Topic 9 Forces and their Effects

• What is an example of interaction of objects at a distance without contact?

  Gravitational attraction
• How does gravitational attraction depend on mass?

  The larger mass gives greater attraction
• What is the effect of a larger charge in electrostatic interactions?

  A larger charge gives greater force
• What happens when like charges interact?

  Like charges repel each other
• What is the behavior of opposite charges in electrostatic interactions?

  Opposite charges attract each other
• What is the effect of a stronger magnet in magnetic interactions?

  A stronger magnet gives a stronger field
• How do like poles behave in magnetic interactions?

  Like poles repel each other
• What is the behavior of opposite poles in magnetic interactions?

  Opposite poles attract each other
• What are examples of interaction of objects with contact?

  • Normal contact force: Force is perpendicular to the plane of contact
  • Friction: Rough surfaces cause friction when moved
• What defines a vector?

  A vector has size and direction
• Give an example of a vector.

  A force of 10N directed downwards
• What is a scalar?

  A scalar has just size
• Give an example of a scalar.

  Mass
• What do free body diagrams represent?

  • They show the direction of forces present in a situation
  • The reaction force acts normal to the line of contact
  • Friction acts opposite to movement along the line of contact
  • Weight always acts downwards from the Centre of Mass
• What does the length of each arrow in a scale drawing represent?

  The length represents its size in relation to other forces acting on the object
• What does it mean if arrows in a scale drawing are in opposite directions with equal length?

  The forces cancel out, and the object is in equilibrium

  
• What happens when an object is in equilibrium?

  The object travels at a constant velocity
• What happens to the drag force at point B in the diagram?

  The drag is a lot less than the weight, causing acceleration
• What does a smaller resultant force at point C indicate?

  It indicates smaller acceleration
• What does an isolated solid system mean?

  No forces are present that come from a source outside the system
• What forces act on a skydiver?

  Air resistance and weight
• What happens to the forces acting on a skydiver as they fall?

  Initially, only weight acts, but air resistance increases as speed increases
• What is the relationship between resultant force and acceleration for a skydiver?

  As the resultant force decreases, acceleration decreases according to $F=$$ma$
• What occurs when weight and air resistance are equal for a skydiver?

  There is no resultant force, so no acceleration and terminal velocity is reached
• What is the initial condition for a vehicle regarding air resistance and thrust?

  Initially, low air resistance and thrust is only hindered by friction
• What happens to the resultant force as air resistance increases for a vehicle?

  The resultant force decreases
• What is terminal velocity for a vehicle?

  It is when thrust is balanced by drag and friction, resulting in no resultant force
• What happens when more thrust is added to a vehicle at terminal velocity?

  It becomes the resultant force until drag increases to balance it again
• What conditions are necessary for rotation to occur?

  • An object must be attached to a pivot point
  • A force must be applied not towards the pivot
  • If the force is applied perpendicular, it will rotate about the pivot
  • If not perpendicular, find the perpendicular distance from pivot to line of force
• What is the formula for moment of a force?


  Moment = $F \cdot d$
• What is the unit of moment?

  Newton metre (N m)
• How does pressing your foot down on a bike pedal create a moment?

  It causes a moment about the pivot, turning the pedal arms
• What is the condition for equilibrium in terms of moments?

  Equilibrium occurs when:

  • Sum of anticlockwise moments = sum of clockwise moments
• How do gears function in terms of speed, force, and direction?

  • Gears can change speed, force, or direction by rotation
  • Smaller gear turns faster but with less force
  • Larger gear turns slower but with more force
  • The second gear always turns in the opposite direction
• What happens when a larger gear is used for the secondary gear?

  The momentum of the larger gear is greater due to the force being further from its pivot
• What is the purpose of lubrication in mechanical systems?

  • Reduces friction
  • Decreases unwanted energy transfer (less heat loss)
  • Increases efficiency