Verified Solution • 2025–26 Session
moderate
Q6.3: NCERT Physics - Part 1 SYSTEMS OF PARTICLES AND ROTATIONAL MOTION 6.3 Solution: A child sits stationary at one end of a long trolley moving…
⚠️ Common Exam Traps & Conceptual Pitfalls
Students frequently assume that the child’s running exerts a reaction on the trolley that alters the system’s centre‑of‑mass velocity, or they add the child’s speed to V as if it were external. In reality, with no horizontal external forces the total linear momentum is conserved, so the CM of trolley + child continues to move at the original speed V regardless of internal motions.
A child sits stationary at one end of a long trolley moving uniformly with a speed V on a smooth horizontal floor. If the child gets up and runs about on the trolley in any manner, what is the speed of the CM of the (trolley + child) system?
Step-by-Step Verified Solution
**Answer:** The speed of the centre of mass remains V.
**Step-by-Step Explanation:**
The child and trolley form a single system. The child moving inside the trolley constitutes purely internal motion. Since there is no external force on the system (frictionless floor), the velocity of the centre of mass remains unchanged [citation:33].
All Questions in this Chapter
Related Learning Resources & Exam Preparation
Chapter Practice Bank
Practice all topic-wise questions from SYSTEMS OF PARTICLES AND ROTATIONAL MOTION
Reference Textbook Solutions
Explore complete chapter exercises for NCERT Physics - Part 1
Competitive PYQ Question Bank
Browse year-wise authentic previous years' questions with solutions
Authentic CBT Mock Tests
Simulate official exam environment with live timer and percentile scoring