Navigation
Explore why direction matters in physics and how velocity differs from simple speed.
If a pilot flies at 500 mph but doesn't know which way to point the plane, will they ever reach their destination? In physics, knowing 'how fast' is only half the story.
Most people use the words 'speed' and 'velocity' to mean the same thing, but in physics, they are very different! Speed is a scalar quantity, meaning it only measures 'how much' or the magnitude. For example, 20 m/s. Velocity is a vector quantity, which means it has both magnitude (how fast) AND direction (where it is going). To describe velocity, you must include a direction, like '20 m/s North.' If you change your speed, your velocity changes. But here is the kicker: if you keep the same speed but turn a corner, your velocity also changes because your direction changed!
A bowling ball rolls down a lane. It travels 18 meters toward the pins in exactly 3 seconds.
Quick Check
If a car is traveling at 50 mph East, and it turns to travel 50 mph South, has its speed changed? Has its velocity changed?
Answer
The speed has not changed (it is still 50 mph), but the velocity HAS changed because the direction changed from East to South.
To calculate velocity, we don't use the total distance traveled. Instead, we use displacement (). Displacement is the straight-line distance from your starting point to your ending point. Imagine you walk 10 meters forward and then 10 meters backward to where you started. Your total distance is 20 meters, but your displacement is 0! Because velocity is based on displacement, if you end up exactly where you started, your average velocity for the whole trip is actually zero, no matter how fast you were running.
An athlete runs one full lap around a 400-meter circular track in 50 seconds.
Quick Check
What is the displacement of a hiker who walks 3 km North and then 2 km South?
Answer
1 km North.
This is a concept that even experts sometimes trip over. An object can have a constant speed while having a changing velocity. How? By moving in a circle! Think of a moon orbiting a planet or a child on a carousel. Even if the ride moves at a steady 5 mph, the child is constantly changing direction to stay in the circle. Because the direction is always shifting, the velocity is always shifting. For an object to have constant velocity, it must move at a steady speed in a perfectly straight line without ever turning.
A car enters a circular roundabout. It maintains a steady reading of on its speedometer as it drives halfway around the circle to exit on the opposite side.
1. Is the speed constant? Yes, the speedometer stays at . 2. Is the velocity constant? No. Even if the speed is steady, the car is turning, which means the direction vector is rotating. 3. If the car takes seconds to go halfway around a roundabout with a diameter of meters, the displacement is the straight line across (m). 4. Average Velocity: in the direction of the exit.
Which of the following is a measurement of velocity?
A drone flies 100 meters North in 20 seconds. What is its velocity?
An object moving at a constant speed in a circle has a constant velocity.
Review Tomorrow
Tomorrow morning, try to explain to a friend why a race car driver has an average velocity of zero at the end of a race.
Practice Activity
Next time you are in a car, watch the speedometer. When the car turns a corner at a steady speed, remind yourself that the velocity is changing even though the speed is not!