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Question

# (i) If a balloon filled with air and its mouth untied, is released with its mouth in the downward direction, it moves upwards. Why? ​(ii) An unloaded truck weighing 2000 kg has a maximum acceleration of 0.5 m/s2. What is the maximum acceleration when it is carrying a load of 2000 kg?

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Solution

## (a) The air filled in the balloon ejects at very high speed, which exerts an upward thrust force on the balloon to conserve the momentum of the whole system. Hence, the balloon moves in an upward direction. (b) Mass of the unloaded truck = 2000 kg Maximum acceleration = 0.5 m/s2 The maximum force developed by the truck = $\mathrm{mass}×\mathrm{acceleration}=2000\mathrm{kg}×0.5\mathrm{m}/{\mathrm{s}}^{2}=1000\mathrm{N}$ Now, the mass of the loaded truck = 4000 kg Maximum acceleration of the loaded truck = $\frac{\mathrm{Maximum}\mathrm{Force}}{\mathrm{Total}\mathrm{Mass}}=\frac{1000\mathrm{N}}{4000\mathrm{kg}}=0.25\mathrm{m}/{\mathrm{s}}^{2}$ Hence, the maximum acceleration of the loaded truck is 0.25 m/s2

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