Wait, aren't speed and velocity the same thing? What about distance and displacement, aren't they the same too?

Both pairs are similar but they are not exactly the same. In order for you to tell the difference you need to learn what vector and scalar quantities are. It is really easy so don't worry.

In physics there are these important things called vectors. A vector is simply something that has magnitude and direction. So let's say you're throwing that same ball from before. You can represent it as a vector because it is going up into the air at an angle (direction) and a certain speed (magnitude).

With vectors, direction matters and it matters a lot. You may throw the ball with the same speed (magnitude), but if you throw it down into the ground (direction), it will not be able to reach your friend across the park.

Just like with regular numbers, you can add vectors together. However, you need to take direction into account. For example, using vector addition, if two vectors point in opposite directions and have the same magnitude, then they cancel each other out. The result of a vector addition is called the resultant vector, or the net vector.

Let's say one vector with magnitude 20 is pointing to the right and another vector with magnitude 10 is pointing to the left. Now add them. What's the resultant vector? The answer is a vector of magnitude 10 pointing right.

Vector addition applies to all directions, not just left and right. For example, the resultant vector of a vector with a magnitude of 30 to the right and another with a magnitude of 40 upward is a vector with a magnitude of 50 pointing diagonally at about 53° above the right (see illustration at right).

Speed and distance are both scalars, they have no direction because when you calculate them, direction is not important. Velocity and displacement are both vectors; they have direction, and it is very important. For example, if you walk 1 km east and then walk 1 km west, the distance you traveled is 2 km. However, the displacement is 0 km since it takes direction into account.

Speed is calculated by taking the distance traveled divided by the time traveled. The magnitude of velocity is calculated by taking the displacement made divided by the time traveled.

Let's use an example to explain the differences between the four different things. Your father tells you that he lived about 10 km away from his school and he walked through a blizzard in his bare feet to school and it takes him about 5 hours. So what was the distance he traveled? This is sometimes hard to calculate. If he walked in a straight line, through trees and houses and anything else in the way, it would be 10 km. But the most accurate way to measure it is to connect all of his footprints and measure how long it is. Let's say we connected all the footprints and found out he actually walked 15 km.

So his distance is 15 km...
...and his displacement is 10 km in the direction of his school (note that the direction must be stated for vector quantities, unless it's 0).
Therefore, his speed was 3 km/hr...
...and his velocity was 2 km/hr in the direction of his school.

Then he treks all the way back home from school. Since he ended up back at the place he started, his displacement becomes 0 km. Therefore, his velocity was zero too.

So in the all years of school he has gone too, his velocity has been zero. His displacement was also zero. Now go tell anyone who has given you that story that they actually had a displacement of zero and see how red their face becomes.

An object that is changing its velocity is accelerating. This is more confusing because of how acceleration is used commonly. Something that slows down or speeds up, is said to accelerate. Usually, you can say something is speeding up if it has a positive acceleration, and slowing down (decelerating) if it has a negative acceleration. However, that is if you are moving in a positive direction (walking to school, using the previous example). If you are moving in a negative direction though (walking home from school), then a positive acceleration is actually a slowing down while a negative acceleration is actually a speeding up. Again, this is because acceleration is a vector quantity as well.

So if you start running to school through the snow and you figure out that you reached a speed of 2 meters per second (m/s) in about 10 seconds, then your acceleration is 0.2 m/s² (2 m/s divided by 10 s). It is usually read as 0.2 meters per second per second. I know that doesn't really roll off your tongue very easily but who ever said science does. But at least you get the idea.

All accelerations are caused by forces. A force is basically a push or a pull on an object. For example, if you wanted to accelerate (or decelerate) a 1000 kg car at 2 m/s², you would need 2000 Netwons (N) of force (1000 kg multiplied by 2 m/s² = 2000 N).

hat's a falling body? Drop a ball on the ground, the ball is now a falling body. Not that hard to understand right? Well, one thing you might not notice is that when the ball falls, it accelerates. One major thing that gives that away is that when it's in your hand, it doesn't move, right? So its velocity is zero, but since it begins to move, then the velocity is not zero anymore. So it accelerates!

And it keeps accelerating. Aristotle, one of the first "scientists," thought that a falling body immediately gained a velocity and then remained at this constant velocity until it was stopped. He also believed that a heavier body fell faster than a light object. Do you think that makes sense? A sheet of paper falls slower than a ball, right?

Another scientist named Galileo tried to imagine what would happen if it was an ideal situation. He thought that air pushed up against the sheet of paper, which explained why it fell slower. He thought air acted as friction--air friction. So he crumpled up the paper and dropped it, it fell a lot faster than before. But since the paper's weight did not change, the crumpled paper was not any heavier than before. Therefore, he proved Aristotle wrong. He believed that everything on Earth accelerated towards Earth at a constant acceleration if there was no air (in a vaccuum). An elephant would fall just as fast as a sheet of paper if there was no air. He was right.

At a given location on the earth and in the absence of air resistance, all objects fall with the same uniform acceleration. This acceleration is called the acceleration due to gravity and it is refered to commonly as g.
g = 9.8 m/s² (metric)
g = 32 ft/s² (English)

Get used to the metric system because from here on in, that is all that is going to be used because using the English system is just plain annoying, and no one uses it in science anyhow.

What exactly are orbits? From what you know, when something "enters" orbit it circles around the Earth, right?

This is what actually happens when something goes into orbit: Imagine what happens if you are standing on the tallest mountain in the world and you pick up your trusty ball and throw it as hard as you can. Well, you now know it will accelerate towards the ground until it hits the Earth. Since the Earth is round, if you throw it fast enough, it will never touch the ground. As the ball falls towards the ground, it falls with the bend of the Earth so that it will eventually hit you in the back of the head after coming completely around the Earth! (By the way, don't actually try to throw a ball around the Earth. You would need to throw it at about 7900 m/s on the surface of the Earth.)

That is actually what happens when you "enter" orbit. When you see a spaceship go into orbit, it is actually constantly falling towards the Earth except it keeps, well, missing it. Now if the spaceship is always falling then that means the things in the spaceship must be falling. They don't have weight but the still have the same mass (this is the difference between mass and weight--mass is how much "stuff" there is in an object, while weight is how much force is created because of gravity).

The person or object falling feels as if they have no weight at all since they are continually falling in the orbit. If you were in an elevator that suddenly began to free fall (Heaven forbid), you would feel weightless as well since you are falling with the elevator. This basically explains the feelings of weightlessness of astronaunts when they are in orbit. They are weightless, but they are not weightless because there is no gravity. They are weightless because they are falling with the spaceship.