RAF Airman/Airwoman Selection Test (AST) Mechanical Comprehension Syllabus

Mechanical Comprehension syllabus — full chapter and topic list

Expand any chapter to see its topics and sub-topics. This is the whole examinable outline for Mechanical Comprehension in RAF Airman/Airwoman Selection Test (AST), not a summary of it.

1. 1

   Forces and Motion

   4 topics

   • Balanced and unbalanced forces
   • Mass, weight and gravity
   • Friction and its effects
     • Reducing and increasing friction
   • Pressure in solids, liquids and gases
2. 2

   Simple Machines

   4 topics

   • Levers and the principle of moments
     • Fulcrum, effort and load
     • Classes of lever
   • Pulleys and mechanical advantage
     • Single and multiple pulley systems
   • Gears and gear trains
     • Direction of rotation
     • Speed and torque trade-offs
   • Inclined planes, wedges and screws
3. 3

   Energy, Work and Materials

   5 topics

   • Work done and the concept of energy
   • Kinetic and potential energy
   • Springs and elasticity
     • Compression and extension
   • Hydraulics and pneumatics
     • Transmission of pressure in fluids
   • Centre of gravity and stability

Mechanical Comprehension flashcards for RAF Airman/Airwoman Selection Test (AST)

25 of 55 cards from the Mechanical Comprehension deck — real questions with worked answers.

1. What is the difference between balanced and unbalanced forces?

   Balanced forces are equal in size and opposite in direction, producing zero resultant force, so an object stays at rest or moves at constant velocity. Unbalanced forces produce a non-zero resultant force, causing the object to accelerate (change speed or direction).

2. State Newton's first law of motion.

   An object remains at rest, or continues moving at constant velocity in a straight line, unless acted upon by a resultant (unbalanced) external force. This property is called inertia.

3. State Newton's second law as an equation, defining each symbol and unit.

   $$F = ma$$ where $F$ is the resultant force in newtons (N), $m$ is mass in kilograms (kg), and $a$ is acceleration in $\text{m/s}^{2}$.

4. State Newton's third law of motion.

   For every action there is an equal and opposite reaction. When body A exerts a force on body B, body B exerts a force equal in magnitude and opposite in direction on body A.

5. What is the difference between mass and weight?

   Mass is the amount of matter in an object, measured in kilograms (kg), and is constant everywhere. Weight is the gravitational force acting on that mass, measured in newtons (N), and changes with gravitational field strength.

6. Give the formula linking weight, mass and gravity, including units.

   $$W = mg$$ where $W$ is weight in newtons (N), $m$ is mass in kilograms (kg), and $g$ is the gravitational field strength ($\approx 9.81\ \text{N/kg}$ on Earth).

7. What is the value of gravitational field strength $g$ on Earth, and what is the weight of a 10 kg mass?

   $g \approx 9.81\ \text{N/kg}$ (often approximated as $10\ \text{N/kg}$). Weight $= mg = 10 \times 9.81 = 98.1\ \text{N}$ (or about $100\ \text{N}$ using $g=10$).

8. What is friction, and in which direction does it act?

   Friction is a resistive force that arises between two surfaces in contact. It always acts parallel to the surfaces and in the direction opposite to motion (or attempted motion).

9. What is the difference between static friction and kinetic (dynamic) friction?

   Static friction acts on a stationary object and must be overcome to start motion; it is generally larger. Kinetic (sliding) friction acts on an object already moving and is usually slightly smaller than the maximum static friction.

10. List two useful effects and two harmful effects of friction.

    Useful: provides grip for walking/braking, allows tyres to grip the road. Harmful: causes wear on moving parts and generates heat, wasting energy. Friction can be reduced with lubrication, bearings, or streamlining.

11. Define pressure and give its formula and SI unit.

    Pressure is force per unit area. $$P = \frac{F}{A}$$ where $P$ is pressure, $F$ is force in newtons (N) and $A$ is area in $\text{m}^{2}$. The SI unit is the pascal (Pa), where $1\ \text{Pa} = 1\ \text{N/m}^{2}$.

12. For a fixed force, how does pressure change as the contact area changes?

    Pressure is inversely proportional to area. A smaller area gives a higher pressure (e.g. a sharp knife), and a larger area gives a lower pressure (e.g. snowshoes spreading weight).

13. Give the formula for pressure at a depth in a liquid (fluid pressure due to a column).

    $$P = \rho g h$$ where $\rho$ is the fluid density ($\text{kg/m}^{3}$), $g$ is gravitational field strength ($\text{N/kg}$), and $h$ is the depth below the surface (m).

14. How does pressure in a liquid vary with depth and direction?

    Pressure increases with depth (it is proportional to depth). At any given depth, the pressure acts equally in all directions and does not depend on the shape or width of the container.

15. State Pascal's principle (law of transmission of pressure in fluids).

    A pressure applied to an enclosed, incompressible fluid is transmitted equally and undiminished to every point in the fluid and to the walls of its container. This is the basis of hydraulic systems.

16. How does the pressure of a gas behave compared with a liquid, and what is Boyle's law?

    A gas is compressible, so its pressure depends on volume and temperature. Boyle's law (constant temperature) states pressure and volume are inversely related: $$P_1 V_1 = P_2 V_2$$

17. What is a lever, and what are its three key parts?

    A lever is a rigid bar that turns about a fixed pivot (fulcrum) to transmit force. Its three parts are the effort (input force), the load (output force/resistance), and the fulcrum (pivot point).

18. Define a moment of a force and give its formula and unit.

    A moment is the turning effect of a force about a pivot. $$M = F \times d$$ where $F$ is the force (N) and $d$ is the perpendicular distance from the pivot to the line of action of the force (m). The unit is the newton metre (N m).

19. State the principle of moments for an object in equilibrium.

    For a body in rotational equilibrium, the sum of clockwise moments about any pivot equals the sum of anticlockwise moments about that pivot: $$\sum M_{\text{clockwise}} = \sum M_{\text{anticlockwise}}$$

20. A 20 N load sits 0.5 m from a pivot. What effort applied 2 m from the pivot on the other side balances it?

    Using moments: $20 \times 0.5 = F \times 2$, so $10 = 2F$, giving $F = 5\ \text{N}$.

21. Describe the three classes (orders) of levers with the arrangement of fulcrum, load and effort.

    Class 1: fulcrum between effort and load (e.g. seesaw, scissors). Class 2: load between fulcrum and effort (e.g. wheelbarrow, nutcracker). Class 3: effort between fulcrum and load (e.g. tweezers, human forearm).

22. Define mechanical advantage (MA) and give its formula.

    Mechanical advantage is how much a machine multiplies the input force. $$\text{MA} = \frac{\text{load (output force)}}{\text{effort (input force)}}$$ It is a ratio with no units.

23. How is mechanical advantage related to the arm lengths of a lever?

    For a lever the mechanical advantage equals the ratio of the effort arm to the load arm: $$\text{MA} = \frac{\text{effort distance from pivot}}{\text{load distance from pivot}}$$ A longer effort arm gives a greater MA.

24. In a single fixed pulley, what is the mechanical advantage and what is its purpose?

    A single fixed pulley has a mechanical advantage of 1 (it does not multiply force). Its purpose is to change the direction of the force, so you can pull down to lift a load up.

25. In a single movable pulley, what is the mechanical advantage and what is the trade-off?

    A single movable pulley has a mechanical advantage of 2 (the load is shared by two rope sections). The trade-off is that you must pull twice the distance (or rope length) to raise the load.

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