Sometimes the universe too complicated to analyze.
Damn, if you take a tennis ball and throw it across the room, even that is actually too complicated. After it leaves your hand, the ball has an interaction with the Earth, causing it to accelerate towards the ground. The ball rotates as it moves, which means there can be more friction on one side of the ball than on the other. The ball is also colliding with some oxygen and nitrogen molecules in the air — and some this molecules that eventually interact with even more atmosphere. The air itself isn’t even constant – the density changes as the ball moves higher and the air can move. (We often call it wind.) And once the ball hits the ground, even the floor isn’t completely flat. Yes, it looks flat, but it is on the surface of a spherical planet.
But not everything is lost. We can still make this model of the thrown tennis ball. All we need is some idealization. These are simplified approximations that turn an impossible problem into a solvable problem.
In the case of a tennis ball, we can assume that all mass is concentrated at a single point (in other words, the ball has no real size) and that the only force acting on it is the constant pull-down gravity. . Why ignore all other interactions? That’s because they don’t make a significant (or even measurable) difference.
Is this even legal in physical court? Well, science is all about the process of building models, including the equation for the trajectory of a tennis ball. At the end of the day, if the empirical observations (where the ball lands) agree with the model (the prediction of where it will land), then we should go. To idealize the tennis ball, everything works very Good. In fact, the physics of a handball becomes a test question in an introductory physics class. Other idealizations are more difficult, such as trying to determine the curvature of the Earth just by looking at this. super long terminal at Atlanta airport. But physicists always do it this way.
Perhaps the most famous idealization was made by Galileo Galilei in his study of the nature of motion. He’s trying to figure out what happens to a moving object if you don’t apply a force to it. At that time, almost everyone followed the teachings of Aristotle, who said that if you don’t apply a force to a moving object, it will stop and stay still. (Although his work is about 1,800 years old, people think that Aristotle is too cool to be wrong.)
But Galileo disagreed. He thought it would continue to move at a constant speed.
If you want to study a moving object, you need to measure both position and time so that you can calculate its velocity or its change in position divided by the change in time. But there’s a problem. How do you accurately measure time for fast moving objects over short distances? If you drop something from a relatively small height, such as 10 meters, it takes less than 2 seconds for it to hit the ground. And back around 1600, when Galileo was alive, that was a pretty hard time to measure. So instead, Galileo watched a ball roll on the rails.
