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š Revise, Reflect, Refine, Q10 (Page 138)
A ball of mass 2 kg is thrown up with a velocity of 20 m sā»¹.View answer
Answer- (i) Sign of work done by gravity —
Motion Direction of gravity Direction of displacement Work done by gravity Upward Downward Upward Negative Downward Downward Downward Positive - (ii) If the ball reaches a height of 19.4 m, how much work was done by air resistance? (g = 10 m sā»²)
Position Potential energy Kinetic energy Total mechanical energy Ground (thrown up at 20 m sā»¹) 0 J ½ × 2 × (20)² = 400 J 400 J Highest point (19.4 m) 2 × 10 × 19.4 = 388 J 0 J 388 J - Work done by air resistance = Final energy − Initial energy
- = 388 − 400
- = −12 J
- Negative — air resistance opposes motion and takes away energy. (Mechanical energy was NOT conserved here because air resistance acted.)
Q10 - A ball of mass 2
Last updated at September 16, 2026 by Teachoo
