Physics Class: Force and Its Effects

Force and its effects

• Force causes a change in a body's state of motion
• Force application can: start, stop, accelerate or decelerate a mass
• Work can be performed if energy is available

Work and Resultant Force

• Forces can act in different directions
• Resultant force is calculated by resolving individual forces
• Equilibrium occurs when the resultant force on an object is zero; the object is not changing its state of motion or rest

Moments and Levers

• Moment is the force multiplied by the distance from the pivot point (fulcrum)
• Lever system is balanced when load moment equals effort moment
• Increasing effort force raises the load
• Lever is a simple machine providing mechanical advantage (MA)
• MA= Load/Effort
• Levers are used to perform work, moving a load with an effort, pivatig around a fulcrum.
• First-class levers example: crowbar
• Second-class levers example: cockpit control levers, wheelbarrow

Third-Class Lever

• Effort is between the fulcrum and the load,
• Example: retraction mechanism on an aircraft landing gear

Velocity Ratio

• Velocity Ratio (VR) is the direct ratio of two speeds in the same system
• Example: pulley system with 4 MA, rope moves 4 times faster than load is raised

Couples

• Couple is a moment derived from two equal forces acting in parallel but opposite directions on two different points of a body
• Produces a torque, or twisting force on a body

Centre of Gravity (CG)

• CG is the point where the weight of a body appears to act
• CG is the geometric centre of uniformly dense bodies
• CG of irregularly shaped objects is the intersection of verticals passing through different suspension points

Stress, Strain, and Elasticity

• Stress is the force acting through a section of a solid material, defined as force per unit area.
• Strain shows deformation of material due to stress
• Elasticity allows a material to return to its original shape below the elastic limit.
• Hooke's Law states stress and strain are proportional below elastic limit: doubling stress doubles strain.

Tension

• Tension describes forces that pull apart an object
• Example: steel cable in aircraft control systems

Compression

• Compression is the resistance to pushing a body together.
• Example: weight of an aircraft compressing runway, rivets.

Shear

• Shear stress occurs when external forces cause adjacent layers to slide over each other.
• Example: oil or grease on sliding metal surfaces

Torsional Stress

• Torsion or torque is a form of shear stress caused by twisting force
• Fixed at one end, material sections slide over each other
• Twisting results in compression stress in one direction and tension stress in opposing direction.

Residual stress

• Abrupt or uneven temperature changes cause internal stress (common in heat-treating metals)
• Can lead to component failure, even with low external stress.

Pressure and Buoyancy

• Liquids and gases are fluids
• Liquids are incompressible; gases are compressible
• Pressure is force per unit area
• Pressure in fluids is due to molecules bombarding container walls.
• Pressure from a fluid column depends on height, gravity, and fluid density.
• Archimedes' principle: an object immersed in fluid displaces equal volume of fluid, buoyed up to weight of displaced fluid.
• Floating if buoyant force is greater than weight

Density and Specific Gravity

• Density is mass per unit volume
• Specific gravity is a substance's density compared to water.
• Gasoline's specific gravity is .72; meaning weight is 72% of the same amount of water
• Gases compared to air yield SG.

Solids, Liquids, and Gases

• Solids have definite shape and volume, with strong interatomic forces

• Liquids have definite volume, but take shape of container

• Gases take both volume and shape of container; comparatively easy to compress.