A car decreases its speed from $ 40\ \text{m/s}$ to $ 20\ \tex... | General Science MCQ | Examoogle

General SciencePhysics: Motion

A car decreases its speed from $40\ \text{m/s}$ to $20\ \text{m/s}$ in $5\ \text{s}$ . Find the acceleration of the car.

$4\ \text{m/s}^2$

$-4\ \text{m/s}^2$

$12\ \text{m/s}^2$

$2\ \text{m/s}^2$

Correct Answer

🔬 SC • Science Concept & PrincipleGeneral SciencePhysics: Motion

Option B

$-4\ \text{m/s}^2$

Quick Summary:

Acceleration is defined as the rate of change of velocity with respect to time. When a car's speed decreases from $40\text{ m/s}$ to $20\text{ m/s}$ in $5\text{ s}$ , the final velocity is less than the initial velocity, resulting in a negative acceleration (retardation or deceleration).

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🔬SCScience SolutionConcept & Principle

💡 Explanation

💡 Everyday Analogy (Real-World Intuition)

When you apply brakes in a moving bicycle or car, you feel pushed forward as the vehicle slows down; this backward pull on the vehicle is deceleration (negative acceleration).

🧠 Memory Mnemonic / Shortcut Aid

Remember 'Final minus Initial over Time' ( $FIT$ : $\frac{v - u}{t}$ ) for acceleration.

🔢 Key Formulas

$a = \frac{v - u}{t}$ — Acceleration formula

$v = u + at$ — First equation of motion

⚙️ Working Principle

According to the formula for acceleration, $a = \frac{v - u}{t}$ , where $u$ is the initial velocity ( $40\text{ m/s}$ ), $v$ is the final velocity ( $20\text{ m/s}$ ), and $t$ is the time interval ( $5\text{ s}$ ). Substituting these values gives $a = \frac{20 - 40}{5} = \frac{-20}{5} = -4\text{ m/s}^2$ . The negative sign indicates deceleration.

📌 Key Points

▸
Acceleration can be positive (acceleration) or negative (retardation/deceleration).
▸
Initial velocity $u = 40\text{ m/s}$ , Final velocity $v = 20\text{ m/s}$ .
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Time taken $t = 5\text{ s}$ .
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Unit of acceleration is $\text{m/s}^2$ .

✅ Advantages

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Helps determine the braking performance of vehicles.

🛠️ Applications / Uses

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Braking systems in automobiles
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Landing of aircrafts

📄 Additional Information

▸
Option A ( $4\text{ m/s}^2$ ) represents acceleration instead of deceleration.
▸
Option B ( $-4\text{ m/s}^2$ ) correctly accounts for the decrease in speed.

📊 Diagram / Illustration

✅

B is correct — The acceleration of the car is $-4\text{ m/s}^2$ , indicating deceleration as speed decreases.

Core Concepts Used

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Acceleration Deceleration Kinematics Equations of Motion

💡 EXAM TIP

Always pay careful attention to the direction of velocity change; if speed decreases, acceleration must come out negative in standard coordinate systems.

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