# Revision history [back]

To answer the first question, the box turns over because of friction coefficient, which has a default value of 1. Update your collision to below to prevent rolling:

    <collision name="collision">
<geometry>
<box>
<size>1 1 1</size>
</box>
</geometry>
<surface>
<friction>
<ode>
<mu>0</mu>
<mu2>0</mu2>
</ode>
</friction>
</surface>
</collision>


And remember, even though the tutorial moves the block with Model::SetLinearVel, which forces CG's linear motion in a straight line, physics dictates that there is still a downward gravity force, and there is still an upward normal contact force and its corresponding tangential friction forces. Ultimately, the combination of forces imparts a rolling moment about CG of the box, causing it to tumble.