What is a lever?

A lever is a stiff rod that can rotate about a fulcrum or pivot point. This rotation transmits force.

When the lever is at equilibrium the tendency for it to rotate clockwise is balanced by the tendency to rotate counterclockwise.

What factors determine the equilibrium point of a lever?

• Add 25 gm weights to the lever and balance it.
• Note the distance from the weights to the fulcrum. These distances are the lengths of the two lever arms (L₁ and L₂).
• Calculate the product of the weight (force = F) and length (L) of the lever arm on each side of the fulcrum.

What is the relationship of these two products? Check yourself

What happens if one of the weights moves closer to the fulcrum? Check yourself

• Replace one of the 25gm weights with a 50gm weight.
• Balance the lever and compare L₁and L₂
• How long should the lever arm with the 50 gm weight be? Check yourself

In the muscle/skeletal system a contracting muscle functions as a weight. When it contracts it creates an in-force that is translated into an out-force by the lever system.

• Write the equation for this relationship when the system is at equilibrium. Check yourself.
• Solve the equation for F_out. Check yourself

Changes in the length of skeletal elements can result in more efficient transmission of an in-force to an out-force. Based on the equation there are several ways to increase the force efficiency of a lever:

• increasing the length of the in-lever arm
• decreasing the length of the out-lever
• or doing both of the above

Make sure you are comfortable with the above ideas before you explore the different types of levers. Return to beginning

Explore the types of levers