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# sample_problems_answer - Sample Problems 1 a = v vf vi = t...

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Sample Problems. 1. a = Δ v Δ t = v f - v i 10s v f = 0 , v i = 100km / h = 100 × 1000m 3600s = 27 . 78m / s a = - 27 . 78m / s 10s = - 2 . 778m / s 2 2. Free fall motion, a = g = 9 . 8m / s 2 i) a = Δ v Δ t = v f - v i 10s , v f = a × 10s = 9 . 8m / s 2 × 10s = 98m / s ii) average velocity = displacement / time = d/t d = 1 2 at 2 = 1 2 × 9 . 8m / s 2 × (10s) 2 = 490 m d t = 490m 10s = 49m / s it will fall 490m . 3. a = 25m / s - 0m / s 10s = 2 . 5m / s 2 4. average speed = total distance total time t 1 = 600km 200km / h = 3 hr t 2 = 600km 300km / h = 2 hr total time = 3hr + 2hr = m 5hr average speed = 1200km 5hr = 240km / h 1

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5. Let the distance L t 1 = L 40km / h , t 2 = L 60km / h Total time = L 40km / h + L 60km / h = 100 L 2400km / h Average speed = 2 L 100 L 2400km / h = 2 × 2400km / h × L 100 × L = 48km / h 6. F = ma, 200N = 40kg × a, a = 5m / s 2 7. we have two force on it. One is 300N lifting fore, the other gravity(= mg = 20kg × 9 . 8m / s 2 ) the net force = 300N - 196N = 104N = 20kg × a, a = 5 . 2 m/s 2 8. a) a = Δ v/ Δ t = (7m / s - 6m / s) / 2s = 0 . 5m / s 2 . b) F = ma = 60kg × 0 . 5m / s 2 = 30N 9. - 40N - 40N = 80kg × a, a = 0 . 5m / s 2 10. W = F || d = mgd = 300kg × 9 . 8m / s 2 × 6m = 17640J 11. W 1 = 10N × 5m = 50J W 2 = 20N × 2m = 40J W 1 > W 2 12. Let the length of the rod L and the distance from the left (1kg) to the support
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