GRADE 8 Science FORCE AND ENERGY – Pressure 1 Notes
Science — Force and Energy
Subtopic: Pressure 1 (Age 13, Kenya)
- Define pressure in simple terms.
- Use the formula P = F / A to calculate pressure, force or area.
- Understand units of pressure (Pascal, Pa) and simple conversions.
What is pressure?
Pressure is the amount of force pushing on a surface divided by the area of that surface. In short: pressure = force ÷ area.
Formula: P = F / A
Where: P = pressure, F = force (in newtons, N), A = area (in square metres, m2).
Units of pressure
- The SI unit is Pascal (Pa). 1 Pa = 1 N / m2.
- Often we use kilopascal: 1 kPa = 1000 Pa.
- Example: a pressure of 5 000 Pa is the same as 5 kPa.
Simple picture
Worked examples
A book exerts a force of 20 N on a table. The area of the part of the book touching the table is 0.05 m2. What is the pressure on the table?
Solution:
P = F / A = 20 N ÷ 0.05 m2 = 400 Pa.
So the pressure is 400 Pa (or 0.4 kPa).
A person of weight 700 N stands on one foot. If the area of the sole of a flat shoe is 0.03 m2, and the area of a high heel is 0.002 m2, find the pressure in both cases.
Flat shoe: P = 700 ÷ 0.03 ≈ 23333 Pa ≈ 23.3 kPa.
High heel: P = 700 ÷ 0.002 = 350000 Pa = 350 kPa.
Because the heel has a much smaller area, the pressure is much larger — this can damage soft floors.
Classroom activities / experiments
- Press a thumb on a potato: compare pressure when thumb is flat vs when pressing with a pencil tip.
- Place a board on sand: stand with shoes and then with bare feet; observe how deep you sink (smaller area → larger pressure → sink more).
- Make a simple nail and wood experiment: try pushing a nail with a hammer vs pressing a flat metal plate with the same force — nail penetrates because pressure is higher at the point.
Rearranging the formula
From P = F / A we can find:
- F = P × A (to find the force if pressure and area are known)
- A = F ÷ P (to find the area if force and pressure are known)
Common mistakes to avoid
- Forgetting to convert area to m2 (e.g., cm2 → m2 divide by 10 000).
- Using force in kilograms instead of newtons — remember weight in newtons = mass (kg) × g (≈ 9.8 N/kg).
- Confusing pressure units — always state Pa or kPa.
Practice questions
- A box applies a force of 150 N on the floor. The area in contact is 0.25 m2. Find the pressure in Pa.
- A nurse applies a force of 20 N with a syringe plunger over an area of 2 cm2. What pressure does the nurse produce (give your answer in kPa)?
- A sharp nail pushes into wood with a force of 50 N. The tip area is 0.5 mm2. Calculate the pressure in Pa. (Hint: convert mm2 to m2.)
- If a force of 180 N creates a pressure of 60 000 Pa, what is the contact area?
- P = 150 ÷ 0.25 = 600 Pa.
- Area = 2 cm2 = 2 ÷ 10 000 = 0.0002 m2. P = 20 ÷ 0.0002 = 100 000 Pa = 100 kPa.
- 0.5 mm2 = 0.5 × 10-6 m2. P = 50 ÷ 0.5×10-6 = 100 000 000 Pa = 1.0×108 Pa.
- A = F ÷ P = 180 ÷ 60 000 = 0.003 m2.
Pressure tells us how concentrated a force is on an area. Use P = F ÷ A, keep units in newtons and metres squared, and express pressure in pascals (Pa) or kilopascals (kPa).