AQA A Level Physics Paper 3 Section B: Engineering physics June 2018 question paper 7408/3BC

State the law of conservation of angular momentum.

Figure 1 shows an orbiting satellite fitted with two small instrument pods attached to the ends of telescopic arms. The arms can be extended or retracted by a motor in the body of the satellite.

With the telescopic arms fully extended, the centre of mass of each instrument pod is at a radius of 4.1 m from the axis of rotation.

moment of inertia of satellite body about axis = 71 kg m² mass of each instrument pod = 5.0 kg

The mass of the telescopic arms is negligible.

Show that the total moment of inertia of the satellite with the arms fully extended is

The satellite is initially rotating slowly about its axis with the arms fully extended. The arms are slowly retracted so that the instrument pods move closer to the body of the satellite.

State and explain the change in the angular speed of the satellite as the arms are retracted.

The satellite is initially rotating at 1.3 rad s⁻¹ with the telescopic arms fully extended. When fully retracted the instrument pods are at a radius of 0.74 m from the axis. The satellite contains sensitive equipment that may be damaged if the rotational speed exceeds 4.2 rad s⁻¹

Deduce whether the arms can be retracted fully without the satellite exceeding its maximum permitted angular speed.

State one function of a flywheel.

A student does an experiment to determine the frictional torque acting at the bearings of a steel flywheel. The flywheel has a radius of 0.075 m and is perfectly balanced.

The student places a small magnet of mass 0.020 kg at point A on the circumference of the flywheel on a horizontal line through the axis of rotation as shown in Figure 2a The student releases the flywheel. The flywheel first comes to rest when it has moved through an angle of 3.00 rad (172°), with the magnet now in position B as shown in Figure 2b.

Figure 2a
Figure 2b

The loss in gravitational potential energy of the magnet equals the work done against the frictional torque acting at the bearings.

Show that the frictional torque is about 7 × 10^–4 N m

The student goes on to determine the moment of inertia of the flywheel.

The magnet is removed and the flywheel is made to spin. Measurements show that the flywheel makes 573 rotations as its angular speed reduces uniformly from 25.0 rad s^–1 to zero. Assume the frictional torque at the bearings is constant and the same as in question 02.2.

Determine the moment of inertia of the flywheel about its axis of rotation.

Figure 3 shows the p – V diagram for an idealised diesel engine cycle. In this cycle a fixed mass of air is taken through four processes 1 2 3 4 1.

Which statement about this cycle is true? Tick (✓) the correct answer.

The cycle in Figure 3 may be modified by allowing the air to continue to expand adiabatically from state 4 until it is at atmospheric pressure at state 5. Figure 4 shows the modified cycle.

The expansion stroke 3 5 is now longer than the compression stroke 1 2. Process 5 1 takes place at constant pressure.

It has been claimed that, compared to the cycle in Figure 3, the modified cycle of Figure 4 gives

A an increase in work done per cycle of 130 J B an increase in efficiency of more than 15%

Deduce whether these claims are true.

Claim A

Claim B

For the air in process 5 1 in Figure 4, ΔU = –374 J

Calculate the energy that must be removed by cooling for process 5 1.

0.060 mol of air is taken through the cycle.

Determine the maximum temperature in the cycle.

The National Grid is supplied mainly from power stations which have overall efficiencies of up to about 40%

Table 1 shows the average power requirements of a large paper-manufacturing business (a paper mill).

The paper mill can either

• take all of its energy from the National Grid, or

• install an electrical generator of output 60 MW driven by a gas turbine of efficiency 36% as part of a combined heat and power (CHP) scheme. The hot exhaust gases

from the turbine are used to produce steam at high temperature and pressure for heating.

The owners of the paper mill are considering the CHP option.

Explain, with reference to the data above and any other factors, the advice you would give them.

In your answer you should

• explain why the maximum theoretical efficiency of a heat engine is much less

than 100%

• use the information above, including the numerical data, to compare the two

options.