Thrust, Pressure, and Buoyancy – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Differentiate between thrust and pressure with suitable examples. Explain how area affects pressure.
Answer:
- Thrust is a force acting on a surface in a particular direction.
- Pressure is the thrust per unit area. Its unit is Pascal (Pa).
- Formula: Pressure = Force / Area.
- A thumbtack point has small area, so it creates high pressure and goes into the wall.
- A broad chair leg has large area, so it creates low pressure and does not sink into the ground easily.
- Thus, for the same thrust, a smaller area means greater pressure, and a larger area means less pressure.
Q2. Why is it painful to carry a heavy school bag with a thin strap? Suggest a practical solution.
Answer:
- A thin strap touches the shoulder over a small area.
- The bag’s weight acts as a thrust on the shoulder.
- Due to the small area, the pressure becomes high.
- High pressure causes pain and can leave a mark on the skin.
- Use a wide and padded strap. It increases the area of contact.
- A larger area reduces pressure, so the bag feels more comfortable.
Q3. Explain pressure in fluids. Why does pressure increase with depth?
Answer:
- A fluid (liquid or gas) exerts pressure on the walls of its container and on objects inside it.
- Fluid pressure acts in all directions.
- Deeper in a fluid, there is more fluid above that point.
- More fluid above means more weight, so pressure is greater at greater depth.
- This is why dams are made thicker at the bottom.
- Divers feel more ear pressure as they go deeper, showing pressure increases with depth.
Q4. What is buoyancy? State Archimedes’ Principle and connect it to floating and sinking.
Answer:
- Buoyancy is the upward force a fluid exerts on an object placed in it.
- Archimedes’ Principle: The buoyant force equals the weight of the fluid displaced by the object.
- If the buoyant force is greater than the object’s weight, the object floats.
- If the buoyant force is less, the object sinks.
- The buoyant force depends on the volume displaced and the density of the fluid.
- So, in denser fluids (like saltwater), the buoyant force is higher for the same object.
Q5. An iron nail sinks but an iron ship floats. Explain using density and buoyancy ideas.
Answer:
- An iron nail is small and solid, so its overall density is more than water. It displaces little water, so buoyant force is small.
- An iron ship has a hollow shape. It encloses air and displaces a large volume of water.
- The buoyant force becomes large because of the large displaced volume.
- The ship’s overall density (iron + air) becomes less than water.
- So the upward buoyant force can balance the ship’s weight.
- Hence, the ship floats, while the nail sinks.
High Complexity (Analysis & Scenario-Based)
Q6. A sealed empty plastic bottle floats in water. Predict what happens if you fill it with dry sand and explain why.
Answer:
- The empty bottle has air inside, so its overall density is less than water. It floats.
- When you add dry sand, the mass increases a lot, but the volume hardly changes.
- The overall density can become greater than water.
- If density becomes greater than water, the weight exceeds the buoyant force, so it sinks.
- If you add only a little sand, it may float but sit lower in water, displacing more water.
- Thus, floating or sinking depends on the balance between weight and buoyant force.
Q7. A wooden block floats higher in seawater than in freshwater. Explain this observation using buoyant force and density.
Answer:
- Seawater is denser than freshwater because it has salt dissolved in it.
- For the same volume displaced, seawater provides a greater buoyant force.
- To balance the block’s weight, it needs to displace less seawater than freshwater.
- So, in seawater, the block is less submerged and floats higher.
- In freshwater, it must displace more water, so it goes deeper.
- This shows buoyant force depends on the density of the fluid.
Q8. A diver goes deeper in a lake. Describe changes in pressure on the diver and explain any effect on buoyancy.
Answer:
- As depth increases, the pressure of water on the diver increases.
- This is because there is more water above, adding more weight.
- The diver may feel ear pain and needs to equalize pressure.
- The buoyant force mainly depends on the volume displaced and water density.
- In water, density changes very little with depth, so buoyancy is almost the same.
- But if gear or suit compresses slightly, the diver’s volume reduces a little, so buoyant force can decrease slightly.
Q9. You must move a heavy person across soft mud without sinking. Propose a method using pressure concepts and explain.
Answer:
- Use a wide board or plank under the feet to increase area.
- The person’s weight (thrust) is the same in both cases.
- With a larger area, pressure on the mud becomes less.
- Low pressure stops the feet from sinking deeply.
- This is the idea behind snowshoes and wide tires.
- It applies the formula Pressure = Force / Area in a practical way.
Q10. A stone is weighed in air and then in water using a spring balance. The reading in water is less. Explain fully using Archimedes’ Principle.
Answer:
- In air, the balance shows the stone’s true weight.
- In water, the stone experiences an upward buoyant force.
- This force equals the weight of water displaced by the stone.
- The balance shows an apparent weight that is less than in air.
- The loss in weight equals the buoyant force.
- This is a direct illustration of Archimedes’ Principle and buoyancy.
Q11. A rectangular metal block (fixed mass) is pressed against clay with its face A, then with its face B (A > B). It sinks deeper with face B. Explain.
Answer:
- The thrust on the clay is the block’s weight in both cases.
- With face A (larger area), the pressure is lower, so it sinks less.
- With face B (smaller area), the pressure is higher, so it sinks more.
- This shows how area affects pressure for the same force.
- It is the same reason knife edges are thin to cut easily.
- It applies the rule: Pressure = Force / Area.
Q12. An aluminum cube floats in mercury but sinks in water. Analyze using density and buoyant force.
Answer:
- Mercury is much denser than water.
- For the cube, the displaced mercury weighs more than the displaced water for the same volume.
- In mercury, the buoyant force can be greater than the cube’s weight, so it floats.
- In water, the buoyant force is less than its weight, so it sinks.
- Thus, floating depends on the density of the fluid, not just the object.
- This directly uses Archimedes’ Principle.
Note: Practice with simple experiments, like pushing objects under water, to feel the change in pressure and buoyancy. Hands-on learning makes concepts stick!
