Exercise 3.5

Class 11 Physics · Laws of Motion · NCERT Solutions

Q 3.5🟡 Moderate

A constant retarding force of $50\text{ N}$ is applied to a body of mass $20\text{ kg}$ moving initially with a speed of $15\text{ m/s}$ . How long does the body take to stop?

Solution

Step 1 — Find acceleration using Newton’s Second Law

The retarding force acts opposite to motion, so it’s negative:

$a = \frac{F}{m} = \frac{-50}{20} = -2.5\text{ m/s}^2$

Step 2 — Apply first equation of motion

The body stops when final velocity $v = 0$ . Using $v = u + at$ :

$0 = 15 + (-2.5)t \implies t = \frac{15}{2.5} = 6\text{ s}$

✅ Final Answert = 6 seconds

⚠️ Common Mistake

Forgetting the negative sign on the retarding force. A retarding force always opposes motion — if the body moves in the positive direction, the force (and therefore acceleration) is negative. Using $a = +2.5$ gives a wrong answer and makes no physical sense.

🎯 JEE Tip

In JEE, “retarding force” and “braking force” always mean the force opposes the velocity direction. Always assign signs based on your chosen positive direction before substituting — don’t do it by feel. Sign errors are the single biggest source of silly mistakes in NLM.

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