HESI A2 · Physics · 7 min read
Friction, Torque and Levers
- Distinguish between static and kinetic friction, describing how the normal force, surface characteristics, and lubricants influence frictional resistance.
- Explain the conditions required for rotational and complete mechanical equilibrium using the concepts of torque, lever arm, and center of mass.
- Analyze the three classes of levers by comparing their component configurations, mechanical advantages, and everyday applications.
The Fundamentals of Friction and Contact Forces
Friction is a fundamental contact force that opposes the relative motion or the tendency of motion between surfaces. When contacting surfaces slide or tend to slide relative to one another, friction can act parallel to the surfaces to oppose that motion or tendency. In physical systems, friction is categorized primarily into static and kinetic forms depending on whether the contacting bodies are stationary relative to one another or actively in motion.
Static friction acts between surfaces that are not sliding relative to one another, functioning to resist the initiation of sliding. Rather than maintaining a single fixed value, static friction adjusts to oppose the tendency to slide, matching the applied force up to a maximum magnitude. For many dry surfaces, this maximum static friction is approximately proportional to the normal force pressing the surfaces together. Once an applied force exceeds this maximum threshold, the surfaces begin to slide against each other.
Kinetic friction acts between surfaces that are actively sliding past one another. Unlike static friction, kinetic friction maintains a nearly constant magnitude for a given pair of surfaces sliding at moderate speeds. During sliding, kinetic friction converts mechanical energy primarily into thermal energy. The relationship between frictional resistance and the surfaces in contact is characterized by the coefficient of friction, which is a dimensionless ratio that depends on the properties of the contacting materials. For any given pair of materials, the coefficient of static friction is typically greater than the coefficient of kinetic friction. As a result, for many surfaces, more force is needed to start an object sliding than to keep it sliding at constant velocity under the same conditions.
The magnitude of friction is strongly tied to the normal force, which is the contact force exerted perpendicular to the surfaces in contact. On a horizontal surface, the normal force equals an object's weight if the object experiences no vertical acceleration and no other vertical forces act upon it. However, for an object resting on an inclined plane with no other forces acting perpendicular to the plane, the normal force is mg cos θ. As the incline angle θ increases, the normal force decreases, correspondingly reducing the maximum available friction. Interestingly, to a first approximation, sliding friction is independent of the apparent surface area of contact. Frictional forces can also be modified by external interventions or movement types: lubricants reduce friction by separating solid surfaces with a thin fluid film, and rolling friction is generally much smaller in magnitude than sliding kinetic friction.
Common mistakes
- Assuming that wider contact surfaces produce greater sliding friction; to a first approximation, sliding friction is completely independent of the apparent surface area of contact.
- Believing that simple machines like levers can multiply both force and total work simultaneously, whereas levers cannot multiply work or energy and require the effort to move a greater distance whenever force is reduced.
- Confusing the conditions for equilibrium by checking only net force; complete mechanical equilibrium requires both net external force and net external torque to be zero.
- Assuming all levers produce an output force larger than the input force, overlooking that third-class levers have an ideal mechanical advantage less than one in the usual model with parallel effort and load forces.
Key points
- Friction acts parallel to contact surfaces to oppose relative motion or sliding tendencies, with static friction adjusting up to a maximum value and kinetic friction remaining nearly constant during moderate-speed sliding.
- The normal force acts perpendicular to surfaces, equaling an object's weight on horizontal planes without vertical acceleration, and equaling mg cos θ on an inclined plane, decreasing as the incline angle θ increases.
- Torque represents the rotational analogue of linear force, equals the applied force multiplied by the lever arm, and is maximized when the applied force is perpendicular to the line from the pivot to the application point.
- Complete mechanical equilibrium demands that both the net external force and the net external torque acting on the system equal zero.
- The ideal mechanical advantage of a lever equals the ratio of the effort moment arm to the load moment arm, with equilibrium occurring when effort torque matches load torque.
- Levers are categorized by the central component: first-class levers place the fulcrum in the middle, second-class levers place the load in the middle, and third-class levers place the effort in the middle; the usual parallel-force model gives second-class levers an ideal mechanical advantage greater than one and third-class levers one less than one.
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