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This series of lessons is designed to help you learn, or review, the fundamentals of physics. This part focuses on gravity and normal force.

Didn't we cover gravity already? 9.82, right?

Yes, gravity is 9.82 N, on Earth. But there's a bit more to it than that.

We already looked at gravity in terms of acceleration- gravity is what causes falling objects (like apples) to fall down (like on Newton's head). But don't confuse the force of gravity, , with the acceleration caused by gravity, g. Force and acceleration are not the same thing!

To keep things simple, we use the equation . As you can see clearly, this is a direct application of Newton's Second Law of Motion. Since g is always the same, on Earth, we can also write , where m is the mass of an object.

Why do you keep saying "on Earth"? The entire concept of gravity is, once more, an attraction between any 2 objects that have a mass. The bigger the mass, the more attraction there is (so fat people, don't lose hope on dating. No, that's a joke). The biggest thing around us is the Earth, and since relative to the Earth we all weigh pretty much the same, the magnitude of gravity will be the same on Earth.

But if we're on the moon, there's less gravity, because the moon is smaller. So on the moon, g would have a lower value than 9.8. Similarly, if you're on Jupiter, g would be bigger. In fact, since Jupiter is so big, g would be so great that the force of gravity will pull you down towards the centre of the planet and crush you like a bug. Fun!

Also, the gravitational pull depends on the distance between the two objects. So g would have a different value if you're far away from the centre of the Earth. This means that if you're in the Dead Sea, lowest point on Earth, g will be higher than on mount Everest. Of course, the different would be pretty tiny and you won't start flying around, but that's how it works.

Ok, and now it's normal force time

The normal force is what we call the force that opposes gravity. As Newton's Third Law states, every action has a reaction, and every force has an inverse. So, if I'm standing up, gravity is acting on my downwards. Why am I not moving downwards? Simple, because the floor is projecting a force on my upwards that equals the gravity- that's your normal force.

The normal force always acts perpendicular to the surface you're moving across (or are standing on if you're stationary). It's always equal to the force that acts in the direction perpendicular and towards the surface, but in the opposite direction.

In cases of horizontal motion on a flat surface, gravity and normal force are always equal, and always cancel each other out.

And the cool part- slopes Here's where we can really apply our stuff. Say a rock of 1 kg is on a slope 45 degrees to the horizontal. Your FBD looks like, with only the normal force and gravity: Now, you'll notice that the magnitude of gravity is . Does this mean the normal force is also 9.8 N?

No. In fact, the diagram is missing one force- static friction, which will get to later on. The important part here is to realize that the normal force now acts in a diagonal, and has 2 component- vertical, which cancels out gravity, and horizontal, which cancels out friction. This reminds you of something? That's right, projectile motion. Forces, like velocities, are vectors. Their components can be added and directions can be found.

Gravity and normal force are really cool

Next time, we'll look at kinetic forces.