In Brief
Physics can be a complicated subject to learn, but we all have at least some idea about how these forces work. Here are nine common physics beliefs that aren't actually true — and why.

Yoda once said while teaching Luke Skywalker that: “You must unlearn what you have learned.” As much as it is a science fiction movie, this also makes sense in the real world, especially when learning physics. Unlike every other science field, physics is different in the sense that we kind of have our own ideas about it even before we learn it in the classroom. We know that everything that goes up must come down, that gravity prevents us from flying, and that things can be moved if we push them hard enough. The problem is, some of these ideas are wrong.

1. Everything that moves will eventually come to a stop. Rest is the “natural” state of all objects.

Of all physics misconceptions, this is the most common. Even the great philosopher Aristotle included it in his most important contribution to the field, the famous Laws of Motion. But now we know it is wrong, because Newton’s First Law of Motion tells us that “everything at rest will stay at rest, and everything in motion will stay in motion, unless acted upon by an external force.”

The first statement seems reasonable enough, but the second part is a little bit murky because friction, the force that stops all motion, is often left out of the equation. Friction is a force that acts between two objects that are in contact and are moving relative to each other. When we roll a ball, it stops because of the frictional force acting between it and the floor.

2. A continuous force is needed for continuous motion.

This misconception is a direct consequence of the first one. While this is true, if you are, for example, pushing a grocery cart in a supermarket, again this is only because there is friction involved. The force you apply to keep an object moving is only to counteract the frictional force. If you were to throw a rock in outer space, it would travel with a constant velocity forever, unless it hit something, of course. This is because space is mostly empty (it has trace elements of gas and dust), and there would not be any frictional force acting on that rock.

3. An object is hard to push because it is heavy.

This is one of the most common misconceptions because it’s something we see and feel everyday. While a heavy object is really hard to push, it is not because of its weight, but because of its inertia or mass. Inertia is an object’s resistance to change in motion. It is important to note that inertia is resistance to “change motion” rather than just motion itself. When I was a kid, I imagined that it would be easy to carry and push massive objects when in outer space but, not surprisingly, my younger self was wrong. Since these objects still have mass despite being weightless, this mass represents the object’s inertia.

4. Planets revolve around the sun because they are pushed by gravity.

We have to remember that gravity — the weakest of the four fundamental forces — is an attractive force. The reason why planets revolve around the Sun can be chalked up to the fact that the planets were already spinning within the protoplanetary disk encircling a young Sun. Gravity keeps the planets in orbit around the Sun, but it isn’t necessarily the one thing pushing the planets along their orbital plane.

How Objects Fall (via CNX)
How Objects Fall   (via CNX)

5. Heavier objects fall faster than lighter ones.

This misconception was debunked long ago by Galileo in an experiment when he dropped two objects with different masses from the Leaning Tower of Pisa. He showed with that experiment that objects move downward with the same acceleration.

Again, the problem comes from not being aware of another force that is involved, which is air resistance. All objects moving through air, and hence, all falling objects, experience air resistance. This force is proportional to the area of the object in the direction of motion. Usually, this force is negligible, but for light objects — with weight comparable to the air resistance, like a feather — it will have a big effect. This was ultimately confirmed by the famous hammer and feather drop experiment on the moon.

6. There is no gravity in outer space.

There is gravity in outer space, it is just weaker than what we experience here on Earth. Astronauts that are orbiting the Earth don’t experience gravity because they are free-falling (yes, you read that right). All satellites, including the Moon and the planets, are in a constant state of freefall.

They just also have a tangential velocity along with their free fall, which is why they don’t crash into what they are orbiting. When something is in free fall, it becomes weightless. This is why Kate Upton could do a photo shoot in zero gravity here on Earth. The plane that she was riding in went into free fall to do that.

7. Planets move in circular orbits around the Sun.

Planets move in elliptical orbits around the sun (with the Sun being the focus of the ellipse). This is the first of Kepler’s Three Laws of Planetary Motion, which deals with precisely how planets orbit the Sun.

One misconception deals with our seasons. Some might wrongly come to the conclusion that Earth’s proximity to the Sun dictates the seasons (summer is when Earth is closest to the Sun and winter is when it’s farthest away), but that’s not entirely true. In reality, our seasons are caused by the tilt of Earth’s axis.

Photo Credit: bradleystockwell.files.wordpress
(via: Starts With A Bang)

8. Gravity is a force of attraction between two objects with mass.

This is a concept that has been accepted by the scientific community for a very long time thanks to one of the greatest physicists of all time, Sir Isaac Newton. But, another physicist by the name of Albert Einstein proved him wrong. Einstein showed through the use of complex mathematics that gravity is not a force per se, but a consequence of the curvature of spacetime. Einstein discovered that massive objects bend the spacetime around them, and we perceive this bending of spacetime as a force.

In the Newtonian view of gravity, light would not be affected by gravity, because it does not have mass. As for Einstein’s general theory of relativity, light will also be affected by gravity, and that is confirmed specifically by an experiment led by Sir Arthur Eddington. They measured the real and apparent positions of stars behind the sun during a total solar eclipse. The difference between these measurements was exactly predicted by Einstein’s general theory of relativity.

Still, Newton’s idea of gravity is being taught in schools because it is a good approximation of gravity. It only fails when large forces are involved (large masses and/or small distances, for instance), like with Mercury and its orbit around the Sun.

9. Electrons orbit around the nucleus like planets orbiting the Sun.

Electrons reside in shells inside the atom. They exist as a standing wave of probability in these shells. This is because of Heisenberg’s uncertainty principle, which states that: “you cannot predict with 100% accuracy both the position and the momentum of an electron.” Bohr’s model puts the electron in discrete orbits around the nucleus, with a constant speed (which violates this law).

This was discovered by Young’s Double Slit Experiment. This experiment consists of firing a beam of electrons in two very small slits, and looking at the resulting pattern at the back of the slit. Instead of seeing two slits in the back, like what you would expect if an electron is a particle, Thomas Young saw a diffraction pattern, which would only happen if the electron is a wave.

Hopefully, this article helped shed some light on physics topics that are shrouded in confusion.