KCSE Physics — Practice Paper 1

Short-answer practice across mechanics, pressure, thermal physics, waves, electricity and radioactivity. Every numerical answer is carried through with units, because in Physics the unit is part of the mark.

How to use this: attempt each question first. Then check not only whether your number matched, but whether your working would have earned the method marks on its own.

Section A — Short answer

1. Define density and state its SI unit (2 marks)

Density is the mass per unit volume of a substance.

$$\rho = \frac{m}{V}$$

SI unit: kg/m³ (kg m⁻³)

Where the marks sit: 1 for the definition, 1 for the unit. "Mass over volume" scores; "how heavy something is" does not.

2. A block of mass 2.5 kg is raised through a height of 4.0 m. Calculate the work done against gravity (3 marks)

(Take g = 10 N/kg)

Work done  = force × distance
           = weight × height
           = (m × g) × h
           = 2.5 × 10 × 4.0
           = 100 J

Where the marks sit: 1 for identifying the force as weight ($mg$), 1 for substituting, 1 for the answer with the unit J.

Common mistake: using the mass directly as the force. Weight is $mg$, not $m$ — this drops two of the three marks.

3. State the principle of moments (2 marks)

For a body in equilibrium, the sum of clockwise moments about a point equals the sum of anticlockwise moments about the same point.

Where the marks sit: 1 for equating the two sums, 1 for about the same point. The second clause is routinely omitted and routinely costs the mark.

4. A uniform metre rule is balanced at its centre. A 200 g mass hangs 20 cm from the pivot. Where must a 50 g mass hang to balance it? (3 marks)

Taking moments about the pivot:

clockwise  = anticlockwise
200 × 20   = 50 × d
4000       = 50d
d          = 80 cm

The 50 g mass hangs 80 cm from the pivot, on the opposite side.

Where the marks sit: 1 for applying the principle of moments, 1 for the arithmetic, 1 for the distance with the side stated.

Note: the masses need not be converted to newtons here — $g$ appears on both sides and cancels. Saying so earns you nothing extra, but it saves you time.

5. Explain why a sharp knife cuts more easily than a blunt one (2 marks)

1. A sharp knife has a smaller area of contact
2. For the same force, pressure = force / area, so a smaller area gives a greater pressure, which cuts more easily

Where the marks sit: 1 for the smaller area, 1 for linking it to increased pressure via the relationship. Stating "it is sharper" restates the question.

6. Calculate the pressure exerted by a column of water 5.0 m deep (3 marks)

(density of water = 1000 kg/m³, g = 10 N/kg)

P = ρ g h
  = 1000 × 10 × 5.0
  = 50 000 Pa
  = 5.0 × 10⁴ Pa   (or 50 kPa)

Where the marks sit: 1 for the formula, 1 for substitution, 1 for the answer with Pa.

7. Distinguish between transverse and longitudinal waves, giving one example of each (4 marks)

Where the marks sit: 1 for each direction of vibration, 1 for each example.

Common mistake: "transverse waves go up and down". Relative to what? The mark requires the vibration to be described relative to the direction of energy travel.

8. A wave has frequency 50 Hz and wavelength 6.0 m. Calculate its speed (2 marks)

v = f λ
  = 50 × 6.0
  = 300 m/s

Where the marks sit: 1 for the equation, 1 for the answer with m/s.

9. Three resistors of 2 Ω, 3 Ω and 6 Ω are connected in parallel. Calculate the effective resistance (3 marks)

 1     1     1     1
--- = --- + --- + ---
 R     2     3     6

      3     2     1     6
   = --- + --- + --- = --- = 1
      6     6     6     6

 1
--- = 1     ⟹     R = 1 Ω
 R

Where the marks sit: 1 for the parallel formula, 1 for a common denominator, 1 for the final resistance.

Sense check: the effective resistance in parallel is always smaller than the smallest individual resistor. Here 1 Ω < 2 Ω ✓. If your answer is bigger than the smallest, you have used the series formula.

10. State two differences between alpha and beta particles (2 marks)

Where the marks sit: any 2 contrasted pairs, 1 mark each, both sides stated.

The three marks most students leave on the table

1. Units. Every quantitative answer needs one, and it is usually a mark in its own right. 100 is not the same answer as 100 J.
2. The formula, written down. Write it before substituting. If your arithmetic slips, the formula mark still stands.
3. Sense checks. Parallel resistance below the smallest resistor; a speed that is not absurd; a pressure that grows with depth. Ten seconds of checking catches most errors.

Where to go next

• KCSE Mathematics Practice Paper 1 — the algebra underneath these rearrangements
• IGCSE Physics Practice Paper 1 — the same physics under Cambridge command words

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