What Forces Are Acting On You?

What are the forces acting on you right now? I want to answer this question by introducing something called a free body diagram. This is a sketch that scientists make that shows all the forces acting on an object. Each force is represented by an arrow; the size of the arrow represents the magnitude of the force, and its direction indicates the direction of the force.

So, for example, right now there is a downward gravitational force on me that pulls me towards the center of the Earth. There's another force that a lot of people don't think about, but is pretty important, and it's the force from the ground pushing me upward. For most people, it's kind of strange to think that the ground, which is something inanimate, could be putting a force up on me. But let me try to convince you that the Earth really is pushing up on me by stepping on this toy here that is supported by a spring.

As I step onto the spring, it contracts. So, you can imagine the spring must be trying to expand out and put a force outwards. That force, in effect, supports me, pushes up on my feet, and balances the gravitational force down so that I remain motionless. Now, the ground is very similar to this spring. The ground is just a very stiff spring, but every time you step, you compress the ground a little bit, and that is enough to make the ground create a force on you, pushing you back up.

The force of the ground upwards must be equal to the gravitational force downwards so that you stay at rest. For simplicity, we draw all forces acting through the center of mass of an object, so even though the force is acting on my feet, I can draw that force upwards as acting through my center of mass.

So, there are really only two main forces on you: the force of gravity downwards and something we call the normal force, which pushes you back up. The reason we call it the normal force is that the word normal means perpendicular, and the force is perpendicular to the surface that you're standing on.