable.
For example, the chart below, shows equation 1 doesn’t use displacement ([katex]\Delta x[/katex]).
| Equation | Variable not in equation |
| [katex] v_f = v_0 + at [/katex] | [katex]\Delta x[/katex] |
| [katex] (v_f)^2 = (v_0)^2 + 2a\Delta x [/katex] | [katex]t[/katex] |
| [katex]\Delta x = v_0t + \frac{1}{2}at^2 [/katex] | [katex]v_f[/katex] |
| [katex]\Delta x = v_ft – \frac{1}{2}at^2 [/katex] | [katex]v_0[/katex] |
| [katex] \Delta x = \frac{1}{2} t (v_f + v_0) [/katex] | [katex]a[/katex] |
PRO TIP: Memorizing these five equations are a must in helping you solve problems rapidly!
1D Kinematic Problems
One-dimension (1D), refers to an object moving in ONLY in one direction: either horizontally (left and right) OR vertically (up and down). We’ll cover two dimensional (2D) problems in the next lesson.
For now, take a look at this 1D problem: An airplane accelerates down a runway at [katex] 3 \, \frac{m}{s^2}[/katex] for [katex]20[/katex] seconds until it lifts off the ground. Determine the distance traveled before takeoff.
This might seem hard at first. So let’s use a framework to break down how to solve these questions.
Framework for kinematic problems
Think of a framework as a template for solving a specific type of problem. This will be the first of many frameworks you will learn in this course.
Here’s my simple framework for solving 1 dimension kinematic problems:
- Read the word problem and identify 4 kinematic variables. The problem will ALWAYS give you 3 variables, and ask you to solve for 1 variable
- Pick a kinematic equation. To make this easy, look for the variable the problem doesn’t even mention. Find the equation that also doesn’t have it, using the equation table from above.
- Plug and chug! Plug in all the given numbers and solve for the unknown variable
This might still sound a bit confusing. So let’s put in into practice.
Once you catch on, revisit this framework, and everything will make much more sense.
Applying the framework
Problem: An airplane accelerates down a runway at [katex] 3 \, \frac{m}{s^2}[/katex] for [katex]20[/katex] seconds until is finally lifts off the ground. Determine the distance traveled before takeoff.
- Identify the 4 variables in the problem: [katex]a = 3 \, m/s^2[/katex], [katex]t = 20 \, s[/katex], [katex]v_0 = 0 \, m/s[/katex], [katex]\Delta x = [/katex] what we need to find.
- Pick an equation: Notice that this question does not involve the ‘[katex]v_f[/katex]’ variable. Thus, we will pick the equation that does not have [katex]v_f[/katex] : [katex]\Delta x = v_0t + \frac{1}{2}at^2 [/katex]
- Plug the numbers into the equation and solve for [katex]\Delta x[/katex]:
- We find that [katex]\Delta x = 600 \, m[/katex].
PQ – Kinematics in the x and y directions
Using the framework to solve following 1D kinematic problems for both the x and y directions.
Check answers by clicking the full version of each question.
Kinematics in the horizontal (x) direction
Kinematics in the vertical (y) direction
Challenge (Bonus) kinematics questions
A car is traveling 20 m/s when the driver sees a child standing on the road. She takes 0.8 s to react then steps on the brakes and slows at 7.0 m/s2. How far does the car go before it stops?
Two students are on a balcony 19.6 m above the street. One student throws a ball vertically downward at 14.7 m/s. At the same instant, the other student throws a ball vertically upward at the same speed. The second ball just misses the balcony on the way down.
PS – 1D Kinematics
In this video we will solve a few problems using my simple 3 step kinematics’ framework. Hopefully, by the end of the video, you’ll see how simple 1D kinematic problems are!
Helpful problem solving tips
- After picking the correct kinematic equation, re-arrange the equation for the variable you are trying to solve for. Then you can plug in the numbers all at once.
- Label your variables using subscripts. For example if you are working in the horizontal direction, you can label, acceleration as [katex] a_x[/katex] and velocity as [katex] v_x[/katex]. This will make more sense once we start solving 2D problems
- Get good at finding “hidden variables.” For example, if a problem tells you a car is moving at constant velocity, they are technically also telling you that acceleration = zero.
- If a problem deals with an object in free fall, you can assume acceleration = [katex]9.81 \, \frac{m}{s^2}[/katex].
Lesson 1.5 Preview
In the next lesson we will apply our problem solving framework to objects moving in two dimensions. We often call these projectiles. Although this may seem harder it’s actually quite simple using the tricks we will show.
