2.9 Circular Motion and Orbits

Critical Questions:

• What keeps the Earth orbiting the Sun instead of shooting off into space?
• How do figure skaters achieve such elegant spins?

As I mentioned earlier, we spend most of our time on a giant hunk of rock that is moving in a circle (approximately) around the sun, just like every other planet in the solar system. And the sun is circling the center of the galaxy. In fact, if you spend enough time looking out into space, you start to realize that almost everything out there is either spinning around, orbiting something else, or both. And understanding that motion represents an important step in the understanding of physics as a whole.

What makes an object move in a circle? Perhaps a more useful question to ask first is, why don’t objects usually move in circles? We know from Newton’s First and Second Laws that objects keep moving in a straight line (or stay stationary) unless an external force causes them to speed up, slow down, or change their direction of motion. And yes, if you’ve been reading carefully, you’ll notice that this answer amounts to a fancy way of saying, “Because.” A slightly better answer might be, “Because space seems to be aligned in straight lines rather than curved ones,” but the truth is, that answer is just yet another way of restating the question.

2.8 Friction and Air Resistance

Critical Questions:

• What is friction, and why does it happen?
• If you drop a penny from a tall building, could it kill someone below?

Before reading this website, you might not have thought of friction as a force. In common language, the word is used to refer to almost anything that happens when two things come in contact, like when you start a fire by rubbing two sticks together or when two people get in an argument. Just like with all of our physics terms, however, we are going to give this one a much more specific definition.

Friction is the force that results when two objects rub together. We’ve already seen the example of a book moving across a table, but I’ve also briefly mentioned the more interesting example of the frictional force between car tires and the road, which actually moves the car forward.

So what causes friction? I’ve hinted at that, too. In order to solve this problem, we have to go down to the microscopic level. If you’re sitting at a table, run your hand over the surface. It probably feels pretty smooth, doesn’t it? But if you had a powerful enough microscope, you could see that the seemingly solid tabletop is actually made up of billions of smaller particles – molecules, atoms, and subatomic particles.

2.7 Newton’s Third Law

Critical Questions:

• If two astronauts are floating in space and one of them shoves the other, how come they both float away in opposite directions?
• What makes a car move forwards?

Walking is such a simple task that most mammals can do it mere hours after being born. (It takes humans a few years to figure it out, but we get there eventually.)

It’s so simple that the average person, unless they are a very special type of person, does not think very hard about the physics of walking.

But think about it: how does it really work? One leg moves forward, the other moves back, and somehow your entire body gets propelled in the direction you want to travel. It’s a great mystery!

Or perhaps you’re rolling your eyes right now. “Come on,” you’re saying, “don’t be an imbecile! Each time you move a leg backwards, you push yourself forwards at the same time. Easy!”

2.6 Newton’s Second Law, Part 2: Multiple Forces

Critical Questions:

• What happens when more than one forces pushes or pulls an object?
• Why do some objects move at a constant speed even though they’re being pushed by a force?

You might have noticed that in the last section, I was careful to talk about only one force at a time. If a real physicist had been reading, steam would be coming out of her ears, because Newton’s Second Law is usually stated in terms of something called the net force. “Net” in physics means about the same thing as it does in economics: it is the total result once everything has been added and subtracted.

So what happens when there is more than one force? I like to think of net force as if two people were pulling on ropes attached to a big crate. If they pull the crate in the same direction, the crate will accelerate twice as quickly. If they pull in opposite directions with equal forces, the crate won’t move at all — these two forces cancel each other out. If one person pulls northwards and the other pulls eastwards, the crate will move to the north-east.

The net force, then, is just the sum of all of the forces going in various directions. It’s important to remember that in the formula for Newton’s Second Law, the F is not just any one force but the sum of all of the forces acting on the object. If two equal forces pull in opposite directions, the net force is zero, which means the acceleration is zero.

2.5 Newton’s Second Law, Part 1: Effects of One Force

Critical Questions:

• How does something move when it’s being pushed?
• What would it take to move the Death Star?

Newton’s First Law is all about what happens when there are no forces acting on an object. It sets everything up for us to be able to really understand everything else. But the second law is the one that gets the most press, because it tells us what forces actually do, and it even gives us a mathematical formula to let us calculate the results of various forces.

Now, I know I promised not to bring math into this blog, but this is going to be one of the exceptions, because (a) it’s arguably the most important equation in the field of physics, and (b) it’s super easy to understand.

We already know that forces move things around. As Newton’s First Law tells us, things don’t just spontaneously start moving on their own; there has to be a force that causes them to change their motion.

Picture yourself in the driver’s seat of a car. You push down on the gas pedal, and the car starts to move forwards. You push harder and pick up speed. At some point, you might ease off the gas pedal a bit and keep the car moving at a constant speed down a straight stretch of road. And when you want to slow down, you might lift your foot off the pedal for a while before using the brakes.

This is actually a complicated series of events. But Newton’s Second Law is powerful enough to explain all of it — not the inner workings of your car, necessarily, but the relationships between the amount you step on the gas and the car’s changing speeds.

2.4 Newton’s First Law, Part 2: Inertia

Critical Questions:

• How do things move when they’re floating through space?
• What is inertia?
• Why can’t you move your sailboat by pointing a fan at the sail?

Let’s head off into Imaginary Physics Problem Land (see the previous post) in order to understand the basics of Newton’s laws before we go deeper down the rabbit hole.

When discussing Newton’s First Law, the specific IPPL we’ll use is one in which we ignore not only friction, but also all other forces that might interfere. So imagine yourself floating in space, in a spacesuit, with that solid object that you had at the beginning of the previous section.

2.3 Newton’s First Law, Part 1: Imaginary Physics Problem Land

Critical Questions:

• Why do we tend to get Newton’s Laws wrong in our heads?
• Why do physics problems always involve “frictionless slopes” and “massless ropes”?

Imagine an object. Think of something with a bit of weight to it, like a small wooden treasure chest or a fresh loaf of good, dense olive bread. Or, once again, you can go ahead and try this simple experiment in real life.

Now imagine that you start to push this object. You keep pushing until it’s got a good speed going, and then you let go. What happens next?

You have watched this exact scenario play out an uncounted number of times in your life, and so you probably have a pretty good idea of the results. The object will move forward for a little while, and then eventually it will slow down and stop.

After observing this behaviour over and over again, most people develop an unconscious version of Newton’s First Law in their own heads. Their version goes something like this: Everything, no matter how fast it’s going, will eventually slow down and stop. And, surprise surprise, this is wrong.

2.2 Forces

Critical Questions:

• What makes things move?
• What’s happening to the molecules in my hand when I reach out and touch something solid, like a door?

As a teacher, I’ve always found it easy to define the term force. Ready? It is a push or a pull. Identifying forces in real life, though, can a bit more tricky.

Of course there are the obvious ones. Push open a door, and you’ve applied a force to it. Throw a ball, ditto. Pull yourself up a rope, lift a fork laden with pasta to your mouth, or punch someone in the kidney: all forces.

But forces can, of course, get a lot more complicated. For example: if you push down on the gas pedal, that’s an obvious force, but what force pushes the car forward? Of course there’s an engine and the engine spins the wheels, but what is actually pushing the car? Not the wheels, exactly — we’ll have to get back to that one.