Mr Kidman's Classroom
Revise the key ideas
Forces and structural performance
- Loads and forces — A structure supports loads and transfers forces through its members (parts such as beams) and connections. Tension pulls, compression pushes, bending curves and torsion twists. Real components may experience several at once. Identify where a load acts and how it reaches the supports before choosing a material or adding reinforcement; a decorative brace placed away from the load path, the route forces take through parts and joins to the supports, may do little.
- Stability — A product must resist tipping as well as breaking. A wider support base and a suitably positioned centre of mass can improve stability, depending on the load and surface. A tall light stand can tip under a sideways force even if its material is strong. Test the intended loading conditions safely and explain the difference between strength and stability.
- Shape and stiffness — Changing a member’s cross-section, the shape seen if it is cut straight across, can improve resistance to bending without simply using a solid block. Folding card into a channel or tube changes where material is placed in the section and can increase resistance to bending in a particular direction. Orientation matters: the same shape can bend differently when turned. Compare samples of equal material fairly and record load and deformation, meaning change of shape.
- Bracing and triangulation — A triangle keeps its shape when side lengths are fixed, while an unbraced rectangular frame can distort at flexible joints. A diagonal brace can resist that movement. It still needs suitable joints and must work under the relevant force direction. Adding a line to a drawing does not prove that a physical brace can transfer the required load.
- Testing a structural element — Define the test load, span, supports and measurement method before comparing beams or frames. Keep relevant conditions constant and increase load only under safe teacher-controlled arrangements. Compare deformation and any failure location. A short model's success does not automatically prove that a full-size structure will behave identically; scale and material differences matter.
Movement and simple machines
- Movement types — Rotary motion turns about an axis, linear motion follows a straight path, reciprocating motion moves back and forth and oscillating motion swings about a pivot. Mechanisms can transmit or convert these movements. Describe both the input and output, such as a rotating shaft producing a repeated lift, rather than only naming the moving parts.
- Levers and moments — A lever turns about a fulcrum. The turning effect, or moment, equals force multiplied by the perpendicular distance from the pivot. A 10 N force acting 0.2 m from a pivot has a moment of 2 N m. A longer effort arm can reduce the force required for the same load moment, but the effort usually moves farther.
A 10 N downward force acts perpendicular to a horizontal lever, 0.2 m from its pivot, giving a clockwise moment of 2 N m. The schematic is not to scale. Full-size diagram - Mechanical advantage and trade-offs — An ideal mechanism can exchange force for movement distance; it does not create energy. If an ideal lever doubles output force, the input must move farther than the load. Real friction and deformation reduce performance. Explain what is gained and given up rather than claiming that a mechanism makes every aspect of a task easier without cost.
- Linkages — Connected rigid links (parts that keep their shape), joined at pivots, can transmit or alter motion. The lengths and pivot positions determine the output path and range. A parallel-motion linkage can keep an output member approximately parallel as it moves. Make a model of the actual sizes and arrangement and check for restricted movement, weak joints and trapping points under supervision.
Compare two positions of a teacher-selected parallel-motion linkage. Identify which bars stay approximately parallel while the moving part changes position.
- Cams and followers — A rotating cam controls a follower's movement through its changing profile. Different profiles can create gradual rises, pauses or sudden drops. The follower needs suitable guidance and contact, and the design must manage friction and any return force. Name the desired output motion before selecting a profile; a cam is not simply an oddly shaped wheel with no functional relationship to its follower.
The follower is the part moved by the cam. The cam profile is its outside shape. A pause means the follower stays at one height for a while. Follow labelled positions on the model to connect the shape with a rise, pause or fall.
Gears and belt systems
- Meshing gears — Gears transmit rotary motion through teeth. Two external gears in mesh turn in opposite directions. If a 10-tooth driver turns a 20-tooth driven gear, the driven gear makes one turn for every two driver turns in the ideal model. This reduces output speed and can increase turning force, with real losses and strength limits.
- Gear trains and idlers — An idler gear can change the output direction or spacing in a simple gear train without altering the overall speed ratio set by the first and final gears. A mesh is a place where two sets of teeth engage. Count the meshes to track direction. Distinguish the role of each gear rather than multiplying every tooth count indiscriminately; more gears also add friction and alignment demands.
In a simple train with a 10-tooth first gear and 20-tooth last gear, an idler between them does not change the overall speed ratio: two turns of the first give one of the last in an ideal model. Each external mesh reverses direction, so two meshes make first and last turn the same way. The idler must still fit and work in the actual arrangement.
- Belts and pulleys — A belt can transmit rotary movement between separated pulleys. With an uncrossed belt, the pulleys normally rotate in the same direction; different diameters change speed in an ideal no-slip model. Slippage, tension and grip affect real behaviour. Guards and spacing matter because moving belts and pulleys create trapping hazards.
- Rack and pinion — A rotating toothed pinion engaging a straight rack converts rotary to linear motion or the reverse. Guides keep the rack aligned and allow it to carry the intended load. Compare its travel and controllability with a lever or cam for a specific product. The teeth and supports must be designed for force and wear rather than just drawn to fit visually.
Combining and evaluating mechanisms
- Friction and lubrication — Friction opposes relative motion and can cause energy losses, heating or wear, but it can also provide needed grip. Bearings, alignment and suitable lubrication can reduce unwanted friction. A belt drive needs enough friction to transmit movement, so 'remove all friction' is not a sensible universal objective. Use only the school-approved materials and maintenance methods.
- Choosing a mechanism — Start with the required input, output, force, speed, space and user. A slow strong lift and a fast light spinner have different needs. Compare mechanisms against the same criteria, build an appropriate model and measure behaviour. A theoretical ratio predicts an ideal relationship; real tests reveal slipping, flexing and other effects that the simple model omits.
