Forces & Stopping Distance

IGCSE Edexcel Physics
1.15–1.21 Resultant force, Newton's laws, stopping distance, terminal velocity
Key Concepts: Resultant force = sum of all forces acting. $F = ma$. Weight = $mg$. Stopping distance = thinking distance + braking distance. At terminal velocity, resultant force = 0.

Section A — Forces and Newton's Laws

1. State Newton's First Law of Motion. [2]
2. State Newton's Second Law of Motion and write the equation it gives. [2]
3. A 700 N force acts forward and a 250 N force acts backward on a car. Calculate the resultant force and state its direction. [2]
4. A 15 kg trolley accelerates at $1.2\,\text{m/s}^2$. Calculate the resultant force. [2]
5. Calculate the weight of a 65 kg person on Earth ($g = 9.8\,\text{N/kg}$). [2]
6. A resultant force of 360 N acts on a car of mass 900 kg. Calculate the acceleration. [2]

Section B — Stopping Distance

7. State the equation linking stopping distance, thinking distance and braking distance. [1]
8. Give two factors that affect thinking distance and explain how each increases it. [4]
9. Give two factors that affect braking distance and explain how each increases it. [4]
10. A car slows uniformly from $20\,\text{m/s}$ to rest in 4.0 s. Calculate the braking distance. [3]

Section C — Terminal Velocity

11. Describe, with reference to forces, how a skydiver reaches terminal velocity after jumping from a plane. [4]
12. A skydiver opens their parachute. Describe and explain what happens to their speed immediately after opening, and what happens eventually. [3]

Total marks: 31

Mark Scheme

1. An object remains at rest or continues in uniform motion in a straight line [1] unless acted on by a resultant (unbalanced) force [1] [2]
2. Resultant force is proportional to the rate of change of velocity (acceleration) [1]; $F = ma$ [1] [2]
3. $700 - 250 = 450\,\text{N}$ [1]; forward [1] [2]
4. $F = ma = 15 \times 1.2 = 18\,\text{N}$ [2]
5. $W = mg = 65 \times 9.8 = 637\,\text{N}$ [2]
6. $a = F/m = 360/900 = 0.4\,\text{m/s}^2$ [2]
7. Stopping distance = thinking distance + braking distance [1]
8. Any two, e.g.: higher speed → greater distance covered during reaction time [2]; alcohol/tiredness/distractions → increase reaction time → greater thinking distance [2] [4]
9. Any two, e.g.: higher speed → greater kinetic energy → longer distance to stop [2]; wet/icy roads or worn tyres → reduced friction → greater braking distance [2] [4]
10. $a = (0 - 20)/4 = -5\,\text{m/s}^2$ [1]; distance = area under v-t graph = $\tfrac{1}{2} \times 20 \times 4 = 40\,\text{m}$ [2] [3]
11. Initially weight > air resistance so skydiver accelerates downward [1]; as speed increases, air resistance increases [1]; eventually air resistance equals weight [1]; resultant force = 0, so speed is constant (terminal velocity) [1] [4]
12. Parachute greatly increases air resistance [1]; air resistance now exceeds weight, so skydiver decelerates [1]; speed decreases until a new (lower) terminal velocity is reached [1] [3]