SIFT Mechanical Comprehension 2 — Questions and Answers

Question 1: What type of simple machine is a bottle opener?

Inclined plane
Wheel and axle
Lever (Correct answer)
Pulley

Correct answer: Lever

A bottle opener is a first-class lever: the fulcrum is between the effort and resistance. The cap edge is the fulcrum, the handle is the effort arm, and the bottle cap is the resistance.

Levers have three classes based on the positions of fulcrum (F), effort (E), and resistance (R). Class 1 (F between E and R): bottle opener, seesaw, scissors. Class 2 (R between E and F): wheelbarrow, nutcracker. Class 3 (E between F and R): tweezers, human forearm. The bottle opener's fulcrum is the lip of the bottle.

Question 2: Two blocks, A (10 kg) and B (5 kg), rest on a frictionless surface connected by a string. If a 30 N force is applied to block A, what is the tension in the string?

5 N
10 N (Correct answer)
15 N
30 N

Correct answer: 10 N

Total mass = 15 kg. Acceleration a = F/m_total = 30/15 = 2 m/s². Tension = m_B × a = 5 × 2 = 10 N.

The system accelerates together: a = F/(m_A + m_B) = 30/15 = 2 m/s². The string only needs to accelerate block B: T = m_B × a = 5 × 2 = 10 N. This two-body problem tests Newton's Second Law applied to connected objects, a common mechanics question type.

Question 3: Which material has the highest thermal conductivity (transmits heat best)?

Wood
Glass
Rubber
Copper (Correct answer)

Correct answer: Copper

Copper is an excellent thermal conductor, which is why it is used in heat sinks, cooking pans, and electrical wiring. Wood, glass, and rubber are much poorer conductors (good insulators).

Thermal conductivity (k) measures how well a material transmits heat. Metals like copper (k ≈ 385 W/m·K) and aluminum (k ≈ 205 W/m·K) far exceed non-metals. Wood (k ≈ 0.1-0.5), glass (k ≈ 1), and rubber (k ≈ 0.15) are insulators. This is why metal objects feel cold and wooden ones feel warm at the same room temperature.

Question 4: What is 'mechanical advantage' of a single fixed pulley?

0
1 (Correct answer)
2
4

Correct answer: 1

A single fixed pulley only changes the direction of force, not its magnitude. Its mechanical advantage is 1 — you pull with the same force as the load weight.

Mechanical advantage (MA) = Load force / Effort force. For a single fixed pulley, you must pull with the full weight of the load. MA = 1. The advantage is convenience — you can pull downward (with gravity assisting) instead of lifting upward. Movable pulleys and block-and-tackle systems provide MA > 1.

Question 5: If both ends of a horizontal pipe carry equal and opposite compressive forces, the pipe is under what type of stress?

Tension
Compression (Correct answer)
Shear
Torsion

Correct answer: Compression

When forces push inward from both ends, the pipe is compressed — it is under compressive stress, which tends to shorten or buckle the material.

Compression (forces pushing toward each other) tends to shorten and potentially buckle a material. Tension (forces pulling apart) tends to elongate it. Shear involves forces in opposite directions along parallel planes. Torsion is twisting stress. Aircraft structures must withstand all four types; design accounts for which materials resist which stresses best.

Question 6: Water is flowing through a pipe. If the pipe splits into two equal branches, what happens to the flow speed in each branch (assuming incompressible flow)?

Flow speed doubles in each branch
Flow speed stays the same
Flow speed halves in each branch (Correct answer)
Flow speed triples in each branch

Correct answer: Flow speed halves in each branch

By the continuity equation (A₁V₁ = A₂V₂ + A₃V₃), when a pipe splits into two equal pipes, the total flow area doubles, so flow speed must halve to maintain the same volumetric flow rate.

Continuity requires: inflow rate = outflow rate. If the pipe splits into two identical branches, each branch carries half the flow. With half the flow in the same pipe diameter, the velocity halves. If the branch pipes were smaller, speed might stay the same or increase — area and flow rate together determine velocity.

SIFT Mechanical Comprehension 2

What type of simple machine is a bottle opener?