A physics club releases identical paper cupcake cases from a balcony. One case is folded tightly and the other is opened into a wide cup shape. Fig. 1 shows...

A physics club releases identical paper cupcake cases from a balcony. One case is folded tightly and the other is opened into a wide cup shape. Fig. 1 shows the two cases falling through still air. The club predicts that the opened case has a smaller terminal velocity. Both cases have the same mass, so their weight is the same. The result is useful when choosing a design for a lightweight object that must fall slowly without using a coil, cable or motor.

(a) What is meant by terminal velocity? [2]
(b) Give one reason why the opened case experiences more air resistance at the same velocity. [1]
(c) Explain why the opened case has a lower terminal velocity than the folded case. [2]

Fig. 1 shows a wind tunnel used to test a model lorry. The model is fixed to a force sensor while air flows past it. At an air velocity of 15 m/s, the sensor reads 18 N. Engineers compare the result with a streamlined model of the same size. In normal use, a lorry engine must provide a forward force that balances resistive forces when the lorry has constant velocity. The work is separate from testing the vehicle's sound system, light circuit and potential difference.

(a) Name the force measured by the sensor. [1]
(b) Give the forward driving force needed for the model lorry to move steadily at 15 m/s. [1]
(c) Explain how a streamlined cab shape can reduce fuel or battery energy use at a constant road velocity. [3]