Force and Pressure — Important Questions

20 Pa

Step 1: Given – Thrust F = 60 N, Area A = 3 m². Step 2: Formula: Pressure P = Thrust / Area = F / A. Step 3: P = 60 N / 3 m² = 20 N/m² = 20 Pa. Step 4: Option A (180 Pa) comes from multiplying force × area instead of dividing. Option B (57 Pa) comes from subtracting. Always use division: P = F ÷ A.

The sharp knife has a smaller area of contact, so it exerts more pressure

Step 1: Both sharp and blunt knives are pushed with the same force (thrust). Step 2: Using P = F / A – when the area A is smaller, pressure P is larger. Step 3: A sharp knife has a very small area at the edge, so the same thrust produces very high pressure. Step 4: A blunt knife has a larger area, so the same thrust produces less pressure and does not cut easily. Step 5: This is the same reason why a pin can be pushed into wood easily but a nail cannot without hammering.

N / m²

Step 1: Pressure = Thrust / Area. Step 2: Unit of thrust (force) = Newton (N). Step 3: Unit of area = metre² (m²). Step 4: Therefore, unit of pressure = N / m² = N m⁻² = Pascal (Pa). Step 5: N × m is the unit of moment of force (torque), not pressure. 1 Pa is defined as 1 N of thrust acting on 1 m² of area.

3 m

Step 1: Given – Moment of force = 12 N m, Force F = 4 N, Find distance d. Step 2: Formula: Moment of force = F × d. Step 3: Rearranging: d = Moment / F = 12 N m / 4 N. Step 4: d = 3 m. Step 5: Option A (48 m) results from multiplying instead of dividing. Option D (0.33 m) results from dividing 4 by 12 instead of 12 by 4. Always isolate the unknown: d = Moment ÷ F.