lever
Summary
A lever is a rigid bar that pivots about a fixed point (the fulcrum), so a small effort force can balance or move a larger load. At rotational equilibrium the torques balance: Feffort × deffort = Fload × dload. A longer effort arm lowers the force needed but not the work.
Detail
Meaning: A lever is a rigid bar that rotates about a fixed pivot, the fulcrum. An effort force at one point balances or moves a load at another. At rotational equilibrium the net torque about the fulcrum is zero. First class has the fulcrum between effort and load (seesaw); second class has the load in the middle (wheelbarrow); third class has the effort in the middle (forearm lifting a weight).
Equation: Feffort × deffort = Fload × dload, where F is force (N) and d is the perpendicular distance from the fulcrum (m). Mechanical advantage MA = Fload/Feffort = deffort/dload (unitless).
On the MCAT: Typical questions give a bar or board with weights and ask for an unknown force or position using torque balance, or ask for the lever class. Biomechanics passages treat the biceps and forearm as a third-class lever, where MA < 1, so the muscle force exceeds the load.
Common confusion: A lever does not reduce the work. Force is traded for distance: an ideal lever has Win = Wout, so halving the force requires moving twice the distance. Also use the perpendicular lever arm; if the force is angled, torque = rF sinθ.
Sources
- Fundamentals of Physics
- College Physics
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.