The mysterious sliding stones. Along the remote Racetrack...

The mysterious sliding stones. Along the remote Racetrack Playa in Death Valley, California, stones sometimes gouge out prominent trails in the desert floor, as if the stones had been migrating (Fig. 6-16). For years curiosity mounted about why the stones moved. One explanation was that strong winds during occasional rainstorms would drag the rough stones over ground softened by rain. When the desert dried out, the trails behind the stones were hard-baked in place. According to measurements, the coefficient of kinetic friction between the stones and the wet playa ground is about 0.80. What horizontal force must act on a 20 kg stone (a typical mass) to maintain the stone’s motion once a gust has started it moving? (Story continues with Problem 37.)

The mysterious sliding stones. Along the remote Racetrack
Playa in Death Valley, California, stones sometimes gouge out
prominent trails in the desert floor, as if the stones had been migrating (Fig. 6-16). For years curiosity mounted about why the
stones moved. One explanation was that strong winds during occasional rainstorms would drag the rough stones over ground softened by rain. When the desert dried out, the trails behind the
stones were hard-baked in place. According to measurements, the
coefficient of kinetic friction between the stones and the wet playa
ground is about 0.80. What horizontal force must act on a 20 kg
stone (a typical mass) to maintain the stone’s motion once a gust
has started it moving? (Story continues with Problem 37.)

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Key Concepts

Newton's First Law and Equilibrium of Forces

Newton's First Law states that an object will continue in its state of motion unless acted upon by a net external force. In problems involving constant velocity, this means the applied horizontal force must exactly balance the kinetic friction, resulting in zero net force.

Normal Force

Normal force is the force exerted by a surface perpendicular to itself, which counteracts the weight of an object resting on it. In horizontal motion problems, the normal force is typically equal to the gravitational force acting on the object, and it is crucial for calculating frictional forces.

Coefficient of Friction

The coefficient of friction is a dimensionless constant that represents the ratio of the frictional force between two surfaces to the normal force pressing them together. It is used to quantify how much frictional resistance one can expect between the surfaces under consideration.

Kinetic Friction

Kinetic friction is the resistive force that acts opposite to the direction of motion when two surfaces slide past each other. Its magnitude is determined by multiplying the coefficient of kinetic friction by the normal force, and it remains constant for a given pair of surfaces in relative motion.

Transcript

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