Types of Forces: Contact & Non-Contact

Types of Forces

• Forces include pushes or pulls on an object due to interaction
• When interaction stops, forces stop
• Forces are split into Field (Non-contact) and Contact Forces
• Forces can be described on a mass, charge and current-carrying conductor in gravitational, electric, and magnetic fields

Non-Contact Forces

• Non-contact forces don't require physical contact to push or pull
• Examples encompasses gravitational, electric, and magnetic forces

Different Types of Non-Contact Forces

• Gravitational force exists between masses and is always attractive

• Electric Force exists between charges, which can be attractive or repulsive:

  • Like charges repel
  • Unlike charges attract

• Magnetic Force exists between magnets, attracting or repelling:

  • Like poles repel
  • Unlike poles attract

Force Fields

• Force Field is how objects interact at a distance
• Three types of fields includes gravitational, electric, and magnetic fields

Gravitational Fields

• Gravitational Field produced by a mass in space experiences gravitational force from another mass
• Gravitational field strength measures gravitational force per unit mass at a point
• Gravitational force and field are in the same direction

Electric Fields

• Electric Field around a charge experiences electric force from another charge
• Electric force direction on a positive charge matches the electric field direction
• Polarity reverses for negative charges

Magnetic Fields

• Magnetic Field results from magnets or current-carrying conductors
• Poles inside experience magnetic force
• Current-carrying conductors or moving charges inside of this field experiences magnetic force, if the field and direction of current is not parallel
• Magnetic force is perpendicular to the field and motion vectors

Contact Forces

• Contact Forces occur with physically touching interactions
• Encompasses normal contact force, frictional force, tension, viscous forces (air resistance), buoyant force (upthrust)

Normal Contact Force

• Stone at rest on a road has rough surfaces making contact at few points
• The road exerts force at contact points
• Magnitude of normal contact force depends on forces acting on a surface
• Direction of normal contact force is perpendicular to the contact surface, through the object of interest

Frictional Force

• Frictional Force occurs from surface resistance, when an object moves or tries to move across it

• Magnitude of friction depends on contact surface roughness

• Friction direction opposes intended motion, parallel to the surface

• Stone slides on the road to the left with frictional forces against its motion

• Reaction force results from combined normal contact and frictional forces

• For wheels, when moving forward, the contact point with the ground tries to move backwards

• Hence, friction acts forward, pushing the wheel

• Reaction force is the resultant of both friction and normal contact force

Tension

• Tensional Force transmits through objects pulled tight at ends

• Force applied in equilibrium is constant throughout the massless string

• Microscopic level reveals atoms joined via molecular bonds acting as tiny springs

• Rope tension and tensional force experienced by an object are the net spring force from molecular bonds

• Tension varies according to extension for springs

• Tension direction is along the string/rope/cable, away from the object

Forces in Springs (Tension and Compression)

• Springs exert pushing/pulling contact forces
• Extended spring's pulling force mirrors tension
• Force vector tail is placed on the particle in force diagrams

Hooke's Law

• Magnitude of force F exerted by a spring on an attached body is proportional to extension x from equilibrium, up to the limit of proportionality
• F=kx, where:
  • F is the load exerted by the spring (N)
  • k is the spring or restoring force constant (Nm⁻¹)
  • x is the extension or compression (m)
• Note: Hooke's Law applies to ideal spring compression within proportionality limit

Centre of Gravity (CG)

• Weight W of an object sums gravitational forces on individual parts
• Approximates a single force W acting at a Centre of Gravity
• Centre of Gravity is the point where the entire weight acts
• Centre of gravity for regularly shaped objects of uniform thickness and density lies at the geometrical centre (intersection of symmetry lines).

Turning Effect of Forces

• Forces can create turning effects, also known as moment of force or torque

Moment of a Force (Torque of a Force)

• Moment of Force is the turning effect about a point, equaling force times perpendicular distance from the line of action to the point
• Use T = F x d

Torque of a Couple

• Couple refers to equal and opposite forces on an object, causing rotation
• The Resultant force is zero
• Torque of a Couple: product of one force and perpendicular distance between forces' lines of action

Equilibrium of Forces

• Refers to systems with balanced forces

Equilibrium Systems LO(h)

• Equilibrium requires no net force in any direction and no net torque at any point
• Static Equilibrium: object at rest

Translational Equilibrium

• Translational Equilibrium means the object isn't accelerating linearly
• Sum of forces equals zero
• Vector addition forms a closed polygon

Rotational Equilibrium LO(g)

• Rotational Equilibrium an object is not rotating or rotating at a constant angular velocity.

Principle of Moments

• Principle of Moments in rotational equilibrium dictates that the sum of clockwise moments about the pivot point equals counter-clockwise moments

Static Equilibrium

• Concept to describe a body at rest or at constant velocity
• Static Equilibrium is when the state the body is at rest

Steps in Solving Static Equilibrium problems

• Identify the object and model it simply
• Establish a coordinate system, showing the direction for positive resultant force and acceleration
• Make a free body diagram which includes ALL forces acting on it
• Sum of clockwise moments = sum of anti-clockwise moments
• Set up force equations (conditions for translational equilibrium)
• The sum of positive moments must be equal to the sum of negative moments.
• Solve the equations to find the unknown forces