Imagine you have a ball tied to the end of a string. You hold the other end of the string and swing around. The ball will eventually appear to orbit around you in a circle. This is essentially the exact same thing that is happening with a planet in its orbit with the Sun! Gravity is a force which constantly tries to pull the planet into the Sun, much like the string is the force pulling the ball towards you.

However, because the planet already has some speed, this is enough to keep it from falling into the Sun. If you spin faster, the ball will be harder to hold onto the string so you would have to pull harder. Imagine now that you are spinning and suddenly you let go of the string—the ball will go flying away. If the Sun suddenly lost its gravitational pull on Earth, we would similarly go flying away into space!

If you watch a simulation of planets orbiting the Sun, you may notice that the orbit is not a perfect circle, but instead more like an oval (ellipse). This is because not only does the Sun’s gravity pull on the planets, but the planets’ gravity also pulls on the Sun! Over time, planets develop an elliptical orbit. Imagine this is like using a bowling ball instead of small ball, and having a long elastic rubber band instead of a rigid string—you will be pulled towards the bowling ball because it’s really heavy, and as you spin around the rubber band will stretch and compress to make an oval orbit.

If you throw a ball, it will keep going until it hits a wall or the ground. However, if the ball is on a string, it will stop once the string is completely tight. The reason why the ball stops is because the string exerts a force on the ball stopping it from moving. Forces can make something move or stop something from moving. For a planet in orbit around the sun, the string is invisible. That invisible string is the gravitational force between the Earth and the sun. So imagine that at some point a planet was moving through space, but it got too close to the sun. But the planet wasn’t moving directly towards the sun, it was just moving near it. At this point the invisible string became strong and the planet kept moving around it like a tether ball around a pole. Because the planet had an initial speed near the sun, it kept moving around it rather than just being pulled directly into the sun.

A ball on a string is an example of a model that we use to make unfamiliar phenomena (e.g., a planet's orbit) familiar via an everyday object. In fact, I recommend trying it out yourself by tying some string to a ball and swinging it around (don't let go!). What path does the ball take?

Whenever you start a physics problem, you always want to make sure to get a physical understanding of it. One of the best ways to visualize this is through a free body diagram, where you draw out all the forces acting on the object of interest. A force is a vector, so both direction and magnitude are important. Below is the free body diagram for a planet in circular orbit.

Two things are happening. The planet is moving in straight line with some velocity and gravity is continually pulling in the planet towards the center (e.g., sun). This gravitational pull can also be characterized as a centripetal force, since the resulting motion is circular (technically a planet's orbit is not circular, mostly because it feels the gravitational pull of all the other planets, so it's not a simple two body problem).

Now what if instead of gravity that was connecting the planet and the sun, it was a piece of string? The diagram looks pretty much the same; the only thing we've changed is what is responsible for force that pulls the object towards the center to make a circular motion. With a string attached to a ball, the force is tension.

You can read a little more about it here: click to read and learn a bit about some paradoxes people have been thinking about the development of the universe.

Physics is full of toy models that make understanding complex phenomena easier to understand intuitively. Another famous toy model that we use to describe physical phenomena are springs, known as the harmonic oscillator. The most common example that you probably know is a pendulum (like one on a grandfather clock that swings back and forth to keep the time). The math that describes the motion of a pendulum can also describe bonds in a molecule, lattice vibrations and thermal properties in a crystal, RLC circuits, and many more! It's all the same math with just a different physical interpretation. In fact, you can find the harmonic oscillator model up until quantum mechanics. I use it many times in the research I'm doing to understand how lattice vibrations in a crystal affects electrical conduction. Even though the harmonic oscillator can't capture everything about a system accurately, it is extremely useful to understand the physics without having to do much math, and that is equally as valuable.

Let’s start by talking about “forces”. You might have heard the term before (since forces are all around you), but maybe you don’t know exactly what a force is. Basically, a force takes something moving one way and tries to move it in another way. For example, if you jump in the air, you start moving upward. Eventually, the force of gravity changes your direction from up toward to sky to back toward the ground. If you were chasing a ball and you ran into a wall, you would feel a force from the wall making you move away from the ball.

A ball on a string is also experiencing a force. If you take a ball on a string and dangle it so that it is hovering over the ground, the string will tighten and the ball will stay in the air. The ball feels a force from gravity and an equal and opposite force from the string. When you start spinning the ball over your head, you are making the ball want to go to the right or the left. But as soon as the ball goes too far to the right or left, the force on the string pulls the ball back to you. You could imagine this movement like a staircase that goes in a circle. As the steps on the staircase become very tiny, you eventually can’t see each step.

This is a good analogy for planets in orbit because planets feel like they are attached to the sun by a string. Just like gravity pulls you back toward the ground, planets and stars feel gravity from each other. The sun, being really massive, is like you holding the string to the ball. The force of the string is gravity, and the ball is the earth. Earth flies through space at 67,108 miles per hour but the “string” attached to the sun keeps pulling it back toward the sun – just like tiny staircases again. The string and ball analogy works really well in other ways, too. For example, it can be used to help predict about how fast a planet might be moving. It’s not perfect though; the planets don’t actually orbit the sun in a perfect circle. Still, it helps people understand the basics of how an orbit works.

A planet is pulled by the gravity of its star toward it, but it has enough forward motion of its own to keep from falling into its star. As a result, it does loops around the star, not having enough of its own motion to escape the star completely, but enough to keep it from falling in.

If you want a better analogy, think about a marble rolling around the inside of a funnel. If the marble has enough forward motion, it will orbit around the hole in the bottom of the funnel. This is almost exactly what planets do.

It's because the forces acting on the ball are the same, I'll explain this, but look at this as an example: click here

There are two forces on the ball in this example. One is inertia, the object continuing to move in the direction it was going, in a straight line (shown as v). If the string were to break, the ball would fly away in a straight line. The other is the force applied by the string, holding the ball to your hand so that it can't fly away even though it wants to as you spin. For a planet, it has the same inertial force. If the sun were to disappear (like the string breaking), the planet would fly away into space in a straight line. But gravity keeps a planet pulled in toward the sun like the string, so it moves in a circle.