Chap 3: Forces and Energy

• Forces and motion

  1. Stationary objects start to move

  2. Moving objects move faster

  3. Moving objects slow down (and eventually become stationary)

  4. Moving objects change direction

  5. When a net force acts on the object
• Net force


  The sum of ALL the forces acting on the object
• Net force on a ball rolling down a sloping road

  • Friction + (-weight) = unbalanced
• The weight is added as a negative value
• Other examples of unbalanced forces

  • Parachutes slow down a falling object
• Weight


  Force pulling an object downwards
• Air resistance


  Force acting in the opposite direction of weight as an object falls through air
• The more air particles that brush against the object, the greater the air resistance
• Unbalanced forces and change in direction


  1. Weight pulls the ball down

  2. When it touches the floor, contact force is greater than the weight

  3. Ball bounces upwards
• Average speed

  Total distance / total time
• Objects don't always move at a constant speed
• Speed can change slightly during a journey
• Unless specified by question, unit for speed should always be m/s
• In 1905, Einstein showed that the universe does have a speed limit. Nothing can travel faster than 300,000 km/s, the speed of light in a vacuum
• Peer review

  A form of collaboration between scientists involving checking and confirming the findings/ results of other Scientists
• Distance-time graphs

  Displays the journey of an object, how the distance of an object changes from it's original position, and how long it takes for the object to travel a certain distance
• Time is represented by the X-axis, Distance is represented by the Y-axis
• The gradient of a distance time graph = speed of the object
• Turning forces


  A force applied on an object can cause the object to change direction
• Lever


  The object that turns/rotates
• Pivot


  The fixed point around which the object rotates/turns
• Moment of a force

  The turning effect of that force on an object
• Directions of moment of a force


  • Clockwise
  • Anti-clockwise
• Factors affecting moment of a force


  • The size of the force
  • The distance between the force and the pivot
• Principle of moments


  If an object is balanced: total clockwise moment = total anticlockwise moment (around the same pivot)
• Pressure


  The pushing effect of a force
• • Friction and weight act in opposite directions.
  • Friction and weight acting on the ball are unbalanced.
  • Weight is a larger force acting on the ball compared to friction.
  • The net force is a force acting in the direction of the  weight.
  • The ball moves in the direction of the weight.