Kinetic and Static Friction AP Physics C Mechanics FRQ – Exam Style Questions etc.
Kinetic and Static Friction AP Physics C Mechanics FRQ
Unit 2: Force and Translational Dynamics
Weightage : 20-15%
Question
A 300-kg box rests on a platform attached to a forklift, shown above. Starting from rest at time = 0, the box is lowered with a downward acceleration of 1.5 m/s2
a. Determine the upward force exerted by the horizontal platform on the box as it is lowered.
At time t = 0, the forklift also begins to move forward with an acceleration of 2 m/s2 while lowering the box as described above. The box does not slip or tip over.
b. Determine the frictional force on the box.
c. Given that the box does not slip, determine the minimum possible coefficient of friction between the box and the platform.
d. Determine an equation for the path of the box that expresses y as a function of x (and not of t), assuming that, at time t = 0, the box has a horizontal position x = 0 and a vertical position y = 2 m above the ground, with zero velocity.
e. On the axes below sketch the path taken by the box
Answer/Explanation
Ans:
a. ΣF = ma; using downward as the positive direction, mg – N = may gives N = m(g – ay) = 2490 N
b. Friction is the only horizontal force exerted; ΣF = f = max = 600 N
c. At the minimum coefficient of friction, static friction will be at its maximum value f = µN, giving µ = f/N = (600 N)/(2490 N) = 0.24
d. y = y0 + v0yt + ½ ayt2 = 2 m + ½ (–1.5 m/s2)t2 and x = x0 + v0xt + ½ axt2 = ½ (2 m/s2)t2, solving for t2 in the x equation gives t2 = x. Substituting into the y equation gives y as a function of x: y = 2 – 0.75x
e. 