These questions revise the lessons in pressure. Use g = 10 N kg⁻¹, water density = 1000 kg m⁻³ and atmospheric pressure = 1.0 × 10⁵ Pa.
Try each question on paper first.
Questions
Question 1
A box of weight 240 N rests on a table. The contact area is 0.060 m². Find the pressure on the table.
Answer
P = F ÷ A = 240 ÷ 0.060 = 4000 Pa.
Question 2
A drawing pin is pushed with a force of 30 N. The tip has an area of 0.10 mm². Find the pressure, then say why a pin goes in easily.
Answer
0.10 mm² = 0.10 × 10⁻⁶ m² = 1.0 × 10⁻⁷ m². P = 30 ÷ (1.0 × 10⁻⁷) = 3.0 × 10⁸ Pa.
The force acts on a tiny area, so the pressure is enormous and the pin pushes into the wood.
Question 3
Find the pressure at the bottom of a swimming pool 2.5 m deep. Then find the total pressure there.
Answer
ρgh = 1000 × 10 × 2.5 = 2.5 × 10⁴ Pa.
Total = 2.5 × 10⁴ + 1.0 × 10⁵ = 1.25 × 10⁵ Pa.
Question 4
A tank holds oil (density 900 kg m⁻³) 0.40 m deep. The base measures 0.50 m × 0.30 m. Find the force of the oil on the base.
Answer
P = 900 × 10 × 0.40 = 3600 Pa. Area = 0.50 × 0.30 = 0.15 m². F = PA = 3600 × 0.15 = 540 N.
Question 5
A hydraulic lift has pistons of area 10 cm² and 400 cm². What force on the small piston holds up a car part weighing 2000 N on the large piston?
Answer
F₁ = F₂ × (A₁ ÷ A₂) = 2000 × (10 ÷ 400) = 50 N.
The pressure is the same in both pistons; only the force changes.
Question 6
A metal cube of volume 1.0 × 10⁻³ m³ is fully under water. Its weight in air is 27 N. Find the buoyant force and the apparent weight.
Answer
Buoyant force = ρVg = 1000 × 1.0 × 10⁻³ × 10 = 10 N.
Apparent weight = 27 − 10 = 17 N.
Question 7
A plastic bottle of mass 0.050 kg floats in water. Find the volume of water it displaces, in cm³.
Answer
Weight = 0.050 × 10 = 0.50 N, so the buoyant force is 0.50 N.
V = 0.50 ÷ (1000 × 10) = 5.0 × 10⁻⁵ m³ = 50 cm³.
Question 8
Explain why two boats floating side by side in a river can be drawn toward each other when the water flows between them quickly.
Answer
The water between the boats moves faster than the water on their outer sides. By Bernoulli’s principle, the pressure between them is lower.
The higher pressure on the outer sides pushes each boat inward, so the net force pushes them toward each other.
If you got these wrong
Questions 1 to 2 need calculating pressure in solids and liquids. Questions 3 and 4 need comparing pressure at different depths.
Question 5 needs applying Pascal’s principle. Questions 6 and 7 need applying Archimedes’ principle, and question 8 needs explaining fluid-flow effects.
To work through your own wrong answers with a teacher, see online one-to-one Physics tuition.