-For dynamic bodies, setting the velocity explicitly every frame can cause problems. For example, a problem occurs when a light dynamic body (like a person) is pressed against a heavy dynamic body (like a car), and you set velocity of the small object so that it's pushing it into the big body. To the physics engine, the change in velocity is the same as applying a large impulse (a very short, very large force). Even if the velocity is low, the large force can allow the small body to push the big body, even when it normally wouldn't be able to. For example, a person walking into a car can overpower the car's friction and cause it to creep along the ground slowly. Additionally, when you set the velocity of an object that is already in contact, it can cause the two objects to overlap by a small amount. The easiest way to avoid both of these problems is to make smaller changes to the body's velocity, accelerating it over a fraction of a second instead of a single frame. An even better solution, which is covered more thoroughly later, is to use constraints to move the object.
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