# Common mistakes

## Rigid-body isn't affected by gravity#

If you expect your rigid-body to fall because of gravity but it doesn't, please make sure to double-check the following:

• Your gravity vector is non-zero.
• Your rigid-body is a dynamic rigid-body.
• You didn't lock the translations of the rigid-body.
• The rigid-body has a non-zero mass.

Note that a collider not attached to a dynamic rigid-body will never fall because it won't be affected by forces.

##### note

If the rigid-body has no collider attached to it, its mass will be zero unless you gave it a mass (or mass properties) explicitly. If the rigid-body has colliders attached to it and you didn't give the rigid-body a mass explicitly, make sure that at least one of the colliders has a non-zero density (or non-zero mass if you set it explicitly on the collider).

##### warning

Currently, colliders with triangle-meshes won't have their mass properties computed automatically. So if a rigid-body only has triangle-mesh colliders attached to it, you need to set its mass/angular inertia manually.

## Applying a force to a rigid-body doesn't work#

If applying a force or an impulse to a rigid-body doesn't work, please make sure to double-check the following:

• The rigid-body is a dynamic rigid-body.
• The rigid-body has a non-zero mass (or non-zero angular inertia for torques).
• The force or impulse must be strong enough to actually push the rigid-body. You may for example try with a very high force/impulse value (say, with a magnitude of 100000.0) and see if this stronger force works.
##### note

If the rigid-body has no collider attached to it, its mass will be zero unless you gave it a mass (or mass properties) explicitly. If the rigid-body has colliders attached to it and you didn't give the rigid-body a mass explicitly, make sure that at least one of the colliders has a non-zero density (or non-zero mass if you set it explicitly on the collider).

## Why is everything moving in slow-motion?#

A common mistake, especially in 2D, is to use pixels as the length unit in the physics world. Let's say that in 2D you have a 100x100 pixels sprite for your player. It may be tempting to use a 100x100 cuboid collider for this sprite: ColliderDesc.cuboid(50.0, 50.0) (we set 50.0 because this is the half-width of the cuboid). Doing this will make it look like the simulation runs in slow-motion because the cuboid will be huge compared to the magnitude of the default gravity (-9.81).

The recommended way of using Rapier is to use SI units (meters, seconds, kilograms, etc.) If the player sprite is a 100x100 cuboid, then it is as if your player is 100 meters tall and 100 meters wide, which is huge. Therefore it is recommended ot have a scaling factor between the graphics and the physics. For example we can say that 1 physics meter is equal to 50 pixels. This means that we can initialize our player collider as a 2x2 cuboid while still using a 100x100 pixels sprite.

All we need to do to keep measures in sync is to multiply by our scaling factor 50 all the positions given by the physics engine before rendering.