# Why cars rotate during small overlap crash test?

i was watching a small overlap car crash test video and i noticed that the car rotate to the right side during the collision. my question is why the car rotate?

this is the link of the video https://www.youtube.com/watch?v=dfG4SqPYcSA

As the car hits the obstacle, the latter exerts a force $$F$$ on the former. This force is eccentric with respect to the CoG (the small cross in the center of the car schematic) and this causes a torque (moment) $$\tau$$ about the CoG:

$$\tau=Fd$$

With Newton's Second Law, the torque causes angular acceleration $$\alpha$$:

$$\tau=I\alpha$$

where $$I$$ is the inertial moment of the car about the CoG.

In other words, the car starts rotating about the CoG.

• sorry i didn't understand your answer. a torque causes angular acceleration due to a force. in the video there is nothing pushing the car from the side to make it rotate. – D. Frederico Jul 29 '20 at 13:29
• When the car hits the obstacle, the obstacle (which is immovable) starts exerting a force $F$ on the car.Because of the shape of the obstacle, $F$ is offset with respect to the CoG. This causes torque about the CoG and with Newton's Second angular acceleration. The whole thing is an elementary application of $\text{N2L}$ for rotation. – Gert Jul 29 '20 at 14:35
• So there is no need for a force from the side: torque acts about a point (or about an axis). Here, torque is provided by $F$. – Gert Jul 29 '20 at 14:40