SIFT Mechanical Comprehension Questions and Answers — Questions and Answers
Question 1: In the pulley system shown, a 100 lb weight is being lifted. Assuming the system is frictionless, what is the minimum force (F) required to lift the weight?
- 100 lbs
- 50 lbs
- 25 lbs (Correct answer)
- 20 lbs
Correct answer: 25 lbs
To determine the required force, calculate the ideal mechanical advantage (IMA) by counting the number of rope segments directly supporting the load. In this system, there are four ropes supporting the movable pulleys and thus the weight. Therefore, the IMA is 4. The required force is the total weight divided by the IMA (100 lbs / 4 = 25 lbs).
Question 2: A smaller gear with 15 teeth is driving a larger gear with 45 teeth. If the smaller gear completes 3 full rotations, how many rotations will the larger gear complete?
- 3 rotations
- 9 rotations
- 1 rotation (Correct answer)
- 1.5 rotations
Correct answer: 1 rotation
The relationship between two meshed gears is determined by their gear ratio, which is the ratio of the number of teeth on each gear. The gear ratio here is 45:15, which simplifies to 3:1. This means for every 3 rotations of the smaller (drive) gear, the larger (driven) gear will complete 1 rotation.
Question 3: Which of the following scenarios describes a second-class lever?
- A pair of scissors cutting paper.
- A person using a wheelbarrow to move soil. (Correct answer)
- A person using a fishing rod to lift a fish.
- A seesaw with two children on it.
Correct answer: A person using a wheelbarrow to move soil.
A second-class lever is a simple machine where the load (resistance) is located between the fulcrum (pivot point) and the effort (force). In a wheelbarrow, the wheel acts as the fulcrum, the soil in the basin is the load, and the person lifting the handles provides the effort.
Question 4: According to Bernoulli's principle, if the speed of a fluid moving through a pipe increases, what happens to the pressure within the fluid?
- The pressure increases.
- The pressure remains constant.
- The pressure is unrelated to the speed.
- The pressure decreases. (Correct answer)
Correct answer: The pressure decreases.
Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. In a horizontal pipe, potential energy is constant, so as speed increases, pressure must decrease.
Question 5: A hydraulic lift uses two pistons. The input piston has an area of 2 square inches, and the output piston has an area of 20 square inches. If a force of 50 lbs is applied to the input piston, what is the maximum weight the output piston can lift?
- 5 lbs
- 500 lbs (Correct answer)
- 100 lbs
- 200 lbs
Correct answer: 500 lbs
This problem is an application of Pascal's principle, which is the basis for hydraulics. The pressure (Force/Area) is transmitted equally throughout the fluid. Therefore, F1/A1 = F2/A2. Plugging in the values: 50 lbs / 2 in² = F2 / 20 in². Solving for F2 (the output force) gives F2 = (50 * 20) / 2 = 500 lbs. The mechanical advantage is the ratio of the areas (20/2 = 10), so the output force is 10 times the input force.
Question 6: If Gear A rotates clockwise, what will be the direction of rotation for Gear C?
- Gear C will not rotate.
- Clockwise (Correct answer)
- Counter-clockwise
- The direction cannot be determined.
Correct answer: Clockwise
When two gears mesh directly, they rotate in opposite directions. Gear A rotates clockwise, so Gear B (the idler gear) will rotate counter-clockwise. Since Gear B rotates counter-clockwise, it will cause Gear C to rotate in the opposite direction, which is clockwise.
SIFT Mechanical Comprehension Questions and Answers
In the pulley system shown, a 100 lb weight is being lifted.
Assuming the system is frictionless, what is the minimum force (F) required to lift the weight?