Understanding Types of Forces

Types of Forces

• A force is described as a push or pull that stems from the interaction between objects.
• Forces that exist include: gravitational force (weight), reaction force, friction, drag force, air resistance thrust, and upthrust

Gravitational Force

• The gravitational force, also known as weight, indicates this pull for an object with mass.
• Gravitational force means that all objects with mass attract.
• The more massive an object is, the larger the gravitational force becomes.

Reaction Force

• Whenever an object rests on a surface, the surface exerts a push force on the object.
• Reaction force acts at right angles (perpendicular) to the surface.

Friction

• Friction is defined as a force opposing the motion of an object
• Frictional forces always act in the opposite direction to the object's motion.
• Friction happens when two or more surfaces rub against each other.
• At a molecular level, imperfections in surfaces lead them to push against each other.

Drag Force

• Drag force is a type of frictional force that appears when any object moves through a fluid (liquid or gas)
• Particles in a fluid collide with the object, slowing its motion.

Air Resistance

• Air resistance is a kind of drag force, opposing motion as particles of air collide with an object.
• Air resistance reduces a skydiver's speed.

Thrust

• Thrust, produced by an engine, quickens the motion of an object
• A car's engine applies thrust force, increasing motion.

Upthrust

• When an object is fully or partially submerged in a fluid, the fluid exerts an upward-acting push force on the object
• Upthrust allows boats to float
• Upthrust forces a ball held underwater to surface once released.

Electrostatic Force

• There is an electrostatic force between two objects with charge.
• Opposite charges attract, while like charges repel each other.

Magnetic Force

• Magnetic force exists between objects with magnetic poles.
• Like magnetic poles repel each other, while opposite poles attract.

Tension

• Tension happens in a stretched object like a rope or spring.
• When each end of an object is acted on by a pull force, tension acts across the length of the object.

Effects of Forces

• Can change the speed of an object
• Can change the direction of an object
• Can change the shape of an object
• The effects of forces exerted on an object often depend on the type of force acting
• The push force (thrust) of an engine can cause a car to speed up
• Friction exerted by the brakes can cause a car to slow down
• The Sun's gravitational pull can affect a comet's direction.
• Opposing forces on either end can compress shape.

Scalar & Vector Quantities

• All quantities can be classified as a scalar or a vector.

Scalars

• Scalars are quantities that have magnitude but not direction.
• Mass is classified as a scalar quantity because it has magnitude but no direction

Vectors

• Vectors have both magnitude and direction.
• The quantity of weight is classified as a vector since it has magnitude and direction.

Distance and Displacement

• Distance measures how far an object has travelled regardless of the direction.
• Displacement measures how far it is between two points in space that includes the direction.
• Because it measures total path length, distance has magnitude but no direction, making it a scalar quantity.
• Displacement is a vector since it has magnitude and direction, describing the length and direction of a straight line from start to finish.

Speed and Velocity

• Speed measures the distance travelled by an object over time, regardless of direction.
• Object speed describes how fast it is moving.
• Speed is a scalar quantity due to magnitude and no direction.
• Velocity measures the displacement of an object over time, including the direction.
• The velocity of an object describes how fast it is moving and the direction it is traveling.

Examples of Scalars & Vectors

• Distance is a scalar while displacement is a vector.
• Speed is a scalar, velocity is a vector.
• Mass is a scalar while weight is a vector.

Forces as Vectors

• Vector quantities use arrows.
• Arrow length indicates magnitude.
• Arrow direction shows direction.

Calculating Resultant Force

• Resultant force means a single force describing all forces acting on an object/body.
• Combined forces produce one net force, describing the combined effect of all of the forces.
• Forces in opposite directions subtract.
• Direction of moment is equal to that of the force with the bigger moment
• Forces in the same direction add.
• Equal forces in opposite directions means no resultant force.

Friction Defined

• Friction is a force opposing the motion of an object.
• Molecular level imperfections on surfaces cause Friction forces when two or more surfaces rub against each other

Unbalanced Forces

• Unbalanced forces do not completely cancel, creating a resultant force.
• Combined forces allow the object to accelerate
  • Object might speed up
  • The object might slow down
  • The object might change direction

Balanced Forces

• Balanced forces mean combined forces cancel each other out, so there is no resultant force on the body.
• Weight of book on desk is balanced by the normal force of the desk.

F = m x a

• The relationship between resultant force, mass, and acceleration uses the equation:
• This equation is also known as Newton's second law of motion
• F = Resultant force (N)
• m = Mass (kg)
• a = Acceleration (m/s²)

Weight

• Weight is the force an object experiences due to its mass in a gravitational field.

Weight and mass contrasted

• Weight and mass are different in physics.
  • Mass is a measure of matter in an object.
  • Mass has magnitude but not any direction.
    • Mass is scalar.
  • Weight is a force.
    • Forces have magnitude + direction.
      • Weight is a vector.

Gravitational Field Strength

• Planets have strong gravitational field strengths.
• Planets attract nearby masses with strong gravitational forces.
• Different planets mean that they have different gravitational strengths depending on mass.

W = mg

• Weight, mass, and gravitational field strength use the equation:
• W = weight, measured in newtons (N)
• m = mass, measured in kilograms (kg)
• g = gravitational field strength, measured in newtons per kilogram (N/kg).
• Gravitational field strength on Earth = 10 N/kg
• g acceleration of freefall in gravitational field
• Earth acceleration of freefall = 10 m/s²

Stopping Distance

• Stopping distance is the total distance travelled during an emergency stop.
• Stopping distance equals the sum of the distance travelled as the person decides to stop and travels as the driver applies the brakes.

Stopping Distance Formulas

• The stopping distance is calculated using the equation:
  • Stopping distance = Thinking distance + Braking distance.
  • Thinking distance is the distance travelled when the driver reacts to an emergency.
  • Braking distance is the distance achieved under the braking force.

Thinking Distance

• The primary factors affecting your thinking distance:
  • The speed of the car
  • Driver reaction time
• Reaction time = how much time passes between seeing something and reacting to.
• Average reaction time of a human = 0.25 s

Braking Distance Factors

• The factors affecting the braking distance are:
  • Speed
  • Road Conditions affect braking effectiveness
  • Vehicle mass plays a role in braking efficiency
  • Worn tyres increase braking distance

Terminal Velocity

• Terminal velocity is the fastest speed an object can reach when falling.
• Terminal velocity is reached when the upward and downward acting forces become balanced.
• Resultant force on object = zero.
• Constant terminal velocity means the object no longer accelerates

Falling Objects

• Falling objects experience two forces:
  • Weight
  • Air resistance
• Air resistance increases as the object speeds increases.
• Air resistance increases because the object collides with air particles.

Investigating Force & Extension

• Aims to investigate the relationship between force and extension for a spring and a rubber band

Equipment

• Clamp and stand
• Ruler
• Spring and rubber band
• 5 x 100g masses
• 100g mass hanger
• Pointer (fiducial marker)
• G-clamp

Hooke's Law

• The relation between the extension of an elastic object and the force applied uses Hooke's Law.
• Extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality.
  • Doubling the force, doubles the extension.
  • Halving the force, halves extension.