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Physics Overview
Colliders
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Rigidbody overview

A RigidbodyA component that allows a GameObject to be affected by simulated gravity and other forces. More info
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is the main component that enables physical behaviour for a GameObjectThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
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. With a Rigidbody attached, the object will immediately respond to gravity. If one or more ColliderAn invisible shape that is used to handle physical collisions for an object. A collider doesn’t need to be exactly the same shape as the object’s mesh - a rough approximation is often more efficient and indistinguishable in gameplay. More info
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components are also added, the GameObject is moved by incoming collisionsA collision occurs when the physics engine detects that the colliders of two GameObjects make contact or overlap, when at least one has a rigidbody component and is in motion. More info
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.

Since a Rigidbody component takes over the movement of the GameObject it is attached to, you shouldn’t try to move it from a script by changing the Transform properties such as position and rotation. Instead, you should apply forces to push the GameObject and let the physics engineA system that simulates aspects of physical systems so that objects can accelerate correctly and be affected by collisions, gravity and other forces. More info
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calculate the results.

There are some cases where you might want a GameObject to have a Rigidbody without having its motion controlled by the physics engine. For example, you may want to control your character directly from script code but still allow it to be detected by triggers (see Triggers under the Colliders topic). This kind of non-physical motion produced from a script is known as kinematic motion. The Rigidbody component has a property called Is Kinematic which removes it from the control of the physics engine and allow it to be moved kinematically from a script. It is possible to change the value of Is Kinematic from a script to allow physics to be switched on and off for an object, but this comes with a performance overhead and should be used sparingly.

See the Rigidbody and Rigidbody 2D reference pages for further details about the settings and scripting options for these components.

Sleeping

When a Rigidbody is moving slower than a defined minimum linear or rotational speed, the physics engine assumes it has come to a halt. When this happens, the GameObject does not move again until it receives a collision or force, and so it is set to “sleeping” mode. This optimisation means that no processor time is spent updating the Rigidbody until the next time it is “awoken” (that is, set in motion again).

For most purposes, the sleeping and waking of a Rigidbody component happens transparently. However, a GameObject might fail to wake up if a Static Collider (that is, one without a Rigidbody) is moved into it or away from it by modifying the Transform position. This might result, say, in the Rigidbody GameObject hanging in the air when the floor has been moved out from beneath it. In cases like this, the GameObject can be woken explicitly using the WakeUp function. See the Rigidbody and Rigidbody 2D component pages for more information about sleeping.

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Physics Overview
Colliders