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GCSE Questions: Pressure, buoyancy, upthrust and fluids

Q1. The diagram shows four blocks of different materials floating on water. The four blocks are the same volume.

(a) Which of the blocks has the smallest weight? C

[1 mark]

(b) The diagram shows a lifebuoy next to a deep swimming pool.

(i) The lifebuoy has a mass of 2.5 kg. Given that the gravitational field strength = 9.8 N/kg, calculate the weight of the lifebuoy.

weight = mass x gravitational field strength

w = mg

w = 2.5 x 9.8

w = 24.5 N

[3 marks]

(ii) When thrown into the water the lifebuoy floats. The two forces acting on the lifebuoy are the weight of the lifebuoy downwards and upthrust upwards.

How big is the upthrust on the lifebuoy compared to the weight of the lifebuoy?

They are equal in size/magnitude (and opposite in direction)

[1 mark]

(c)

(i) Write down the equation which links acceleration, mass and resultant force.

F = ma

[1 mark]

(ii) A rope is used to pull the lifebuoy to the side of the swimming pool. A resultant force of 4.0 N acts on the lifebuoy. The mass of the lifebuoy is 2.5 kg.

Calculate the acceleration of the lifebuoy.

a = F/m

a = 4.0/2.5

a = 1.6 m/s2

[3 marks]

[9 Marks TOTAL]