Verified Solution • 2025–26 Session
moderate
Q4.3: NCERT Physics - Part 1 LAWS OF MOTION 4.3 Solution: Give the magnitude and direction of the net force acting on a…
⚠️ Common Exam Traps & Conceptual Pitfalls
Students frequently assume the stone keeps a horizontal force $ma_{\text{train}}$ after it leaves the train, mixing inertial and non‑inertial frames; remember that once the stone is free, only gravity ($mg$) acts in an inertial frame, and any horizontal pseudo‑force appears only when you analyze the motion from the accelerating train’s frame.
Give the magnitude and direction of the net force acting on a stone of mass 0.1 kg, (a) just after it is dropped from the window of a stationary train, (b) just after it is dropped from the window of a train running at a constant velocity of 36 km/h, (c) just after it is dropped from the window of a train accelerating with 1 m s__, (d) lying on the floor of a train which is accelerating with 1 m s__, the stone being at rest relative to the train. Neglect air resistance throughout.
Step-by-Step Verified Solution
**Answer:** (a) 1.0 N downward (b) 1.0 N downward (c) 1.0 N downward (d) 0.1 N in direction of train motion
**Step-by-Step Explanation:**
(a) Stone is dropped from stationary train. Force = mg = 0.1 _ 10 = 1.0 N, vertically downward [citation:1][citation:13].
(b) Train moves with constant velocity, so horizontal acceleration is zero. Force = mg = 1.0 N, vertically downward [citation:13][citation:14].
(c) Just after dropping, the horizontal force due to train acceleration no longer acts on the stone. Net force = mg = 1.0 N, vertically downward [citation:1][citation:13].
(d) Stone lies on floor of accelerating train, so it accelerates with the train. Force = ma = 0.1 _ 1 = 0.1 N, in the direction of train's motion [citation:1][citation:13].
All Questions in this Chapter
Related Learning Resources & Exam Preparation
Chapter Practice Bank
Practice all topic-wise questions from LAWS OF 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