AFCT Mechanical Comprehension 2 — Questions and Answers
Question 1: A lever has a fulcrum placed 2 feet from a 100-pound load and 8 feet from the effort. What effort force is needed to lift the load?
- 50 pounds
- 25 pounds (Correct answer)
- 40 pounds
- 12.5 pounds
Correct answer: 25 pounds
Using the lever principle: Load x load arm = Effort x effort arm. So 100 x 2 = Effort x 8, giving Effort = 25 pounds.
The lever principle states that for equilibrium: Load x load arm = Effort x effort arm. Substituting: 100 lbs x 2 ft = Effort x 8 ft. Solving: 200 = 8 x Effort, so Effort = 25 pounds. The mechanical advantage is 8/2 = 4.
Question 2: Which simple machine is a screw most closely related to?
- Lever
- Inclined plane (Correct answer)
- Pulley
- Wheel and axle
Correct answer: Inclined plane
A screw is essentially an inclined plane wrapped around a cylinder, converting rotational force into linear motion.
A screw is an inclined plane wrapped helically around a central shaft. If you unwrap the threads, they form a ramp. The pitch (distance between threads) corresponds to the slope of the inclined plane. A screw converts rotational force into linear force, providing significant mechanical advantage.
Question 3: If two gears are meshed together, what can be said about their rotation?
- They rotate in the same direction
- They rotate in opposite directions (Correct answer)
- The larger gear always rotates faster
- Their speeds are always equal
Correct answer: They rotate in opposite directions
When two gears mesh directly, they always rotate in opposite directions because the teeth of one push the teeth of the other.
When two gears are meshed, they rotate in opposite directions. The teeth of the driving gear push against the driven gear. To make two gears rotate in the same direction, an intermediate idler gear is needed. The speed ratio depends on the number of teeth.
Question 4: What happens to water pressure as depth increases?
- It decreases
- It increases (Correct answer)
- It stays the same
- It fluctuates randomly
Correct answer: It increases
Water pressure increases with depth because the weight of the water above pushes down, adding to the pressure at lower levels.
Water pressure increases linearly with depth at about 0.433 psi per foot. This occurs because water at any depth must support the weight of all water above it. The formula is: P = density x gravity x height. This is why dams are thicker at the bottom.
Question 5: A fixed pulley is primarily used to:
- Multiply force
- Change the direction of force (Correct answer)
- Increase speed
- Reduce friction
Correct answer: Change the direction of force
A single fixed pulley changes the direction of the applied force but does not provide a mechanical advantage.
A single fixed pulley redirects the pulling force: you pull down on the rope and the load goes up. However, it provides no mechanical advantage: effort equals load. Its benefit is ergonomic convenience. Compound pulley systems provide actual mechanical advantage.
Question 6: What principle explains why a hydraulic jack can lift a heavy vehicle with a small force?
- Newton's Third Law
- Pascal's Law (Correct answer)
- Bernoulli's Principle
- Archimedes' Principle
Correct answer: Pascal's Law
Pascal's Law states that pressure applied to a confined fluid is transmitted equally in all directions, allowing a small force on a small piston to create a large force on a larger piston.
Pascal's Law states that pressure in an enclosed fluid is transmitted undiminished everywhere. In a hydraulic jack, a small piston applies pressure to fluid, which transmits to a larger piston. Since pressure = force/area, the same pressure on a larger area produces greater force.
AFCT Mechanical Comprehension 2
A lever has a fulcrum placed 2 feet from a 100-pound load and 8 feet from the effort.
What effort force is needed to lift the load?