Why Does a Skater Need Ice or Wheels in Order to Skate?

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Из курса от партнера University of Virginia

How Things Work: An Introduction to Physics

818 оценки

University of Virginia

How Things Work: An Introduction to Physics

818 оценки

An introduction to physics in the context of everyday objects.

Из урока

Skating

Professor Bloomfield examines the principle of inertia through skate boarding. Objects at rest tend to remain at rest while objects in motion, tend to remain in motion. Why does a stationary skater remain stationary? Why does a moving skater tend to continue moving? How can we describe the fluid, effortless motion of a coasting skater? How does a skater start, stop, or turn? Why does a skater need ice or wheels in order to skate? Physics concepts covered include Newton's first and second laws and 5 physical quantities: position, velocity, acceleration, force, and mass.

How Can We Describe the Fluid, Effortless Motion of a Coasting Skater?8:33

How Does a Skater Start, Stop, or Turn?16:31

Why Does a Skater Need Ice or Wheels in Order to Skate?4:39

Познакомьтесь с преподавателями

Louis A. Bloomfield
Professor of Physics

Why does skaters need ice or wheels in order to skate?

If you're thinking about friction and about, more or less, eliminating with Ice

wheels, you're right. Skates do a wonderful job of reducing the

effects of friction. But, I'm going to go with the more

general answer, that real world complications often mask inertia and

Newton's first law. When Inertia was first published by Sir

Isaac Newton in his Principia Mathematica in 1687.

It was a remarkable observation. Inertia underlies all of motion.

But, it's largely masked from view by nuisance effects.

First, there is gravity. I'll talk more about gravity in the next

episode. But here's a preview.

According to Newton's first law, if I'm free of all external forces, then I'll

move at constant velocity. I'll be inertial, and I'll coast.

Let's go try it! Look.

No hands. Ahhhhh.

The problem was the gravity was exerting a downward force on me and I wasn't

actually inertial. Since we can't turn off gravity, we have

to cancel it out with another force. When a skater is skating on level ground.

The ground pushes up on the skater by the way of the skates, with just enough

upward force to cancel out gravity's downward force on the skater.

So the skater is experiencing no net force and the skater is inertial.

. We have gravity under control, but that

still doesn't explain the need for ice or wheels on the skates.

So I'm going to ask you a question about this book.

And I want you to answer it truthfully according to your experience.

And according to what you thin in your gut, is the right answer.

I'll give you a chance later on to change that answer, if you like.

Suppose this book is free of external forces except for any that you might

exert on it yourself. How does this book move?

The book's motion is governed by both Newton's first and second laws of motion.

And if you push on the book, and you're exerting the only external force on the

book, it accelerates in the direction of your push and it's velocity changes with

time. If you stop pushing on that book, it

becomes inertial. It's experiencing no overall external

force, and it coasts. Friction, and air resistance as well, are

nuisances, and they interfere with inertial motion.

They make the whole idea of coasting seem counterintuitive.

You've never seen a file cabinet or a carpet or a glass of water even, coast

horizontally across a horizontal surface. You have to push them the whole way, and

when you stop pushing, they very quickly come to a stop.

But that's not because Newton's laws are wrong, it's because of friction.

It sure seems like you have to push on something to make it move, and when you

stop pushing it comes to a stop. But that's just friction messing with

your head. If you use skates to eliminate friction,

so that friction can't interfere with inertial motion.

Then an object, namely the skater, can be truly free of external forces.

Or, least overall external forces. And can move according to Newton's first

law. And when something pushes on that skater.

So the skater experiences a net force. That net force causes the skater to

accelerate and the skater's velocity changes according to that acceleration.

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