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Q4.2: NCERT Physics - Part 1 LAWS OF MOTION 4.2 Solution: A pebble of mass 0.05 kg is thrown vertically upwards. Give the …
⚠️ Common Exam Traps & Conceptual Pitfalls
Students often equate the direction of motion with the direction of net force, thinking that when the pebble moves upward the force must be upward or that at the top the force vanishes because the velocity is zero. In fact, the only force is gravity, giving a constant downward acceleration $a=g$, so the net force is always $F=mg$ downward, regardless of the pebble’s velocity or whether it was launched at an angle.
A pebble of mass 0.05 kg is thrown vertically upwards. Give the direction and magnitude of the net force on the pebble, (a) during its upward motion, (b) during its downward motion, (c) at the highest point where it is momentarily at rest. Do your answers change if the pebble was thrown at an angle of 45� with the horizontal direction? Ignore air resistance.
Step-by-Step Verified Solution
**Answer:** 0.5 N, vertically downward in all cases. No, answers do not change if thrown at 45�.
**Step-by-Step Explanation:**
The only force acting on the pebble is gravity (ignoring air resistance). By Newton's second law:
This force acts vertically downward in all three cases: during upward motion, downward motion, and at the highest point [citation:3][citation:13][citation:14].
If thrown at 45�, the horizontal component of velocity exists throughout, but it produces no vertical force. The net force remains 0.5 N downward [citation:13].
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