The comic. (The fifth panel also applies to postmodernists.)
Another funny one. (This one’s for the physics geeks.)
My physics teacher in high school said there is no such thing as centrifugal force. He said it’s called centripetal force.
I got a D in physics.
Can someone who got an A explain it to me?
Not to overly toot my own horn, but I got an A in physics, so here goes. I must also add that if your teacher explained centrifugal and centripetal force as the same thing, he must not be a very good teacher.
The most important thing to remember here is that there is no absolute viewpoint in physics; things look different depending on where you’re viewing them from, which physicists like to call a frame of reference, being fond of long words where short ones will do. When good old Isaac was formulating his laws back in the 17th century, he postulated that
An Inertial frame of reference (IFoR) is one in which Newton’s laws are valid. Let’s take the curious Canadian pastime of Curling, where you have an object moving down a smooth sheet of ice. No forces act on it, and it keeps travelling in a straight line at constant speed unless something pushes it. (Let’s ignore the very small effects of friction here). So the curling arena is clearly an IFoR - Newton’s laws apply.
As an example of a non-IFoR, consider a plane accelerating down a runway, with you standing in the aisle wearing roller skates. Now if you had no way of knowing that the plane was accelerating, it would appear to you as though some mysterious force was pushing you backwards down the aisle, even though there was nothing doing the pushing. That’s an example of a pseudoforce, something that appears only in a non-IFoR. But that’s only the view from inside the plane. From outside the plane, we have no need to invoke a pseudoforce - it’s apparent to us that the plane is accelerating, and your inertia - resistance to having your velocity changed - is keeping you in the same place with respect to the earth, while the plane accelerates past you.
Now let’s consider a car travelling down a straight road. Imagine you were watching yourself in this car from a helicopter, flying overhead. (All that I am going to say here applies to the person sitting in the car, though). As long as the road is straight, all is well, because your body wants to travel in a straight line at constant speed, which is what the car is doing. Now if the car starts to turn to the left, your body still “wants” to travel in a straight line, and therefore continues to do so, sliding across the seat of the car, to the right.
Again, change your point of view and imagine this from inside the car, without any external reference points. The same thing happens, but in this case, it appears that we have acceleration to the right without any force present. To keep Newton’s Second Law (that objects only accelerate if there is a force) operative, therefore, we invoke a fictictious force called the centrifugal force, pushing you to the right.
In short, centrifugal force is a hack to keep Newton’s laws working in non-IFoRs, so you can apply the same principles and have consistent results no matter which frame of reference you choose. I’ve always found it easier to “step back” and consider things from the IFoR point of view rather than keep the centrifugal force and see things from inside the non-IFoR, but YMMV.
Centripetal force, on the other hand, is very simply the force needed to keep an object moving in a circle. In the case of the car turning, the seatbelt or door provides the necessary centripetal force to keep you going round the bend. In the case of you whirling a stone on a string, it’s the tension in the string, and in the case of the Earth orbiting the sun, it’s the gravitational attraction between them.
Hope that helps!
Loved this one. 