For example, it's common to represent the shape of complex things such as a person as a much simpler shape such as a box or a cylinder. Some things are too much work for computers to do the math on though so often games will 'cheat' and simplify things so that games can run at a reasonable rate so that it feels smooth. The actual math can get quite complicated but developers (mostly physics engine developers) have come up with ways to do complex calculations like this and very quickly. But the bullet skipped right over the wall! So it's not enough that we simulate the change in position of the bullet we also have to simulate it's path through space in that timestep. Say the wall is at 50,0,0 and the bullet started at 0,0,0 so in one timestep this bullet has gone from 0,0,0 to 100,0,0 (simple addition). The naive approach would be to step the bullet forward in time and check if it's currently inside the wall. Well now we have a problem when we want to make this bullet hit the wall. Well you can do what we talked about above and give it a very large velocity such as (100,0,0). For example, if you tried to simulate a bullet as a physical object and you wanted it to hit something, say a 10cm wall.

The tricky part comes when you're not just simulating a single thing but instead a bunch of different objects of varying size, mass, shape, texture, and velocities. So we know that if a ball is at 0,0,0 and it's velocity is 1,0,0 then each time step it will advance in the x direction by 1. Similarly we track things like velocity in the same way so we know the direction of physics related things by looking at which component of position they affect. They give us a way to locate things in 2D or 3D space. In a game we track the position of objects with 2-3 numbers, (x, y, z). There are already equations for these things such as acceleration being equal to force/mass. As other posters mentioned simple motion of a single object without the presence of gravity is quite simple.

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