Time vs. Distance: How Physics Redefines Speed
I’m Ken Kuwako, your science trainer. Every day is an experiment.
The Olympics vs. The Lab: Is Our Concept of Speed Upside Down?
When you watch sports on TV, what moments make you exclaim, “Wow, that’s fast!”? Whether it’s the 100-meter dash, a marathon, or a freestyle swimming race, we watch athletes compete based on speed in many different ways.But did you know there’s a crucial difference between the “speed we feel every day” and the “speed we study in science class”? Today, I want to talk to those of you who are about to study physics about this slightly curious difference in how we measure speed.
In the World of Sports, Distance is King
Think about a school sports day or the Olympics. The 100m dash, the 42.195km marathon, the 200m swim. What’s the common rule in all of them? It’s that the distance (the finish line) is predetermined.In everyday life and in sports, we measure how much “time” it takes to cover a “unit distance” (a fixed distance). The competition is to see “who can reach the finish line in the shortest amount of time.” Therefore, the less time taken, the faster the athlete is judged to be.In essence, we are looking at the time per unit distance. This is highly intuitive for us. After all, when we say “Let’s race to that spot!”, the finish line isn’t moving.
In the World of Physics, Time is the Standard
On the other hand, in the world of “Physics” that you are about to study, we shift our thinking. Physics is based on a system where we consider how much “distance” is covered per “unit time” (a fixed amount of time). Why do we intentionally make this change?Take a look at a car’s speed gauge. If it displays “60 km/h (60 kilometers per hour),” it tells you that the car has the ability to travel 60 km in a fixed time of 1 hour. If we tried to express this using the “sports method,” we would have to calculate “the speed at which it takes 1 minute to travel 1 km,” which makes it difficult to grasp the situation instantly.When analyzing physical phenomena, it is crucial to predict “where an object will be after 1 second, or 2 seconds.” Therefore, it is overwhelmingly more convenient for calculations and predictions to fix time as the standard (per second) and compare the distance covered (in meters).
Viewing the 100m Dash with “Physics Eyes”
Let’s convert the iconic 100-meter dash, a staple of sports, into the physics concept of “distance per unit time.” Imagine an athlete runs 100m in 10.0 seconds. When we calculate this using the rules of physics (Speed $v$ = Distance $x$ $\div$ Time $t$), we get the following:
100m/10s=10 m/s
This means a speed of “10.0 meters traveled per second.” In sports, we feel thrilled by “He finished in 10 seconds!” but in physics, we analyze it as “He is moving 10 meters every single second.”As you begin your physics journey, you will put on these new glasses—this idea of “distance traveled per unit time”—which is slightly different from your everyday sense of speed. By adopting this perspective, you’ll be able to predict the “future position” of everything, from the movement of a ball to the orbits of planets.
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