In the movie The Dark Knight (2008) Batman faces off against his arch nemesis the Joker. In this scene the Joker is in an 18-wheel truck truck and Batman, on a motorcycle, is trying to stop him.
Is this realistic that the wire could flip the Joker’s truck?
Solution: In order for the truck to rotate there must be a net torque in the direction of rotation. One torque is produced by gravity. While on the ground this is offset by a torque from the normal force of the ground. To flip the truck the torque caused by the wire must be greater than the torque from gravity (the normal force torque will disappear as soon as the truck leaves the ground). If we look on the internet for typical properties of an 18-wheel truck we find they can have a mass of 15000kg unloaded, a length of 16m and a height of 4m. Let’s assume that the wire is 1m below the axis of rotation of the truck and that the axis of rotation is 1m from the front of the truck and that gravity acts in the middle of the truck. The from the formula for torque we get
the force exerted by the wire would need to exceed 1 million Newtons. The reason for this is that the lever arm for the torque by the wire is seven times shorter than that for the gravitational torque. Thus overcoming the gravitational torque requires a huge force. You can watch a video about how this stunt was done and you’ll see that in fact the wire doesn’t flip the truck. Instead the stunt crew set off an explosion near the back of the truck that flips it. Because the explosion is near the back the lever arm is long and the force is required is much less.
