Retardation (or deceleration) is negative acceleration that occurs when an object slows down over time. Learn its definition, examples, formulas, numerical problems, and MCQs with answers for JEE, NEET, and CBSE Class 11 Physics.
What is Retardation?
Retardation, also known as deceleration or negative acceleration, occurs when the velocity (speed with direction) of an object decreases over time.
When an object speeds up, it has positive acceleration. When it slows down, it has negative acceleration, which is called retardation.
Why Does Retardation Happen?
Retardation happens when:
- A moving vehicle applies brakes (e.g., a car stopping at a traffic light).
- A ball thrown upwards slows down due to gravity.
- A train reduces speed while approaching a station.
How is Retardation Measured?
Retardation is measured in the same way as acceleration, using the formula: $$a = \frac{\text{Change in velocity}}{\text{Time taken}}$$
Since velocity is decreasing, the value of acceleration is negative, indicating retardation.
Example: A Car Braking
Let’s take an example where a car is moving at an initial velocity of 10 m/s. The driver applies the brakes, and the car comes to rest (0 m/s) in 5 seconds.
Using the formula for acceleration:
$$a = \frac{\text{Final velocity} – \text{Initial velocity}}{\text{Time taken}} $$
$$a = \frac{0 – 10}{5} = -2 \text{ m/s}^2$$
Interpretation:
- The negative sign (-2 m/s²) shows that the car is slowing down.
- This means the car is undergoing retardation at 2 m/s².
📌 Conceptual Questions with Answers
Q1: Why is retardation called negative acceleration?
Answer: Retardation is called negative acceleration because it represents a decrease in velocity over time. Acceleration is given by:
$$a = \frac{\text{Final velocity} – \text{Initial velocity}}{\text{Time taken}}$$
When the final velocity is less than the initial velocity, the result is a negative value, indicating a decrease in speed, which we call retardation.
Q2: Can an object have acceleration when it is slowing down?
Answer: Yes, an object can have acceleration even when it is slowing down. In this case, the acceleration is negative (retardation), meaning the object is decelerating rather than speeding up.
Q3: A ball is thrown upwards. Is it accelerating or decelerating?
Answer: When a ball is thrown upwards, it slows down due to the force of gravity acting downward. Since its velocity is decreasing, it undergoes retardation or negative acceleration.
Q4: What is the difference between acceleration and retardation?
Answer:
| Acceleration | Retardation (Deceleration) |
|---|---|
| Velocity increases over time. | Velocity decreases over time. |
| It has a positive value. | It has a negative value. |
| Example: A car starting from rest and speeding up. | Example: A car applying brakes to stop. |
Q5: Can an object have retardation even if it is moving forward?
Answer: Yes, an object can move forward while experiencing retardation. For example, if a car is moving in a forward direction but applies brakes, it is still moving forward while slowing down.
📝 Multiple Choice Questions (MCQs)
Q1: What is retardation?
🔘 (A) Acceleration in the direction of motion
🔘 (B) Acceleration in the opposite direction of motion
🔘 (C) Increase in velocity over time
🔘 (D) A force that pushes an object forward
✅ Correct Answer: (B) Acceleration in the opposite direction of motion
Q2: A car is moving at 20 m/s. The driver applies brakes, and it stops in 4 seconds. What is the retardation?
🔘 (A) 5 m/s²
🔘 (B) -5 m/s²
🔘 (C) 4 m/s²
🔘 (D) -4 m/s²
✅ Correct Answer: (B) -5 m/s²
📌 Solution:
$$a = \frac{\text{Final velocity} – \text{Initial velocity}}{\text{Time taken}}$$
$$a = \frac{0 – 20}{4} = -5 \text{ m/s}^2$$
Q3: If an object has a negative acceleration, then its speed must be:
🔘 (A) Increasing
🔘 (B) Decreasing
🔘 (C) Constant
🔘 (D) None of these
✅ Correct Answer: (B) Decreasing
Q4: Which of the following is NOT an example of retardation?
🔘 (A) A car stopping at a red light
🔘 (B) A bus slowing down before a speed breaker
🔘 (C) A rocket launching into space
🔘 (D) A ball thrown upwards slowing down
✅ Correct Answer: (C) A rocket launching into space
📌 Explanation: A rocket launching is an example of positive acceleration, not retardation.
Q5: What is the unit of retardation?
🔘 (A) m/s
🔘 (B) m²/s
🔘 (C) m/s²
🔘 (D) s/m²
✅ Correct Answer: (C) m/s²
Key Points to Remember
✅ If velocity increases, acceleration is positive.
✅ If velocity decreases, acceleration is negative (retardation).
✅ Retardation is just acceleration with a negative value.
✅ It is measured in metres per second squared (m/s²).
Real-Life Examples of Retardation
🔹 A bicycle slowing down when you stop pedaling.
🔹 A rocket moving upwards but slowing due to gravity.
🔹 A bus approaching a stop and decreasing speed.
📄 Retardation (Deceleration) – Practice Worksheet
📌 Section 1: Conceptual Questions
1️⃣ Define retardation and explain how it differs from acceleration.
2️⃣ What happens to the velocity of a body when it undergoes retardation?
3️⃣ Can an object moving forward experience retardation? Explain with an example.
4️⃣ A stone is thrown vertically upward. Explain why it undergoes retardation.
5️⃣ How is retardation related to acceleration? What is its sign convention?
6️⃣ Give three real-life examples where retardation occurs.
7️⃣ What would happen if a car had a very high retardation while braking?
📌 Section 2: Numerical Problems
8️⃣ A train moving at 72 km/h slows down to 18 km/h in 10 seconds. Find the retardation of the train.
📌 Hint: Convert km/h to m/s using: $1 \text{ km/h} = \frac{5}{18} \text{ m/s}$.
9️⃣ A cyclist moving at 15 m/s comes to rest in 5 seconds. Find the retardation.
🔟 A ball is thrown upward with a velocity of 20 m/s. If acceleration due to gravity is 9.8 m/s², find how much time the ball takes to stop.
1️⃣1️⃣ A bus stops from a speed of 25 m/s in 5 seconds. Find:
- The retardation of the bus.
- The distance covered before stopping.
📌 Hint: Use the equation:$s = ut + \frac{1}{2} a t^2$
Multiple Choice Questions (MCQs)
1️⃣2️⃣ What is the SI unit of retardation?
🔘 (A) m/s
🔘 (B) m²/s
🔘 (C) m/s²
🔘 (D) s/m²
✅ Correct Answer: (C) m/s²
1️⃣3️⃣ A car is moving at 20 m/s and applies brakes. It stops in 4 seconds. What is the retardation?
🔘 (A) 5 m/s²
🔘 (B) -5 m/s²
🔘 (C) 4 m/s²
🔘 (D) -4 m/s²
✅ Correct Answer: (B) -5 m/s²
1️⃣4️⃣ Which of the following is an example of retardation?
🔘 (A) A rocket launching upwards
🔘 (B) A ball rolling down a slope
🔘 (C) A bus slowing down before a red light
🔘 (D) A car accelerating on a highway
✅ Correct Answer: (C) A bus slowing down before a red light
1️⃣5️⃣ If an object has a negative acceleration, then its speed must be:
🔘 (A) Increasing
🔘 (B) Decreasing
🔘 (C) Constant
🔘 (D) None of these
✅ Correct Answer: (B) Decreasing
📝 Extra Challenge Question
A motorcycle is moving at 30 m/s. The rider applies brakes and comes to a stop in 3 seconds.
- What is the retardation?
- How far does the motorcycle travel before stopping?
📌 Hint: Use the equations:
- $a = \frac{v – u}{t}$
- $s = ut + \frac{1}{2} a t^2$
🧐 Do You Know? (Interesting Fact)
🚗 A Formula 1 car can decelerate from 200 km/h to 0 km/h in just 2 seconds, experiencing an extreme retardation of nearly -27 m/s²!
