Levers & Wheels: Quick Reference (Last-Minute)

Levers Overview

• Lever terms: Force (F), Resistance/Load (R), Force Arm (F arm), Resistance Arm (R arm).

• Torque (turning effect): $\text{Torque}=F\cdot\text{F arm}$

• Lever equation (balance): $F\cdot\text{F arm}=R\cdot\text{R arm}$

• Mechanical Advantage: $\text{MA}=\frac{\text{F arm}}{\text{R arm}}=\frac{R}{F}$

• Long force arms reduce the muscle force needed to move the resistance.

Lever Classes

First Class – FAR

• Axis between F and R (pivot between force and load).

• Farm = R arm ⇒ MA = 1 ⇒ Balance.

• Axis closer to F: \text{F arm}<\text{R arm},\quad\text{MA}<1\, d/ROM advantage).

• Axis closer to R: \text{F arm}>\text{R arm},\quad\text{MA}>1\, eful movement).

Second Class – ARF

• Axis closer to R; F arm > R arm ⇒ MA > 1.

• Note: Farm is always longer than Rarm, so less force is needed to move a larger resistance.

Third Class – AFR

• Axis closer to F; F arm < R arm ⇒ MA < 1.

• Note: F arm is always shorter than R arm, so more force is required to move the resistance.

Equations & Relationships

• Inverse Relationship:

  • Increase F arm ⇒ decrease force needed to move resistance.

  • Increase R arm ⇒ decrease resistance that can be moved.

• Proportional Relationship:

  • If one or both resistance components increase, one or both force components must increase to balance.

• Torque: $\text{Torque}=F\cdot\text{F arm}$

• Lever Equation: $F\cdot\text{F arm}=R\cdot\text{R arm}$

• Mechanical Advantage: $\text{MA}=\frac{\text{F arm}}{\text{R arm}}\quad\text{or}\quad\text{MA}=\frac{R}{F}$

• Key takeaway: Long force arms are advantageous (less muscle force required).

Wheels & Axles

• If Wheel Radius > Axle Radius:

  • Longer force arm ⇒ mechanical advantage (like a 2nd Class lever).

  • Requires less force to move the resistance.

• If Wheel Radius < Axle Radius:

  • Shorter force arm ⇒ mechanical disadvantage.

  • Requires more force, but provides speed/ROM of the wheel (like a 3rd Class lever).