AP Physics C: Mechanics 2.7 B Describe Static Friction Between Two Surfaces Questions

Describe static friction as an adjusting contact force up to a maximum and apply impending-slip conditions to stacked blocks, inclines, rolling contact, and experiments.

Exam points

treat static friction as an adjusting force whose magnitude does not automatically equal its maximum
apply the maximum static-friction condition only at impending slip or a limiting case
determine the minimum coefficient needed to prevent relative motion between stacked blocks
solve equilibrium or motion limits for objects held on a rough inclined surface
use static friction to maintain rolling without slipping or shared acceleration at a contact

[Maximum number: 2]

Students design an experiment using blocks of adjustable mass to investigate friction using the setup shown. Block 1 of initial mass 0.44 kg is placed on a rough horizontal surface and connected by a string to block 2 of initial mass 0.20 kg. The string extends over a pulley that has negligible mass and friction.

Calculate the minimum value of the coefficient of static friction $\mu_{s}$ that would keep the two-block system at rest.

The coefficient of friction is such that when block 2 is released from rest, block 1 travels across the surface. The acceleration a of each block is recorded with motion detectors 1 and 2, as shown in the figure. The data for the motion detectors as functions of time t are shown on the graphs. For each motion detector, the positive direction is away from the detector.

Figure for Question AP Physics C: Mechanics 2.7 B Describe Static Friction Between Two Surfaces Questions question 1 — AP Physics C: Mechanics

For correctly evaluating Newton's second law equations for the system at rest
$m_{2} g-f=\left(m_{1}+m_{2}\right) a=0 \therefore m_{2} g=f$

1 point

For correctly substituting for static friction into above equation:
$\begin{aligned} m_{2} g=f=\mu_{s} F_{N}=\mu_{s} m_{1} g \mu_{s}=\frac{m_{2}}{m_{1}}=\frac{(0.20 \mathrm{~kg})}{(0.44 \mathrm{~kg})}=0.45 \end{aligned}$

1 point

Total for part (a)

2 points

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AP Physics C: Mechanics 2.7 B Describe Static Friction Between Two Surfaces Questions question 1