Question

A constant retarding force of 50 N is applied to a body of mass 20 kg moving initially with a speed of 15 m $s^{-1}$. How long does the body take to stop? }

• A. 6s
• B. 2.5 s
• C. 15 s
• D. 50 s

Hint:

Retarding force always produces negative acceleration (deceleration).

Answer 1

The Correct Option is A

Step 1: Concept
Use Newton's Second Law ($F = ma$) and Kinematic Equations ($v = u + at$).
Step 2: Analysis
Force ($F$) = $-50$ N (retarding force). Mass ($m$) = 20 kg. Initial speed ($u$) = 15 m/s. Final speed ($v$) = 0 m/s (body stops).
Step 3: Calculation
Acceleration ($a$) = $F / m = -50 / 20 = -2.5$ m/$s^2$. Using $v = u + at$: $0 = 15 + (-2.5)t$. $2.5t = 15 \Rightarrow t = 15 / 2.5 = 6$ s.
Step 4: Conclusion
Hence, the body takes 6 seconds to stop.
Final Answer:(A)