Stopping Distance: Formula, Reaction Distance, Braking Distance & Questions
 |
| Stopping Distance consists of Reaction Distance and Braking Distance. Learn the formula with an easy solved example. |
Internal Links
- Motion in a Straight Line
- Equations of Motion
- Uniform and Non-Uniform Motion
- Speed, Velocity and Acceleration
- Distance and Displacement
- Newton's Laws of Motion
- Friction in Physics
- Work, Energy and Power
- Projectile Motion
- Free Fall Equations
- Average Speed Formula
- Relative Motion
- SUVAT Equations
- Kinematics Notes
- Physics Formula Sheet
Stopping Distance (Reaction Distance + Braking Distance)
What is Stopping Distance?
When a driver sees an obstacle, the car does not stop immediately. The car travels some distance before stopping completely.
The stopping distance has two parts:
- Reaction Distance → Distance travelled while the driver reacts.
- Braking Distance → Distance travelled after applying the brakes.
Stopping Distance = Reaction Distance + Braking Distance
S = Sr + Sb
S = Sr + Sb
1. Reaction Distance
Sr = u × t
Where
- u = Initial speed (m/s)
- t = Reaction time (seconds)
2. Braking Distance
Use the equation of motion:
v² = u² + 2as
When the car stops:
- v = 0
- a = Retardation
Sb = u² / 2a
Solved Example
Question
A car is moving with velocity 20 m/s. The driver's reaction time is 0.5 s. The maximum retardation is 4 m/s². Find the stopping distance.
Step 1 : Reaction Distance
Sr = u × tSr = 20 × 0.5
Sr = 10 m
Step 2 : Braking Distance
Sb = u² / 2a= 20² / (2 × 4)
= 400 / 8
= 50 m
Step 3 : Stopping Distance
S = Sr + Sb= 10 + 50
= 60 m
Answer = 60 m
Shortcut Formula
Stopping Distance
S = ut + u² / 2a
S = ut + u² / 2a
Important Points
- Reaction distance depends on reaction time.
- Braking distance depends on speed and braking force.
- Higher speed means longer stopping distance.
- Wet roads increase braking distance.
- Always maintain a safe distance while driving.
Practice Questions
Q1.
A car moves at 10 m/s.
Reaction time = 1 s
Retardation = 5 m/s²
Find stopping distance.
Q2.
A car moves at 30 m/s.
Reaction time = 1 s
Retardation = 6 m/s²
Find stopping distance.
Q3.
A car moves at 15 m/s.
Reaction time = 0.4 s
Retardation = 3 m/s²
Find stopping distance.
Q4.
A car moves at 25 m/s.
Reaction time = 0.8 s
Retardation = 5 m/s²
Find stopping distance.
Answers
| Question | Answer |
|---|---|
| Q1 | 20 m |
| Q2 | 105 m |
| Q3 | 43.5 m |
| Q4 | 82.5 m |
Quick Revision
| Quantity | Formula |
|---|---|
| Reaction Distance | Sr = ut |
| Braking Distance | Sb = u² / 2a |
| Stopping Distance | S = Sr + Sb |
Memory Trick
Think → Brake → Stop
- Think = Reaction Distance
- Brake = Braking Distance
- Stop = Stopping Distance
