Physics Statics and Moments

Statics

• A force causes a change in a body's state of motion. It can start, stop, accelerate, or decelerate a mass.
• Forces can do work if energy is available. Forces acting in different directions can be resolved to calculate a resultant net force.
• Equilibrium occurs when the forces acting on an object sum to zero, meaning the object's state of motion or rest doesn't change.

Moments and Levers

• The moment of a force is the force multiplied by the distance from the fulcrum (pivot).
• A system is balanced when the load moment equals the effort moment.
• A lever is a simple machine that can be used to gain mechanical advantage (MA). MA is calculated as Load/Effort.
• The purpose of a lever is to perform work, moving a load with less effort, pivoting around a fulcrum. A positive MA means the load moved is larger than exerted effort.
• A crowbar is an example of a first-class lever. Cockpit control levers (throttle and thrust) and wheelbarrows are examples of second-class levers.

Third Class Levers and Velocity Ratio

• A third-class lever has the effort between the fulcrum and the load (e.g., aircraft landing gear retraction mechanism).
• A velocity ratio is the direct ratio of two speeds in the same system.
• If a pulley system has an MA of 4, the operator would move 4 times faster than the raised load.

Couples

• A couple is a moment derived by two equal forces acting in opposite directions on two different points of a body.
• Couples produce a torque (twisting force) causing rotation.

Centre of Gravity (CG)

• The center of gravity (CG) of a body is the point where the weight appears to act, regardless of the body's position.
• The CG of regularly shaped bodies with uniform density is at the geometric center.
• The CG of irregularly shaped solids is the intersection of lines drawn vertically from different points of support.

Stress, Strain, and Elasticity

• Stress is the force acting through a section of a solid material, defined as force per unit area.
• Strain is the deformation of a material due to stress.
• If strain is less than the elastic limit, the material will return to its original shape. Within the elastic limit, strain is proportional to stress (Hooke's Law). Doubling stress doubles the strain.
• Tension describes forces pulling an object apart (e.g., flexible steel cables in aircraft).
• Compression describes forces pushing an object together (e.g., aircraft weight on a runway).
• Shear stress occurs when external forces cause adjacent material layers to slide over one another.

Torsional Stress

• Torsion or torque is a form of shear stress involving twisting forces.
• Twisting a rod fixed at one end can make sections of material slide over each other.
• This results in compression stress in the direction of the twist and tension stress in the opposite direction.
• Alternating compression and tension stresses on the wing require careful material design to prevent fatigue.

Residual Stress

• Abrupt or uneven temperature changes can cause internal stress in metals, potentially causing component failure even with low external stress.

Pressure and Buoyancy

• Pressure is defined as force per unit area. Pressure in fluids is caused by the continuous bombardment of molecules against the container.
• Pressure exerted by a liquid column depends on the vertical height of the column, gravity, and the liquid's density (i.e., liquid mass per unit volume).
• Density isn't affected by shapes or sizes of substances.
• Liquids are incompressible; gases are compressible.
• Buoyancy is the upward force on an object in a fluid. An object floats if the buoyant force is greater than its weight. Archimedes' principle states that an object submerged in a fluid displaces a volume equal to its own volume and is supported by an upward force equal to the weight of the displaced fluid.