6.6
Static and Kinetic Frictional Force
One of the simpler characteristics of sliding friction is that it is parallel to the contact surfaces between systems, and is always in a direction th…
The magnitude of frictional force has two forms, namely static frictional force and kinetic frictional force.
The frictional forces can be explained using the applied force versus the frictional force graph. In the case of static frictional force, the applied force is exactly counterbalanced by the frictional force.
Hence, the net horizontal force is zero and the object remains at rest. Within the static frictional region, the graph is linear. Once the applied force exceeds the maximum value, the object accelerates and the frictional force decreases.
Now, the kinetic frictional force takes over and the object begins to move. Within the kinetic frictional region, it is easier to keep the object moving than to start moving it.
Hence, for any given pair of surfaces, the magnitude of the kinetic frictional force is always less than that of the maximum static frictional force.
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Q1: What is the difference between static and kinetic friction?
Static friction acts on stationary objects and prevents motion when force is applied, while kinetic friction acts on moving objects and opposes their motion. Static friction is typically greater than kinetic friction for the same pair of surfaces. Once an object overcomes maximum static friction and begins moving, kinetic friction takes over and is easier to maintain than the initial force needed to start motion.
Q2: Why is it easier to keep an object moving than to start its motion?
Kinetic friction is always less than maximum static friction for any given pair of surfaces. Static friction increases to match applied force until it reaches its maximum value. Once this threshold is exceeded and the object moves, kinetic friction takes over at a lower magnitude, requiring less force to maintain motion than was needed to initiate it.
Q3: How does an applied force graph show the transition from static to kinetic friction?
The applied force versus friction graph shows a linear region where static friction counterbalances applied force, keeping net horizontal force zero. Once applied force exceeds maximum static friction, the object accelerates and kinetic friction takes over. The graph shows a drop in friction magnitude at this transition point, illustrating why kinetic friction is less than maximum static friction.
Q4: What happens to an object when applied force equals static frictional force?
When applied force exactly equals static frictional force, the net horizontal force becomes zero and the object remains at rest. Static friction responds dynamically to the applied force, increasing to match it in magnitude while acting in the opposite direction. This equilibrium persists until the applied force exceeds the maximum static friction threshold.
Q5: How does friction direction relate to the motion of surfaces in contact?
Friction is parallel to contact surfaces and always opposes the motion or attempted motion of systems relative to each other. For moving objects, kinetic friction acts opposite to the direction of motion. For stationary objects, static friction acts opposite to the direction of applied force, preventing relative motion between surfaces.
Q6: What does the linear region of the static friction graph represent?
The linear region represents the static friction zone where applied force is exactly counterbalanced by static frictional force. Throughout this region, the object remains at rest with zero net horizontal force. The graph remains linear until applied force reaches the maximum static friction value, at which point the object begins to accelerate.
Q7: Why does kinetic friction decrease compared to maximum static friction?
Once an object overcomes maximum static friction and begins moving, kinetic friction takes over at a lower magnitude. This occurs because the contact between surfaces changes during motion. The reduced kinetic friction explains why maintaining motion requires less force than initiating it, making it easier to keep objects moving once they start accelerating.