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Q4.5: NCERT Physics - Part 1 LAWS OF MOTION 4.5 Solution: A constant retarding force of 50 N is applied to a body of mass …

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often treat the retarding force as a positive magnitude in the equation $v = u + at$, leading to an incorrect calculation of time. You must explicitly assign a negative sign to the acceleration ($a = -F/m$) because the force opposes the initial velocity, ensuring the velocity decreases to zero.

A constant retarding force of 50 N is applied to a body of mass 20 kg moving initially with a speed of 15 m s__. How long does the body take to stop?

Step-by-Step Verified Solution

**Answer:** t = 6 s **Step-by-Step Explanation:** Mass m = 20 kg, retarding force F = 50 N. Acceleration:
a=Fm=5020=2.5 m/s2 (opposite to motion)a = \frac{F}{m} = \frac{50}{20} = 2.5 \text{ m/s}^2 \text{ (opposite to motion)}
Using v = u + at with u = 15 m/s, v = 0, a = -2.5 m/s_:
0=15−2.5t  ⟹  t=152.5=6 s0 = 15 - 2.5t \implies t = \frac{15}{2.5} = 6 \text{ s}
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All Questions in this Chapter

Q4.1 Q4.2 Q4.3 Q4.4 Q4.5 Q4.6 Q4.7 Q4.8 Q4.9 Q4.10 Q4.11 Q4.12 Q4.13 Q4.14 Q4.15 Q4.16 Q4.17 Q4.18 Q4.19 Q4.20 Q4.21 Q4.22 Q4.23

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