Question 1

A penguin slides on its belly across ice, as shown in the image. If the mass of the penguin is $$m$$ and the coefficient of kinetic friction between the penguin and the ice is $$\mu_k = 0.03$$, what is the magnitude of the frictional force acting on the penguin?

Accepted Answer

$$f_k = 0.03 m g$$

Question 2

A block is at rest on a ramp. The angle of the ramp is slowly increased until the block just begins to slide at $$	heta = 25^\circ. $$What is the coefficient of static friction between the block and the ramp?

Accepted Answer

$$\mu_s = 	an 25^\circ$$

Question 3

A person pushes a box across a horizontal floor at constant velocity. Which of the following statements is correct about the forces acting on the box?

Accepted Answer

The kinetic friction force is equal in magnitude and opposite in direction to the applied force.

Question 4

A block is pulled horizontally with a force $$T on a $$table. The block starts moving when $$T$$ exceeds a certain value. What happens to the frictional force at the instant the block starts moving?

Accepted Answer

The frictional force decreases from its maximum static value to the kinetic value.

Question 5

Which of the following best describes the difference between static and kinetic friction?

Accepted Answer

Static friction prevents motion up to a maximum value, while kinetic friction acts when there is relative motion and is constant.

Question 6

A racing cyclist and a running person are moving at the same speed through air. According to the drag coefficients table, which experiences a greater drag force, assuming equal cross-sectional area?

Accepted Answer

The running person, because $$C_D is $$larger.

Question 7

A sphere of mass $$m$$ and radius $$r$$ falls through air at high Reynolds number. The drag force is given by $$D = \frac{1}{2} C_D \rho A v^2$. What is the terminal speed v_t$ of the sphere?

Accepted Answer

$$v_t = \sqrt{\frac{2mg}{C_D \rho \pi r^2$}}$$

Question 8

A block is at rest on four different ramps with different coefficients of static friction but the same angle. How do the magnitudes of the static friction forces compare?

Accepted Answer

They are all equal.

Question 9

A skydiver reaches terminal velocity during free fall. Which of the following is true?

Accepted Answer

The drag force equals the gravitational force.

Question 10

A sphere (Sphere 1) has twice the radius and eight times the mass of another sphere (Sphere 2). Both fall at high Reynolds number. What is the ratio of their terminal speeds $$v_1$$/$$v_2$$?

Accepted Answer

$$v_1 = 2$$ $$v_2$$

Question 11

Which of the following best describes the Reynolds number $$Re$$ for an object moving through a fluid?

Accepted Answer

$$Re = \frac{\rho v L}{\eta}$$, where $$\rho is $$fluid density, $$v is $$speed, $$L is $$characteristic length, and $$\eta is $$viscosity.

Question 12

A block is pulled across a table at three different speeds. How does the magnitude of the kinetic friction force compare at each speed?

Accepted Answer

It is the same at all speeds.

Question 13

Which of the following materials has the lowest coefficient of kinetic friction according to the table provided?

Accepted Answer

Ice on ice

Question 14

A synovial joint in the human body has a very low coefficient of friction due to which of the following?

Accepted Answer

The presence of articular cartilage and synovial fluid.

Question 15

At low Reynolds number, the drag force on a sphere is given by which equation?

Accepted Answer

$$D = 6 \pi \eta r v$$

Question 16

A block is at rest on a horizontal surface. Which of the following statements is true about the normal force $$n$$ acting on the block?

Accepted Answer

$$n = mg$$

Question 17

A Toyota Prius and a commercial airliner are moving at the same speed and have the same cross-sectional area. Which experiences a greater drag force, assuming the same air density?

Accepted Answer

The Toyota Prius, because its drag coefficient $$C_D is $$larger.

Friction and Drag: Static & Kinetic Friction, Drag Forces, and Terminal Speed

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