AS Level · Physics 9702 · Mechanics & materials

Forces, density & pressure: AS Level Physics questions with answers

Free Forces, density & pressure questions for AS Level Physics (9702), part of Mechanics & materials. Each question has the answer and the marking points an examiner would look for.

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Forces, density & pressure practice questions

Question 1Multiple choice · 1 markA* challenge

A mercury barometer reads 0.76 m. What is atmospheric pressure? (ρ of mercury = 13 600 kg m−3, g = 9.81 m s−2)

  1. A 1.01 kPa
  2. B 10.3 kPa
  3. C 76 kPa
  4. D 101 kPa
Show answer

Answer: D

Question 2Multiple choice · 1 mark

Which condition is needed for an object to be in equilibrium?

  1. A constant velocity only
  2. B no resultant force and no resultant moment
  3. C no resultant force only
  4. D no resultant moment only
Show answer

Answer: B

Question 3Multiple choice · 1 mark

Which statement describes centre of gravity?

  1. A Work done per unit positive charge bringing a charge from infinity
  2. B The upward force on an object in a fluid due to the pressure difference
  3. C The resultant displacement is the sum of the individual displacements where waves meet
  4. D The point at which the whole weight of an object may be considered to act
Show answer

Answer: D

Question 4Structured · 5 marksA* challenge

A block floats in water with part of its volume submerged.

(a)

Explain the origin of the upthrust on the block.

[2]
(b)

State Archimedes’ principle.

[1]
(c)

The block has density 750 kg m−3. Calculate the fraction of its volume below the surface of water (density 1000 kg m−3).

[2]
Show answers and mark scheme
PartAnswer and marking pointsMarks
(a)pressure greater at bottom than top
pressure in a fluid increases with depth B1
so pressure on the bottom surface is greater than on the top surface B1
2
(b)upthrust = weight of fluid displaced
upthrust equals the weight of fluid displaced B1
1
(c)0.75
weight = upthrust: ρbV = ρwVsub C1
0.75 A1
2
Question 5Structured · 10 marks

A solid cube of side 0.10 m and density 700 kg m−3 floats in water of density 1000 kg m−3.

(a)

Define density.

[1]
(b)

Calculate the weight of the cube.

[2]
(c)

Explain the origin of the upthrust on the cube.

[2]
(d)

Use your answer to (b) to determine the depth of the cube below the water surface.

[3]
(e)

Calculate the extra downward force needed to push the cube so that it is just completely submerged.

[2]
Show answers and mark scheme
PartAnswer and marking pointsMarks
(a)mass per unit volume
mass per unit volume B1
1
(b)6.87 N
m = 700 × 0.103 C1
W = 6.87 N A1
2
(c)pressure increases with depth; force on bottom face greater than on top
pressure in a fluid increases with depth B1
so the upward force on the lower face is greater than the downward force on the upper face B1
2
(d)0.0700 m
upthrust = weight C1
1000 × 0.102 × d × 9.81 = 6.87 C1
d = 0.0700 m A1
3
(e)2.94 N
full upthrust = 9.81 N C1
extra force = 2.94 N A1
2
Question 6Structured · 5 marks

A uniform beam of weight 40 N and length 1.2 m is hinged at a wall. It is held horizontal by a cable attached to its far end, making 30° with the beam.

(a)

State the principle of moments.

[2]
(b)

Calculate the tension in the cable.

[3]
Show answers and mark scheme
PartAnswer and marking pointsMarks
(a)sum clockwise = sum anticlockwise about any point
for equilibrium, sum of clockwise moments about a point = sum of anticlockwise moments about the same point B2
2
(b)40 N
moments about hinge: T sin 30° × 1.2 = 40 × 0.6 C1
correct substitution C1
40 N A1
3

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