Grade 11 Physical Sciences · Mechanics

Calculate and explain frictional force

Calculate friction using f = μN with the correct normal force, on flat ground or a slope (μ = 0,3 and N = 100 N gives 30 N).

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Grade 11 Newton Laws: FrictionKevinmathscience

Grade 11 Newton Laws: Friction on a slopeKevinmathscience

Grade 11 and 12 Forces Friction: Static and kinetic frictionMiss Martins Maths and Science

Newton's Laws Practice Question Gr 11 and 12 Physics: Finding the coefficient of frictionMiss Martins Maths and Science

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Question 2.5 · May/June 2025 · Paper 1 · National (DBE) · 3 marks

A crate of mass

25

kg slides down a rough inclined plane at a constant speed of

2

m·s

^{- 1}

towards point A while force F acts on it, parallel to the incline, as shown below. A constant kinetic frictional force of

40

N acts on the crate when the plane is inclined at an angle of

30°

with the horizontal. Diagram for this question

2.5 The coefficient of kinetic friction between the crate and the inclined plane is

0, 19

. Using the identity

\frac{s i n θ}{c o s θ} = tan θ

, calculate the frictional force as the crate slides from point A to point B.(3)

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More past paper questions: calculate and explain frictional force

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3. Question 2.3.1 · May/June 2024 · Paper 1 · National (DBE) · 3 marks
Block A of mass $4, 1$ kg is connected to block B of mass $2, 3$ kg by a light inextensible string passing over a frictionless pulley. Block A is at rest on a rough horizontal table and block B hangs vertically, as shown in the diagram below.

A force F of magnitude $49$ N is applied on block A at an angle of $50°$ to the horizontal, causing block A to accelerate TO THE LEFT from rest along the table.

The coefficient of kinetic friction between the surface of the table and block A is $0, 35$ .
2.3 Calculate the magnitude of the:
2.3.1 Kinetic frictional force exerted on block A(3)
Show the memo (3 ticks)Memo
1. ✓
  Formula $f_{k} = μ_{k} F_{N}$
  
  Any one: $f_{k} = μ_{k} (F_{g} - F_{y})$ or $f_{k} = μ_{k} (m g - F sin θ)$
  1 mark · Calculate and explain frictional force
2. ✓
  Substitution $f_{k} = (0, 35) [(4, 1) (9, 8) - 49 sin 50°]$
  1 mark · Calculate and explain frictional force
3. ✓
  Answer with unit $f_{k} = 0, 93$ N
  1 mark · Calculate and explain frictional force
4. Question 2.4 · November 2024 · Paper 1 · National (DBE) · 2 marks
In an experiment, a crate of mass $8, 5$ kg, lying stationary on a rough horizontal table, is connected to a mass hanger by means of a light inextensible string passing over a frictionless pulley, as shown in the diagram below. Mass pieces are added to the mass hanger and the acceleration of the crate is measured. The experiment is repeated several times by adding different masses to increase the hanging mass each time.

Ignore the effects of air friction.

The results obtained were used to draw the sketch graph below.
2.4 A $5$ kg block is now placed inside the crate and the experiment is repeated. How will this affect the maximum static frictional force now experienced by the crate? Choose from INCREASES, DECREASES or REMAINS THE SAME.

Give a reason for the answer.(2)
Show the memo (2 ticks)Memo
1. ✓
  ChoiceINCREASES
  1 mark · Calculate and explain frictional force
2. ✓
  Reason $f_{s}^{ma x} \propto N$ OR $f_{s}^{ma x} \propto m$ OR $f_{s}^{ma x} = μ_{s} N$ OR the normal force acting on the crate increases OR increase in mass/weight of crate (increases the normal force)
  1 mark · Calculate and explain frictional force
5. Question 2.4.2 · November 2023 · Paper 1 · National (DBE) · 2 marks
Two blocks, $A$ , of mass $4$ kg, and $B$ , of mass $9$ kg, are connected by a light inextensible string. The blocks are held at rest on a plane which is inclined at an angle of $35°$ with the horizontal.

A constant force, $F$ , acting parallel to the plane, is applied to block $B$ , as shown in the diagram below. The blocks now accelerate up the plane at $2$ m·s $^{- 2}$ .
2.4 The angle that the plane makes with the horizontal is now decreased.

Answer the questions that follow.
2.4.2 Explain the answer to QUESTION 2.4.1.(2)
Show the memo (2 ticks)Memo
1. ✓
  Normal force increasesSince $μ_{k}$ and m are constant, as $θ$ decreases, normal force / $w_{⊥}$ / $m g cos θ$ will increase.
  
  OR
  
  $N = m g cos θ$
  1 mark · Calculate and explain frictional force
2. ✓
  Friction proportional to normal forceFrictional force is directly proportional to normal force.
  
  OR
  
  $f \propto N$ / $f = μ_{k} N$
  1 mark · Calculate and explain frictional force
6. Question 1.2 · November 2023 · Paper 1 · National (DBE) · 2 marks
Various options are provided as possible answers to the following questions. Each question has only ONE correct answer.
1.2 A crate of mass m is stationary on a plane inclined at an angle $θ$ with the horizontal, as shown below.

Which ONE of the following statements regarding the magnitude of the frictional force acting on the crate is CORRECT?

The magnitude of the frictional force acting on the crate is ...(2)
1. A equal to the component of the weight of the crate which is parallel to the plane.
2. B larger than the component of the weight of the crate which is parallel to the plane.
3. C equal to the component of the weight of the crate which is perpendicular to the plane.
4. D larger than the component of the weight of the crate which is perpendicular to the plane.
Show the memo (1 tick)Memo
1. ✓
  Correct optionThe crate is at rest, so the net force along the plane is zero. The only forces along the plane are static friction (up the plane) and the parallel component of the weight, $m g sin θ$ (down the plane). So $f_{s} = m g sin θ$ . Friction cannot be larger (B), because then the crate would accelerate up the plane.
  
  Answer: A
  2 marks · Calculate and explain frictional force

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