chapter 4

• effects of a force on the motion of a body

  a body at rest moves

  a moving body increases in speed

  a moving body decreases in speed

  a moving body changes in direction
• zero acceleration

  object can be stationery or moving with constant velocity

  although acceleration is zero, it does not mean there are no forces acting on the object, it just means that the resultant of these forces is zero
• balanced forces

  when two forces acting on the object is equal but act in opposite directions

  resultant force is zero and object remains stationery or continues moving at a constant velocity
• inertia

  mass is a property that resists change in motion, also known as inertia

  the greater the mass of an object, the greater its inertia

  the larger the mass of an object, the harder it is for the object to slow down, start moving, move faster or change direction
• unbalanced forces


  if the resultant force acting on an object is not zero, forces acting on an object is unbalanced

  when there is resultant force acting on an object, the object will accelerate in the direction of the resultant force
• action reaction


  the boy's feet exerts a force on the pool wall by pushing against it, the wall then exerts a reaction force on the boy's feet. this force pushes the boy forward

  both action and reaction forces are equal in magnitude but act in opposite directions

  for every action, there is an equal and opposite reaction
• newton third law of motion tells us 4 characteristics of forces

  forces always occur in pairs. each pair made up of an action and a reaction forces

  action and reactions forces are equal in magnitude, but act in opposite directions

  action and reaction forces act on different bodies
• friction

  opposes motion between surfaces in contact due to surface irregularities
• positive effects of friction

  can walk without slipping

  moving objects are able to slow down when needed
• negative effects of friction

  cars are less efficient by up to 20 percent

  moving parts of engines, motors and machines suffer wear and tear
• reducing negative effects of friction


  reduce friction between surfaces in contact

  • using wheels
  • using ball bearings
  • using lubricant
  • using air cushion
• enhancing positive effects of friction


  increasing friction or air resistance between surfaces in contact

  • using threads
  • using parachutes
  • using chalk
• free fall

  can only occur if the only force acting on the object is its own weight, meaning falling of object is happening in a vacuum space

  undergoes constant acceleration of 10m/s^2

  direction of motion is downwards, towards the centre of the earth

  acceleration of the object is not dependent on the mass of the object
• air resistance

  opposes the motion of moving objects

  increases with the speed of objects

  increases with size or surface area of the object

  increases with density of air
• terminal velocity


  when the object is released, the only initial force acting on the box is the weight of the box hence the box accelerates downwards at 10m/s^2. as the box falls, the air resistance of the box increases, causing the resultant force to be less than the weight of the box. this means that acceleration is lesser than 10m/s^2. as air resistance increases with velocity, the air resistance acting against the box will be equivalent to its weight. the box will then travel at constant speed known as terminal velocity, where the box has zero acceleration.
• object falling from a short distance

  may not reach terminal velocity before hitting the ground