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Learning Introductory Physics with Activities

Paul J. Emigh, Rebecka Tumblin, Kathryn Hadley, Danielle Skinner

Section 4.5 The Force of Static Friction

Definition 4.5.1. Static Friction.

The maximum magnitude of any static friction force is related to the corresponding normal force
\begin{equation*} |\vec{F}^{sf}_{12}| \leq \mu_s |\vec{F}^{N}_{12}| \end{equation*}
The static friction will act in a direction that opposes the attempted relative motion of the objects involved.
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The model for static friction only tells you the maximum value; the magnitude can take on any value up to this maximum, depending on the other forces acting on the involved objects. The constant \(\mu_s\) is called the coefficient of static friction; it is a property of the surfaces involved in the force.
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Exercises Activities

1. Sensemaking - Units.

Using the definition above, determine the units for \(\mu_s\text{.}\)
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Answer.
\(\mu_s\) is unitless!
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2. Largest.

A small car might have a mass of around \(1000 \mathrm{~kg}\text{,}\) and the coefficient of static friction between the tires and the road is about \(0.9\text{.}\) What is the largest possible force that static friction can exert on this car?
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3. Smallest.

A small car might have a mass of around \(1000 \mathrm{~kg}\text{,}\) and the coefficient of static friction between the tires and the road is about \(0.9\text{.}\) What is the smallest possible force that static friction can exert on this car?
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4. Largest and Smallest.

Describe situations when each of the previous cases (the largest and smallest possible forces from static friction) would occur.
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