Royal Navy Recruiting Test (RT) Mechanical Comprehension Test Syllabus

What is the purpose of an idler gear in a gear train?

An idler gear sits between driver and driven gears to reverse the direction of rotation (making them turn the same way) without changing the overall gear ratio.

Why is an inclined plane (ramp) considered a simple machine?

It lets a load be raised using a smaller effort over a longer distance. A longer, gentler slope needs less force but more distance to gain the same height.

How are a wedge and a screw related to the inclined plane?

A wedge is two inclined planes back-to-back that converts a pushing force into a splitting/sideways force. A screw is an inclined plane wrapped around a cylinder, converting rotation into linear motion.

What is a wheel and axle, and how does it provide mechanical advantage?

A wheel fixed to a smaller axle so they rotate together. Effort applied at the large wheel's radius moves a larger load on the small axle; MA $= \dfrac{\text{wheel radius}}{\text{axle radius}}$.

How does a winch make lifting heavy loads easier?

A winch uses a wheel-and-axle: a long handle (large radius) winds a rope onto a small drum (small radius), so a small effort over a long handle path lifts a heavy load a short distance.

Define pressure and give its formula and unit.

Pressure is force per unit area: $P = \dfrac{F}{A}$, measured in pascals (Pa), where $1\ \mathrm{Pa} = 1\ \mathrm{N/m^{2}}$.

Why does a sharp knife or a drawing pin cut/pierce more easily than a blunt one?

A small contact area concentrates the same force into a much higher pressure ($P = F/A$), so the material is penetrated more easily.

What is the key principle behind hydraulic systems?

Pascal's principle: pressure applied to an enclosed fluid is transmitted equally throughout. Since liquids are nearly incompressible, force is transmitted through the fluid.

How do hydraulic systems multiply force? Give the relationship.

Pressure is equal in both cylinders, so $\dfrac{F_{1}}{A_{1}} = \dfrac{F_{2}}{A_{2}}$. A larger output piston area gives a larger output force, multiplying force in proportion to the area ratio.

A hydraulic press has an input piston of area $0.01\ \mathrm{m^{2}}$ and output piston of $0.05\ \mathrm{m^{2}}$. An input force of $100\ \mathrm{N}$ gives what output force?

$F_{2} = F_{1}\dfrac{A_{2}}{A_{1}} = 100 \times \dfrac{0.05}{0.01} = 500\ \mathrm{N}$.

State the principle that determines whether an object floats or sinks.

Archimedes' principle: an object floats if the upthrust (weight of fluid displaced) is greater than or equal to its weight; it sinks if its weight exceeds the maximum upthrust. Equivalently, it floats if its density is less than the fluid's.

What is upthrust (buoyancy force) equal to?

Upthrust equals the weight of the fluid displaced by the object. A floating object displaces a weight of fluid equal to its own weight.

What are pneumatic systems and what fluid do they use?

Pneumatic systems use compressed air (a gas) to transmit force and do work. Unlike hydraulics, the gas is compressible, so the system is springier and stores energy.

Give one key difference between hydraulic and pneumatic systems.

Hydraulics use incompressible liquid (high force, rigid, precise); pneumatics use compressible air (faster, lighter, cushioned but less force and less precise).

Define kinetic energy and give its formula.

Kinetic energy is the energy of a moving object: $E_{k} = \dfrac{1}{2}mv^{2}$, measured in joules (J).

Define gravitational potential energy and give its formula.

Stored energy due to height in a gravitational field: $E_{p} = mgh$, measured in joules (J).

State the principle of conservation of energy.

Energy cannot be created or destroyed, only transferred or transformed from one form to another; the total energy in a closed system stays constant.

When an object falls freely, what energy transfer occurs?

Gravitational potential energy is converted into kinetic energy as it falls; ignoring air resistance, the loss in $E_{p}$ equals the gain in $E_{k}$.

Define work done and give its formula and unit.

Work done is force times distance moved in the direction of the force: $W = F \times d$, measured in joules (J).

Define mechanical power and give its formula and unit.

Power is the rate of doing work: $P = \dfrac{W}{t}$, measured in watts (W), where $1\ \mathrm{W} = 1\ \mathrm{J/s}$.

State Hooke's law for a spring.

The extension of a spring is directly proportional to the force applied, up to the limit of proportionality: $F = kx$, where $k$ is the spring constant and $x$ the extension.

What is the difference between tension and compression in a structural member?

Tension is a pulling force that stretches a member (lengthening it); compression is a pushing force that squashes a member (shortening it).

What is the centre of gravity of an object?

The single point through which the whole weight of the object appears to act; the object can be balanced at this point.

What makes an object more stable in terms of its centre of gravity and base?

An object is more stable with a low centre of gravity and a wide base. It topples when its centre of gravity passes outside the edge of its base of support.

Compare the properties of metals and ceramics relevant to mechanical use.

Metals are generally strong, tough, ductile and good conductors. Ceramics are hard and heat-resistant but brittle (they crack rather than bend).

Define the material properties: hardness, toughness, ductility and brittleness.

Hardness: resistance to scratching/denting. Toughness: ability to absorb energy without fracturing. Ductility: ability to be drawn into wire/deform plastically. Brittleness: tendency to shatter with little deformation.

Which hand tool is correctly used for each task: turning a hex nut, checking a surface is level, and driving a nail?

A spanner (wrench) turns a hex nut, a spirit level checks for level/horizontal, and a hammer drives a nail.

In a belt or chain drive between two pulleys/sprockets, how do their sizes affect speed?

The smaller wheel turns faster; speed is inversely proportional to diameter (or teeth for sprockets). A crossed belt also reverses the direction of rotation.

When reading a mechanical diagram of two meshed cogs, if the driver turns clockwise, which way does the driven cog turn?

The directly meshed driven cog turns anticlockwise (the opposite direction). Each additional meshed cog reverses direction again.

What controls the rate of fluid flow in a pipe, and how does a tap/valve work?

A valve or tap controls flow by opening or restricting the passage. Opening it more increases flow; closing it reduces or stops flow. A non-return (one-way) valve allows flow in only one direction.

In a practical lifting problem, why does using a longer ramp or more pulley sheaves reduce the effort needed, and what is the trade-off?

Both increase mechanical advantage so less force is needed, but the trade-off is that the effort must move through a greater distance — total work done (force × distance) stays roughly the same (energy is conserved).