Topic 9: Forces & Their Effects

• What are the Different Ways that Objects Can Interact?

  • Non-contact
  • Contact
  • Producing pairs of forces which can be represented as vectors
• What is Gravitational Force?

  The force of attraction between objects with mass

  • An object with larger mass gives a greater attraction
• What is Electrostatic Force?

  Forces of attraction and repulsion due to their electric charges

  • Opposite charges attract
  • Like charges repel
• What is Magnetic Force?

  A force of attraction or repulsion between two metal objects

  • Opposite poles attract
  • Like poles repel
• What is Contact Force?

  A force which acts between objects that are physically touching
• What are Examples of Contact Forces?

  • Friction
  • Air resistance
  • Tension
  • Reaction force / Normal force
• What is Friction?

  A force that opposes motion, & occurs when two objects rub against each other
• What is Air Resistance?

  A type of friction that occurs when objects move through the air
• What is Tension?

  A force that pulls two objects connected by a length, such as a string or rope, & occurs when a force is applied to the length
• What is Normal Contact Force?

  A force that pushes touching objects apart perpendicular to each other, & occurs when objects are supported by a surface
• What is a Scalar Quantity?

  A quantity that only has magnitude
• What is a Vector Quantity?

  A quantity that has both magnitude & direction.
• What are Some Examples of Scalar Quantities?

  • Mass
  • Distance
  • Speed
  • Energy
  • Temperature
• What are Some Examples of Vector Quantities?

  • Force
  • Displacement
  • Velocity
  • Acceleration
  • Momentum
• What is Non-Contact Force?

  A force which acts at a distance, without any contact between bodies, due to the action of a field
• What is a Free-Body Diagram?

  A diagram used to show the relative magnitude and direction of all forces acting upon an object
• Points to note for Free Body Diagrams:

  • The reaction force always acts at the normal to the line of contact, from the point of contact
  • Friction acts in the opposite direction to movement, along the line of contact
  • Weight always acts downward, acting from the centre of mass
• How Can Scale Diagrams Be Used to Represent Forces?

  • The direction of the arrow represents the direction of the force.
  • The length of the arrow represents the magnitude of the force
• When Does Rotation Occur?

  When an object is attached to a pivot point
• What is the Equation for the Moment of a Force?

  • Moment of a force (Nm) = Force (N) $\times$ Distance (m)
  • $Moment\ =$$\ F\times d$
• What is the Principle of Moments When Rotational Forces Are in Equilibrium?

  The sum of clockwise moments = the sum of anticlockwise moments
• What Does a Lever Consist Of?

  • Pivot
  • Effort
  • Load
• Simple Levers and Rotation:

  • As effort is applied to rotate one end about the pivot, the opposite end is also rotated about the pivot in the same direction.
  • This has the effect of rotating or lifting the load.
  • They allow a larger force to act upon the load than is supplied by the effort, so it is easier to move large or heavy objects.
• What is a Gear?

  Wheels with toothed edges that rotate on an axle or shaft.
• What happens when one gear turns?

  The other gear must also turn
• In which direction must the teeth of two meshing gears move?

  In the same direction
• What is the relationship between the rotation directions of two meshing gears?

  The gears rotate in opposite directions
• How do the forces acting on the teeth of two gears compare?

  The forces are identical for both gears
• What happens to the moment when a larger gear is driven by a smaller gear?

  The large gear rotates slowly but has a greater moment
• What happens to the moment when a smaller gear is driven by a larger gear?

  The smaller gear rotates quickly but has a smaller moment
• How Does Lubrication Increase Efficiency?

  Reduces friction, so reduces unwanted energy transfers (by heat, etc.)