# How to physically move a rigid body up the stairs?

When an object is controlled by a CharacterController, it is easy to make it climb ramps with a certain slope, or climb steps with a certain height.

When the object is controlled by a RigidBody instead, it can still climb ramps naturally - I just use AddForce and it walks on ramps just like on any other surface. I do not have to add any special logic for a ramp. However, it does not climb stairs. Is there a natural "physical" way to make the RigidBody climb stairs, without explicitly checking that there are stairs ahead?

• What shape is your collider? Does it have a rounded edge that can lift it up and over the step, or a hard corner that will snag? May 14 '20 at 11:46
• @DMGregory It is a capsule. May 14 '20 at 13:21

In order to lift a capsule up the stair's riser, the normal force exerted by the stair corner against the rounded bottom must meet or exceed the force of gravity pulling it down.

Given a particular capsule radius $$\r\$$ and riser height $$\h\$$, both sitting on the same floor plane, the corner hits the capsule at a point relative to its center of curvature:

$$\vec p_{contact} = \left( \sqrt{r^2 - h^2}, h - r\right)$$

Which means the normal force will act in the direction given by the unit normal:

$$\vec n = \frac {-1} r \left( \sqrt{r^2 - h^2}, h - r\right)$$

To lift off the ground, the vertical component of this force has to equal or exceed the force of gravity, given by an acceleration $$\a_g < 0\$$:

\begin{align} f \cdot \frac {h - r} r &= m \cdot a_g\\ f &=\frac { m \cdot a_g \cdot r} {h - r} \end{align}

This will then try to push your capsule backward away from the stair by the horizontal component of the force:

$$f_{horizontal} = \frac { m \cdot a_g} {h - r} \sqrt{r^2 - h^2}$$

So you'll need to drive your capsule into the stair with a force greater than or equal to this amount to achieve lifting. Plus some extra to overcome losses to friction.

• Very interesting, thanks! So the reason that my capsule did not climb the stais was just that the force was too small. May 19 '20 at 8:01