The physics system in Unity is what allows us, with just a few points and clicks to add what would be considered “normal” attributes to game objects. It does all the math we need to deal with basic things such as gravity and impact. It is managed through a combination of the Collider component and the Rigidbody component.
In the middle of the Rigidbody component you will see the “Use Gravity” option. This, just like it says will calculate the effects of gravity on your game object in combination with any other behaviors you have programmed. In the below example, you see two objects. Each has an identical behavior set in the attached script. That being, a downward direction at a speed of 5 units. When it reaches the bottom, it respawns at the top on a random ‘x’ coordinate. The only difference is the cube on the right has “Use Gravity” enabled and the block on the left does not. You can see how much this one checkbox changes the behavior as the block with gravity continues to accelerate.
Because of this, you may find some of the default actions of the Rigidbody need to be disabled so you can dictate control of your game objects.
Simply removing the Rigidbody component is not something we can do, because an object must have a Rigidbody in order to detect collisions. Although it goes by fast, the above animation shows how when the red “enemy” cube finally collides with the blue “player” cube it is destroyed.
In fact, we can demonstrate this. We can remove the Rigidbody component from the cube on the right but leave it on the cube on the left. Doing so, we can see how a collision with the left cube is detected causing the red cube to be destroyed. However, collisions with the cube on the right are ignored.
Interestingly, the Rigidbody component is not required on BOTH game objects. The blue player game object does not have a Rigidbody component attached to it. The only game object required to have the Rigidbody component is the one that will trigger some action. In this case, the red enemy cube detects the collision and destroys itself. Had I wanted to, I could have put the Rigidbody component on the blue player cube. But in order to destroy the enemy, the player cube, upon detecting collision would have to send a message to the red enemy cube saying “tag! now destroy yourself.”
There is of course, loads more to this. But this is the most basic of basics and enough to get you started.