Using the Law of Moments

The Law of Moments can be used to calculate the size and position of the counterweight needed to balance the load.

Forces on a crane

The weight of the lorry on this half of the bridge causes it to turn around the support. The force required to secure the bridge can be calculated using the law equating the clockwise and anticlockwise moments.

Forces on a bridge

If the cab was taken away from the trailer it would fall down, pivoting around the rear wheels.

Forces needed to support a lorry trailer







The cab provides the force to turn the load back up. Application of the law helps to calculate load distribution and maximum loads.