Force and Pressure ICSE Class 8: Moment of Force, Pressure, Liquid and Air Pressure

What Force and Pressure is about

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Physics: your school sets and marks the exam. Force and Pressure sits in the Theme 3: Force and Pressure theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

Moment of a force

The moment of a force (its turning effect) about a pivot is
$\text{moment}=\text{force}\times \text{perpendicular distance of the line of action of the force from the pivot}$.

SI unit: newton metre (N m).

The moment is larger when:

• the force is larger, or
• the force acts farther from the pivot.

A force whose line of action passes through the pivot has zero moment, because the perpendicular distance is 0. That is why pushing a door at its hinge does not open it.

A moment can be clockwise or anticlockwise.

Worked example

A force of 20 N is applied at right angles to a door at 0.5 m from the hinges. Find its moment.

Try it first, then show the solutionSolution

1. Moment $=F\times d$.
2. $=20\times 0.5=10$ N m.

Answer: 10 N m

Using a longer arm

For the same moment, a force applied farther from the pivot can be smaller: $F=\frac{\text{moment}}{d}$.

Examples:

• door handles are fixed far from the hinges;
• a long spanner loosens a tight nut more easily than a short one;
• the handle of a tap or a steering wheel gives a large turning effect with a small force.

Worked example

A moment of 10 N m is needed to open a heavy door. What force is needed if it is applied 0.8 m from the hinges?

Try it first, then show the solutionSolution

1. $F=\frac{10}{0.8}$.
2. $=12.5$ N.

Answer: 12.5 N

Pressure

Pressure is the force acting normally (at right angles) on unit area of a surface:
$P=\frac{F}{A}$.

SI unit: pascal (Pa), where $1\text{Pa}=1{\text{N/m}}^2$. Another unit is N/cm²: $1{\text{N/cm}}^2=10000\text{Pa}$.

For the same force:

• smaller area → larger pressure (sharp knives, nails, needles, heels);
• larger area → smaller pressure (wide straps on school bags, broad tyres on tractors, wide feet of camels).

Worked example

A person weighing 600 N stands on both feet, with a total area of 0.04 m² touching the ground. Find the pressure. What happens if she stands on one foot (0.02 m²)?

Try it first, then show the solutionSolution

1. $P=\frac{600}{0.04}=15000$ Pa.
2. On one foot: $P=\frac{600}{0.02}=30000$ Pa, double the pressure.

Answer: 15 000 Pa; 30 000 Pa on one foot

Pressure in different units

Change areas carefully:

• $1{\text{m}}^2=10000{\text{cm}}^2$, so $1{\text{cm}}^2=0.0001{\text{m}}^2$.
• A pressure in N/cm² × 10 000 gives the pressure in Pa.

It is safest to change the area into m² before dividing, so that the answer is directly in pascals.

Worked example

A box weighing 50 N rests on a face of area 25 cm². Find the pressure in N/cm² and in Pa.

Try it first, then show the solutionSolution

1. $P=\frac{50}{25}=2{\text{N/cm}}^2$.
2. $25{\text{cm}}^2=0.0025{\text{m}}^2$, so $P=\frac{50}{0.0025}=20000$ Pa.

Answer: 2 N/cm² = 20 000 Pa

Pressure exerted by liquids

A liquid presses on the bottom and the walls of its container. Observations (qualitative):

• liquid pressure increases with depth: water spurts out farthest from the lowest hole in a bottle;
• at the same depth, the pressure is the same in all directions;
• pressure depends on the depth (and on the liquid), not on the shape of the container.

Applications: a dam is made thicker at the base, where the water pressure is greatest; deep-sea divers wear special suits; water from a tap on the ground floor flows with more force than on the top floor.

Worked example

Three holes are made one above another in a tall plastic bottle filled with water. From which hole does water spurt out farthest, and why?

Try it first, then show the solutionSolution

1. The lowest hole is at the greatest depth.
2. Liquid pressure increases with depth, so water is pushed out hardest there.

Answer: From the lowest hole, because pressure increases with depth.

Atmospheric pressure

Air has weight, so the atmosphere (the layer of air around the Earth) presses on everything: this is atmospheric pressure. At sea level it is about $100000$ Pa ($1\times 10^5$ Pa), which means about 100 000 N on every square metre.

We are not crushed because the pressure inside our bodies balances it, and air pushes equally from all sides.

Evidence and uses:

• drinking through a straw: you lower the air pressure inside the straw, and atmospheric pressure pushes the drink up;
• a syringe or dropper fills when the plunger or bulb lowers the pressure inside;
• a can from which the air is removed is crushed by the air outside;
• a rubber sucker sticks to a smooth wall.

Worked example

Taking atmospheric pressure as 100 000 Pa, find the force of the air on the top of a table of area 0.5 m². Why does the table not collapse?

Try it first, then show the solutionSolution

1. $F=P\times A=100000\times 0.5=50000$ N.
2. Air also pushes up on the underside of the table with almost the same force, so the forces balance.

Answer: 50 000 N; it is balanced by air pressure from below

Physics in real life: Force and Pressure

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Moment of a Force (Turning Effect)
• Pressure in Solids, Liquids and the Atmosphere

Force and Pressure formula sheet

Moment of force

$\text{moment}=F\times d$

d = perpendicular distance from the pivot; unit N m.

Pressure

$P=\frac{F}{A}$

Unit Pa = N/m².

Force from pressure

$F=P\times A$

Rearranged.

Unit link

$1{\text{N/cm}}^2=10000\text{Pa}$

1 cm² = 0.0001 m².

Force and Pressure: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Force and Pressure

The question patterns to expect, and the method that works for each.

Tricks to remember Force and Pressure

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Far and forceful

  A bigger turning effect comes from a bigger force or a bigger distance from the pivot.

  Use it when: Moment questions.

• Sharp = small area = big pressure

  Sharpen to push in, broaden to spread out.

  Use it when: Pressure examples.

• Deeper is stronger

  Liquid pressure grows with depth: dams are fat at the feet.

  Use it when: Liquid pressure.

• Air pushes in

  Air always pushes from high pressure to low: lower the pressure inside a straw and the drink rises.

  Use it when: Atmospheric pressure examples.

Force and Pressure: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 3 + 3 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Force and Pressure

The slips examiners see most often, and what to write instead.

• ✗Mistake: Writing the unit of moment as N/m

  Moment is force × distance.

  Wrong working

  Moment $=10$ N/m

  Right working

  Moment $=10$ N m

  ✓Do this: Multiply: newton × metre.

  Usually costs: 1 mark.

• ✗Mistake: Area in cm² with an answer in Pa

  1 Pa = 1 N/m²; using cm² gives N/cm².

  Wrong working

  $\frac{50}{25}=2$ Pa

  Right working

  $2{\text{N/cm}}^2=20000$ Pa

  ✓Do this: Change cm² into m² first.

  Usually costs: 1 mark.

• ✗Mistake: Using a slanting distance for the moment

  Only the perpendicular distance from the pivot to the line of the force counts.

  ✓Do this: Draw the line of the force and measure at right angles to it.

  Usually costs: The answer.

• ✗Mistake: Saying we 'suck up' a drink

  Sucking only lowers the pressure; atmospheric pressure pushes the drink up.

  ✓Do this: Mention atmospheric pressure in the explanation.

  Usually costs: 1 mark.

• ✗Mistake: Thinking liquid pressure acts only downwards

  At any depth it acts equally in all directions, including on the walls.

  ✓Do this: Remember the jets from holes in the side of a bottle.

  Usually costs: 1 mark.

Practice questions on Force and Pressure

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The moment of a force depends on:
   (a) the force only
   (b) the distance only
   (c) the force and its perpendicular distance from the pivot
   (d) the mass of the body

   Hint

   Moment = F × d.

   Answer

   (c)

2. Q2 · 1 mark · Very short answerExam level

   Write the SI unit of moment of force.

   Hint

   Force × distance.

   Answer

   Newton metre (N m).

3. Q3 · 2 marks · Short answerExam level

   A force of 200 N acts on an area of 0.5 m². Find the pressure.

   Hint

   P = F/A.

   Answer

   400 Pa.

4. Q4 · 2 marks · Short answerHarder exam level

   A moment of 6 N m opens a door. Find the force needed at 0.3 m and at 0.1 m from the hinges.

   Hint

   F = moment ÷ d.

   Answer

   20 N at 0.3 m; 60 N at 0.1 m.

5. Q5 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): The pressure of a liquid increases with depth.
   Reason (R): Deeper layers support the weight of more liquid above them.
   (a) (A) is true and (R) is false.
   (b) (A) is false and (R) is true.
   (c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

   Hint

   Weight of liquid above.

   Answer

   (c)

6. Q6 · 2 marks · Short answerHarder exam level

   Why do tractors have broad tyres, and why are nails pointed?

   Hint

   Area and pressure.

   Answer

   Broad tyres spread the weight over a large area, giving low pressure so they do not sink into soft ground. A pointed nail has a tiny area at its tip, giving high pressure so it goes in easily.

7. Q7 · 3 marks · Short answerHarder exam level

   An elephant weighing 40 000 N stands on four feet, each of area 0.1 m². A woman weighing 500 N stands on one heel of area 1 cm². Who exerts more pressure on the ground?

   Hint

   1 cm² = 0.0001 m².

   Answer

   Elephant: $\frac{40000}{0.4}=100000$ Pa. Heel: $\frac{500}{0.0001}=5000000$ Pa. The heel exerts 50 times more pressure.

8. Q8 · 2 marks · Short answerHarder exam level

   Water is filled in a tall bottle with three holes one above another. Describe what you see and what it shows.

   Hint

   Jet lengths.

   Answer

   The lowest jet goes farthest and the top jet least far. Liquid pressure increases with depth.

9. Q9 · 2 marks · Short answerHarder exam level

   A box weighing 120 N rests on an area of 40 cm². Find the pressure in N/cm² and in Pa.

   Hint

   40 cm² = 0.004 m².

   Answer

   3 N/cm² $=30000$ Pa.

10. Q10 · 2 marks · Short answerHarder exam level

    Explain how a syringe fills with liquid when its plunger is pulled back.

    Hint

    Pressure inside drops.

    Answer

    Pulling the plunger lowers the pressure inside the barrel. Atmospheric pressure on the liquid outside is then greater and pushes the liquid into the syringe.

11. Q11 · 3 marks · Short answerHOTS / competency

    A force of 20 N is applied 0.6 m from a door's hinges, at right angles. Find its moment. What is the moment if the same force is applied along the door, pointing towards the hinges? Explain.

    Hint

    Perpendicular distance.

    Answer

    $20\times 0.6=12$ N m. Along the door, the line of the force passes through the hinge, so the perpendicular distance is 0 and the moment is 0: the door does not turn.

12. Q12 · 3 marks · Short answerHOTS / competency

    A machine weighing 2000 N must not press on the floor with more than 50 000 Pa. What is the least base area it needs?

    Hint

    A = F/P.

    Answer

    $A=\frac{2000}{50000}=0.04{\text{m}}^2$.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Define moment of a force.Show answer

   The turning effect of a force: force × perpendicular distance from the pivot.

2. 2.SI unit of moment?Show answer

   N m.

3. 3.Define pressure.Show answer

   Force acting normally per unit area.

4. 4.1 Pa equals?Show answer

   1 N/m².

5. 5.Why are dams thicker at the base?Show answer

   Water pressure increases with depth.

6. 6.Approximate atmospheric pressure at sea level?Show answer

   About 100 000 Pa.

7. 7.Why do school bags have wide straps?Show answer

   To spread the force over a larger area and reduce pressure on the shoulders.

8. 8.Is there any turning effect if a force passes through the pivot?Show answer

   No; its moment is zero.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Did you use the perpendicular distance for the moment?
• ☐Is your moment in N m and pressure in Pa?
• ☐Did you convert cm² to m² before dividing?
• ☐Can you explain why a smaller area gives more pressure?
• ☐Can you state two facts about liquid pressure?
• ☐Can you explain the straw or syringe using atmospheric pressure?

Force and Pressure: frequently asked questions