AP Physics C Mechanics 2.7: Static Friction

Analyze static friction as a contact force that adjusts to prevent relative slipping, up to the maximum allowed by the surfaces.

[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 2.7.B—Describe static friction between two surfaces 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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2.7.B—Describe static friction between two surfaces question 1