Friction and air resistance：練習問題

Which of the following best describes the effect of friction between two surfaces in contact?

In which of the following everyday activities is friction not considered useful?

A wooden block is placed on a flat board. One end of the board is slowly lifted to create an incline. As the angle of the board increases, the block remains stationary until the angle reaches \(25^\circ\). At an angle of \(15^\circ\), which of the following statements about the forces acting on the block is true?

A box is being pushed across a floor to the left with a force of \( 10 \text{ N} \). Which of the following statements correctly describes the frictional force acting on the box from the floor?

The design of a parachute allows a person to fall slowly and land safely. Why is a parachute with a larger surface area more effective at slowing down a falling object than a smaller one?

A block is pushed across a wooden surface and then across an air track with the same force. Explain why the block on the air track travels a much greater distance, referring to the nature of the contact surfaces.

A wooden box of mass \(M = 50 \text{ kg}\) rests on a rough floor. The maximum static friction force is \(300 \text{ N}\). A person applies a horizontal force of \(200 \text{ N}\) to the box. Calculate the magnitude of the actual frictional force acting on the box and determine the resulting acceleration.

Why do engineers design high-speed vehicles, such as racing cars or high-speed trains, with a streamlined shape, and which specific force is primarily reduced by this design feature?

a) Define friction and air resistance, explaining how each force acts to oppose the motion of an object.

b) Provide one example from everyday life where friction is beneficial and explain how it is increased or utilized. Provide another example where friction is undesirable and explain one method used to reduce it.

c) Explain how the streamlined design of a racing car helps to reduce the effect of air resistance during a race.

Resistive forces like friction and air resistance are major considerations in the design of modern transportation systems. Suppose a group of students is investigating these forces in a laboratory setting.

a) In the first experiment, a block is pushed across a flat wooden surface. In the second experiment, the same block is placed on an air track where a thin layer of compressed air is blown through small holes. Explain why the block on the air track travels a much greater distance than on the wooden surface, referring specifically to the nature of contact surfaces. (2 points)

b) In a wind tunnel study, the students measured the air resistance acting on three different 3D shapes (A, B, and C) of the same frontal area moving at the same speed. The results are recorded below:

Shape A (Cube): \(15.2\text{ N}\)
Shape B (Sphere): \(7.4\text{ N}\)
Shape C (Teardrop/Streamlined): \(0.9\text{ N}\)

Identify which shape is the most streamlined and explain how this specific shape helps reduce air resistance. (2 points)

c) Streamlining is a critical design feature for high-speed bullet trains but is rarely used for heavy-duty waste collection trucks. Using the relationship between speed and air resistance, explain why streamlining is prioritized for the train. (2 points)

d) While engineers often try to reduce friction to save energy, identify one specific component of a vehicle where high friction is essential for the safety of the passengers. (1 point)