Unit 1 Breakdown
You are on Lesson 2 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 [Current Lesson]
- Unit 1.3 | Graphing motion
- Unit 1.4 | Using Kinematic Equations in 1 Dimension
- Unit 1.5 | Projectile Motion: Using Kinematic Equations in 2 Dimensions
In this lesson you will learn:
- Kinematic variables
- Understanding distance vs displacement
- Understanding speed vs velocity
- Understanding acceleration
Kinematics
Kinematics is the study of motion. For anything that moves, we will be able to use kinematics to analyze the motion with graphs and equations.
In this lessons we will simply cover the kinematic variables (displacement, velocity, and acceleration) and what each one means. You might have already head of these variables, but you’ll soon see there is a lot more going on behind the scenes.
Over the next few lessons we will tie all the variables together with equations and start problem solving!
Understanding the kinematic variables
In this unit of kinematics, as well as future units, there are 3 very important variables: Displacement, velocity, and acceleration.
This might seem easy to understand, but it can get be pretty confusing. Be extra careful to read slowly.
Pay attention to bold words and their meanings.
For example speed and velocity do not mean the same thing.
Distance and displacement sound similar. But they are not the same.
Video [RE] – Displacement, velocity, and acceleration simplified
In this video, I’ll attempt to cover this whole lesson in just a few minutes.
However, it is still a good idea to still go through the rest of the lesson and solve all the given problems to get the best understanding of the material.
Displacement
Position (a scalar quantity) is where we are. It’s like your home on a map.
We can specify a position using coordinates. Place your finger anywhere on the solid line above – that will be your position.
Displacement (a vector quantity) is how far we are from our starting point.
Imagine taking a ruler and drawing the dotted green line on the diagram above. This line represents your displacement.
Lets say you run 1 lap around a 100 m. What’s your displacement?
Since you started and ended at the same spot, your displacement is 0.
Your distance traveled is 100 meters.
Displacement is written as ∆x. The “∆” symbol is the greek letter for delta which means “change in.”
Mathematically ∆x = xf =xi, which simply states that displacement is the change in the final position from the initial position.
Lastly, Distance (a scalar quantity) is the total amount traveled.
Take your finger and trace out the entire purple path. The total amount your finger traveled is what we call distance.
Another example: You took a 200-km trip to LA and back. What’s the distance traveled and displacement?
Your displacement (∆x) is zero, because you’re back where you started. Your distance, however, is 400 kilometers (200 km to go and 200 km back)
PQ – Displacement
Find the distance and displacement of each senario below.
Note that displacement is written as ∆x and distance is written as ‘s’.
Velocity
Velocity (a vector quantity) is your how fast you move in a specific direction: riding a bike 5 m/s north.
Then what is speed?
Velocity is how fast displacement changes.
Speed is how fast distance changes.
They are similar, but not same.
Mathematically,
velocity (v) = displacement/time = ∆x/t
speed = distance/time = d/t
Example: suppose you run 100 meters around a track in 100 seconds.
a) your average speed is 100m/100s = 1m/sec.
b) your average velocity is 0m/100s = 0m/sec. (Remember, displacement is 0, when running a full lap!)
PQ – Velocity
Answer the questions below and check your answers given in the bold. Remember to use standard units.
- A high school bus travels 240 km in 6.0 h. What is its average speed for the trip? (11.11 m/s)
- A caterpillar travels, north across the length of a 2.00 m porch in 6.5 minutes. What is the average velocity of the caterpillar? (5.1 x 10-3 m/s)
- A hiker is at the bottom of a canyon facing the canyon wall closest to her. She is 280.5 m from the wall and the sound of her voice travels at 340.0 m/s at that location. How long after she shouts will she hear her echo. (1.65 s)
- A motorist traveling on a straight stretch of open highway sets his cruise control at 90.0 km/h. How far will he travel in 15 minutes? (23,000 m)
- A runner completes a 200 meter circular track in 100 seconds. What is his average speed? His average velocity? (2 m/s, 0 m/s)
- A man walks 7 km East in 2 hours and then 2.5 km West in 1 hour. Find his average speed and average velocity. (speed = .89 m/s, v = 4.2 m/s east)
- Sally drove 120 km south at 60 km/h and then 150 km east at 50 km/h. Determine Sally’s speed and magnitude of velocity. (speed = 15 m/s, |v| = 10.67 m/s)
- Challenge: A cross-country rally car driver sets out on a 100.0 km race. At the halfway marker (50.0 km), her pit crew radios that she has averaged only 80.0 km/h. How fast must she drive over the remaining distance in order to average 100.0 km/h for the entire race? (36.94 m/s)
Acceleration
Acceleration (another vector quantity) is how quick velocity changes.
This can be a bit tricky, but let’s break it down.
Let’s say you are traveling 1 m/s north. The next second you are traveling 2 m/s north. And the next second, 3 m/s north.
We can say that our velocity is increasing by 1 m/s north every second. OR 1 m/s per second. OR 1 m/s/s also written as 1 m/s2 to the north. We call this acceleration.
Mathematically we can write: acceleration (a) = v/t
Now lets say you’re riding a bike.
If you start pedaling faster, you’re accelerating.
If you hit the brakes, you’re also accelerating – just in the opposite direction (this is also called de-acceleration)!
In simple terms: Acceleration means to speed up. De-acceleration means to slow down.
Units for acceleration
Acceleration is denoted by the variable ‘a’ and has the units meters per second squared (m/s²).
Take the acceleration 9.81 m/s2. This tells us, for every second the speed increases by 9.81 m/s.
We call this type of acceleration “gravity.” (more on this later)
A unique instance of acceleration.
Acceleration is a vector. It gets it’s direction from the direction of velocity.
The “magnitude of velocity” does always have to change. Instead the direction of velocity could be changing.
Imagine a a car moving in a circle around a track.
Let’s assume the car moves at constant speed. Would you say it’s accelerating?
The car IS in fact accelerating! Even though the magnitude of velocity remains the same, the direction of velocity changes as the car moves around in a circle.
In summary, acceleration is defined as the rate of change of velocity over time. This means that if an object is accelerating, its velocity is changing, either in magnitude or direction, or both.
PQ – Acceleration
Answer the questions below and check your answers given in the bold. Remember to use standard units.
Using the kinematic variables
You now properly understand the 3 main kinematic variables.
But where are we going to use them?
The remaining lessons in this unit will focus on utilizing the kinematic variables in motion graphs (lesson 1.3) and the big 4 kinematic equations (lesson 1.4) to solve real life problems. This is about to become a lot more interesting!
Conclusion
Let’s take a moment to recap what we’ve learned.
First, we explored how kinematics, the study of motion, would be used.
We thoroughly understood the 3 main kinematic variables of displacement, velocity, and acceleration. And you also got plenty of practice to get an even deeper understanding.
If you are still having trouble understanding, redo the lesson, and remember, practice questions are the key to mastering these concepts. So, keep on exploring and practicing!
PQ – End of lesson quiz
Before moving on to the next lesson it important to test your understanding. Attempt the questions below.
Lesson 1.3 Preview
In the next lesson we will use our knowledge of kinematic variables to create graphs. This may sound boring, but graphs are an incredible tool to help us visualize motion as you will see.
