Pressure in liquids and gasesIB MYP Physics: Subtopic test

1. 1
   A hydroelectric dam in Norway holds back a deep reservoir of fresh water. The wall of the dam is built much thicker at the bottom than at the top. Fresh water has a density of 1000 kg/m³. Use g = 10 N/kg.

   (a)

   Which statement explains why the dam wall is thicker at the bottom?

   [1 mark]

   • AWater at the bottom is hotter, so it expands and pushes harder
   • BPressure in a liquid increases with depth, so the force on the wall is greater near the bottom
   • CPressure in a liquid acts only downwards, so the base of the wall takes all the force
   • DWater at the bottom is moving faster than water at the top

   (b)

   The water in the reservoir is 20 m deep. What is the pressure due to the water at the base, using p = hρg?

   [1 mark]

   • A2000 Pa
   • B20 000 Pa
   • C200 000 Pa
   • D2 000 000 Pa

   (c)

   A second dam holds back sea water, of density 1030 kg/m³, to the same depth. State and explain how the pressure at the base of the second dam compares with the first.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A car garage in Nairobi uses a hydraulic lift. A mechanic pushes a small piston of area 0.004 m² with a force of 150 N. The liquid transmits the pressure to a large piston of area 0.20 m² underneath the car.

   (a)

   Why is a liquid, rather than a gas, used in a hydraulic system?

   [1 mark]

   • AA liquid is almost incompressible, so it transmits the pressure straight to the large piston
   • BA liquid is lighter than a gas, so the pistons move more easily
   • CA liquid has a lower pressure than a gas at the same depth
   • DA liquid can be squashed into a smaller volume to store energy

   (b)

   What is the force exerted by the large piston on the car?

   [1 mark]

   • A3 N
   • B150 N
   • C750 N
   • D7500 N

   (c)

   Explain why the large piston can exert a much greater force than the force applied to the small piston.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates how pressure changes with depth in a liquid. She fills a tall plastic bottle with water and makes three small holes of the same size, one above the other, at heights of 5 cm, 15 cm and 25 cm above the base. She covers the holes with tape, keeps the bottle full, removes the tape and measures how far each jet of water lands from the bottle on a flat table.

   (a)

   State a testable hypothesis for this investigation and give a scientific reason for it.

   [3 marks]

   (b)

   Identify the independent variable and the dependent variable, and state two variables that the student must control.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A port authority in Rotterdam is deciding whether to let very large container ships use its harbour. A ship floats because the sea water pushes up on it with an upthrust. A larger ship can carry more cargo, but it sits lower in the water and needs a deeper channel, which means dredging the sea bed.

   (a)

   Explain, using the ideas of weight, upthrust and displaced water, why a steel ship floats although a steel block of the same mass sinks, and what happens as cargo is loaded.

   [6 marks]

   (b)

   Discuss and evaluate whether the port authority should allow very large ships to use the harbour.

   [6 marks]

   Total for question 4: 12 marks

End of questions