DAA - Defence Aptitude Assessment Mechanical Comprehension Principles Questions and Answers — Questions and Answers
Question 1: In the gear system shown, Gear A has 10 teeth and turns clockwise. Gear B has 20 teeth, and Gear C has 5 teeth. Which of the following statements is true?
- Gear C will turn clockwise and faster than Gear A. (Correct answer)
- Gear C will turn anti-clockwise and slower than Gear A.
- Gear C will turn clockwise and at the same speed as Gear A.
- Gear C will turn anti-clockwise and faster than Gear A.
Correct answer: Gear C will turn clockwise and faster than Gear A.
When two gears mesh, they turn in opposite directions. Since Gear A turns clockwise, Gear B will turn anti-clockwise. Since Gear B turns anti-clockwise, Gear C will turn clockwise. Regarding speed, a smaller gear will always turn faster than a larger gear it is meshed with. Gear C has the fewest teeth (5), so it will rotate the fastest. Gear A has 10 teeth, so Gear C will turn faster than Gear A.
Question 2: A person is using a crowbar to lift a heavy crate. To get the maximum mechanical advantage and make the lift as easy as possible, where should the fulcrum be placed?
- Exactly in the middle of the crowbar.
- As close as possible to the person's hands.
- As close as possible to the crate. (Correct answer)
- It does not matter where the fulcrum is placed.
Correct answer: As close as possible to the crate.
A lever provides the greatest mechanical advantage when the distance from the effort (the person's hands) to the fulcrum is maximized, and the distance from the fulcrum to the load (the crate) is minimized. By placing the fulcrum as close as possible to the crate, the person increases the length of the effort arm, requiring less force to lift the heavy load.
Question 3: Four pulley systems are lifting an identical weight. System A has one fixed pulley. System B has one fixed and one movable pulley. System C has two fixed and two movable pulleys. System D has three fixed pulleys. Assuming the pulleys and ropes are weightless and frictionless, which system requires the least amount of effort to lift the weight?
- System A
- System B
- System C (Correct answer)
- System D
Correct answer: System C
The mechanical advantage of a pulley system is determined by the number of rope segments supporting the load. System A (one fixed pulley) has a mechanical advantage of 1. System B (one fixed, one movable) has 2 supporting ropes, giving a mechanical advantage of 2. System C (two fixed, two movable) has 4 supporting ropes, giving a mechanical advantage of 4. System D (three fixed pulleys) only changes the direction of the force and has a mechanical advantage of 1. System C requires the least effort because it has the highest mechanical advantage.
Question 4: The image shows four interconnected containers of different shapes. If water is poured into the container on the far left, what will be the final water level in all four containers when the water comes to rest?
- The water level will be highest in the widest container.
- The water level will be the same in all four containers. (Correct answer)
- The water level will be highest in the narrowest container.
- The water level will be highest in the container on the far right.
Correct answer: The water level will be the same in all four containers.
This demonstrates the principle of communicating vessels. In a set of interconnected containers, a liquid at rest will balance out to the same level in all containers, regardless of their shape or volume. This is because the pressure exerted by a fluid at a given depth is constant, and the liquid will flow until the pressure is equalized across the bottom of all containers.
Question 5: Which of the following designs for a vehicle would be the most stable and least likely to tip over while cornering?
- A high center of gravity and a wide base.
- A high center of gravity and a narrow base.
- A low center of gravity and a narrow base.
- A low center of gravity and a wide base. (Correct answer)
Correct answer: A low center of gravity and a wide base.
An object's stability is increased by having a low center of gravity and a wide base of support. A low center of gravity means the object is less likely to have its center of gravity move outside its base of support when tilted. A wide base provides a larger area for the center of gravity to remain within, preventing tipping. Racing cars, for example, are built very low and wide for this reason.
Question 6: Two pendulums are set to swing. Pendulum A has a string length of 1 metre and a mass of 10kg. Pendulum B has a string length of 2 metres and a mass of 5kg. Which pendulum will have the shorter period (i.e., swing back and forth more quickly)?
- Pendulum B, because it is lighter.
- They will have the same period.
- Pendulum A, because it is shorter. (Correct answer)
- Pendulum A, because it is heavier.
Correct answer: Pendulum A, because it is shorter.
The period of a simple pendulum is determined primarily by its length and the acceleration due to gravity, not its mass. A shorter pendulum will have a shorter period, meaning it will swing back and forth more quickly than a longer pendulum. The mass of the bob has no significant effect on the period.
DAA - Defence Aptitude Assessment Mechanical Comprehension Principles Questions and Answers
In the gear system shown, Gear A has 10 teeth and turns clockwise.
Gear B has 20 teeth, and Gear C has 5 teeth.
Which of the following statements is true?