Exam 2 Part B Review

Boxes A and B in figure both remain at rest. Is the friction force on A larger than, smaller than, or equal to the friction force on B? Explain

They are equal because both boxes are being pulled by a 30N force to the right. Since at rest the net force is equal to zero, so static friction force is exactly 30 N but opposite in direction the force pulling each box.

The boxes in the given figure are now sliding; both have uk = 0.35. Is the friction force on A larger than, smaller than, or equal to the friction force on B?

Friction force on A is larger than friction force on B. Although they have same coefficient of kinetic friction, block A has greater mass, so block A has greater normal, and thus greater friction force than box B.

Explain for pulley (a), angular velocity is negative (cw) and angular acceleration is positive

Pulley (a) spins clockwise, so a negative angular velocity; however, the heavier left block that is going up slow it down, so angular acceleration is positive

Point B on a rotating wheel is twice as far from the axle as point A. Is wB equal to 1/2wA, wA, or 2wA? Explain

The angular velocities of point A and point B are equal because, even though point B has twice the radius of point A, both points still turn through the exact same angle in the same amount of time.

Point B on a rotating wheel is twice as far from the axle as point A. Is vB equal to 1/2vA, vA, or 2vA? Explain

vB = 2vA because, according to the equation v = wr, point B has twice the radius of point A (rB = 2rA) while sharing the exact same angular velocity.

Torque ranked: 3>2>1=4

torque = Fl, since all four forces have the exact same magnitude, the torque depends entirely on the length of the lever arm(l). Force 3 creates the largest torque because it is applied perpendicularly at the far edge of the door, giving it the largest lever arm. Force 2 creates less torque because it is applied at an angle, which shrinks its lever arm as the nagle shrinks compared to Force 3. Finally, force 1 and force 4 both create zero torque because their force lines pass directly through the axis of rotation, meaning their level arms are zero.

The moment of inertia of a uniform rod about an axis through its center is 1/12ML². The moment of inertia about an axis at one end is 1/3ML². Explain why the moment of inertia is larger about the end than about the center. Assume same mass M.

The moment of intertia is larger at the end because more mass is located farther away from the axis of rotation. Rotational inertia depends on distance, so spreading the mass farther from the pivot makes the rod much harder to spin.