Fig. 1 shows a cargo drone carrying a sealed medical box. The drone hovers 35 m above the ground before lowering the box at constant speed using a cable. Th...
Question 1 Report
Fig. 1 shows a cargo drone carrying a sealed medical box. The drone hovers 35 m above the ground before lowering the box at constant speed using a cable. The mass of the box is 6.5 kg and its volume is 0.0050 m³. The density of air is 1.2 kg/m³.
(a) Calculate the weight of the box. Take g as 9.8 N/kg. [2]
(b) State the resultant force on the box as it is lowered at constant speed. [1]
(c) Calculate the upthrust on the box due to the air. [3]
(d) Explain why the cable tension is slightly less than the weight of the box. [2]
(e) Describe how the cable tension changes if the box accelerates downward. [2]
(f) State one safety reason for lowering rather than dropping the medical box. [1]
Answer Details
(a) Weight is calculated using \(W=mg\).
\[W=6.5\times9.8=63.7\text{ N}\]
The weight is 63.7 N. [2]
(b) Constant speed means zero acceleration, so the resultant force is 0 N. [1]
(c) Upthrust equals the weight of displaced air:
\[U=\text{volume}\times\text{air density}\times g\]
\[U=0.0050\times1.2\times9.8=0.0588\text{ N}\]
The upthrust is 0.0588 N, or 0.059 N. [3]
(d) The air exerts an upthrust upwards. [1] At constant speed, the upward forces balance the weight, so \(\text{tension}+\text{upthrust}=\text{weight}\). Therefore the cable tension is slightly less than the weight. [1]
(e) If the box accelerates downwards, there must be a downward resultant force. Therefore the cable tension decreases, so that weight is greater than the total upward force. [2]
(f) Lowering reduces the impact force when the box reaches the ground, helping to prevent damage to the medical box or injury to people. [1]
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