OAR Mechanical Comprehension: Fluids Questions and Answers — Questions and Answers
Question 1: A hydraulic lift uses an enclosed fluid to lift a heavy object. If a force of 100 N is applied to a small piston with an area of 5 cm², what is the maximum weight that can be lifted by a larger piston with an area of 100 cm²?
- 500 N
- 1,000 N
- 2,000 N (Correct answer)
- 10,000 N
Correct answer: 2,000 N
According to Pascal's Principle, pressure applied to an enclosed fluid is transmitted undiminished throughout the fluid. Pressure (P) is Force (F) divided by Area (A). Therefore, P1 = P2, which means F1/A1 = F2/A2. Plugging in the values: 100 N / 5 cm² = F2 / 100 cm². Solving for F2 gives (100 N * 100 cm²) / 5 cm² = 2,000 N.
Question 2: A cargo ship is fully loaded in a freshwater port and then sails out into the saltwater ocean. Because saltwater is denser than freshwater, how will the ship's position in the water change, assuming no cargo is added or removed?
- The ship will ride higher in the water. (Correct answer)
- The ship will sink lower in the water.
- The ship's position will not change.
- The ship will become unstable and list to one side.
Correct answer: The ship will ride higher in the water.
Archimedes' principle states that a floating object displaces a weight of fluid equal to its own weight. Since the ship's weight is constant, it must displace the same weight of water in both freshwater and saltwater. Saltwater is denser than freshwater, meaning a smaller volume of saltwater is needed to equal the ship's weight. Therefore, the ship will displace less water by volume and will float higher.
Question 3: Water flows through a horizontal pipe that narrows at a certain point. According to Bernoulli's principle, what happens to the speed and pressure of the water as it passes through this constricted section?
- The speed increases and the pressure increases.
- The speed decreases and the pressure increases.
- The speed increases and the pressure decreases. (Correct answer)
- The speed decreases and the pressure decreases.
Correct answer: The speed increases and the pressure decreases.
Bernoulli's principle states that for a fluid in motion, an increase in speed occurs simultaneously with a decrease in pressure. As the pipe narrows, the principle of continuity dictates that the fluid must speed up to maintain the same volume flow rate. This increase in kinetic energy (speed) comes at the expense of pressure energy, so the pressure in the constricted section decreases.
Question 4: Three containers of different shapes are all filled with water to the same height. Which of the following statements about the pressure at the bottom of each container is true?
- The container with the largest volume has the greatest pressure at the bottom.
- The pressure at the bottom of all three containers is the same. (Correct answer)
- The container with the smallest bottom surface area has the greatest pressure.
- The widest container has the greatest pressure at the bottom.
Correct answer: The pressure at the bottom of all three containers is the same.
The pressure exerted by a column of fluid depends only on the height of the fluid, its density, and the acceleration due to gravity (P = ρgh). Since the fluid (water) and the height are the same for all three containers, the pressure at the bottom is identical, regardless of the container's shape or total volume.
Question 5: Which of the following best defines the property of a fluid known as viscosity?
- The force exerted by the fluid per unit of surface area.
- The ratio of an object's density to the fluid's density.
- The weight of the fluid displaced by an object.
- The fluid's internal resistance to flow. (Correct answer)
Correct answer: The fluid's internal resistance to flow.
Viscosity is a measure of a fluid's internal friction or its resistance to flow. A fluid with high viscosity, like honey, flows slowly, while a fluid with low viscosity, like water, flows easily. The other options describe pressure, specific gravity, and buoyant force, respectively.
Question 6: A submarine is neutrally buoyant and suspended motionless at a depth of 100 meters. To begin ascending towards the surface without using its propellers, what action must it take?
- Pump water into its ballast tanks.
- Pump water out of its ballast tanks. (Correct answer)
- Angle its dive planes upwards.
- Increase its internal air pressure.
Correct answer: Pump water out of its ballast tanks.
According to Archimedes' principle, an object is neutrally buoyant when its total weight equals the weight of the water it displaces (the buoyant force). To ascend, the submarine must become positively buoyant, meaning the buoyant force must be greater than its weight. By pumping water out of its ballast tanks, the submarine reduces its overall weight while maintaining the same volume, causing the buoyant force to be greater than its weight and making it rise.
OAR Mechanical Comprehension: Fluids Questions and Answers
A hydraulic lift uses an enclosed fluid to lift a heavy object.
If a force of 100 N is applied to a small piston with an area of 5 cm², what is the maximum weight that can be lifted by a larger piston with an area of 100 cm²?