chap 4 - dynamics

• effects of a force on the motion of a body

  • when object is travelling at zero acceleration, its is either stationary / at rest/ moving or constant velocity ( resultant forces is zero)
• effects of a force on the motion of a body

  • a body at rest move : eg : a football moves when it is being kicked = change in speed
  • a moving body increases in speed : a moving hockey ball moves faster when it is pushed = increase in speed
  • a moving body decreases in speed : a descending parachutist slows down due to air resistance = decrease in speed
  • a moving body changes direction : a moving tennis ball is returned when it is being hit = change in direction
• resultant force

  • force is a vector quantity with both magnitude and direction
  • to determine resultant force for forces acting in a straight line, a direction is assigned as positive
  • when there are more than one force acting on an object, the direction of each force needs to be considered in order to find the resultant force
• effect of a force on the motion of a body

  • when force is applied to an object, there is a change in speed and/or direction overtime which implies that there is a change of velocity
  • since there is a change in velocity, it means that there is an acceleration or deceleration
  • hence, a force can cause an object to accelerate or decelerate
• newton´s law

  1. resultant force

  • force acting on an object is zero, we say the forces acting on the object are balanced (cancel out each other)
  • direction is always positive
  • if the resultant force acting on the ball is zero, the forces are balanced
  • if the resultant force acting on the ball is not zero, the forces are unbalanced
  • every object will continue in its state of rest/ uniform motion in a straight line unless resultant force acts on it
• newton´s law

  2. resultant force not zero

  • resultant force acts in an object of a constant mass, the object will accelerate in the direction of the resultant force
  • formula : resultant force = mass x acceleration
  • resultant acting on the object is not zero = forces acting on the object are unbalanced
• • when forces are balanced, resultant forces is zero and there will be zero acceleration
  • when there is zero acceleration, the object will remain at rest and object in motion will remain in motion at constant velocity
• • when forces are unbalanced, the resultant force is not zero and there will be acceleration
  • when there is acceleration, object at rest will start moving or object in motion will change its velocity (sped and/or direction)
• newton´s third law

  • equal in magnitude but act in opposite directions
  • for every actions, there is an equal and opposite reaction
  • eg : when air being let out of the balloon, balloon moves upwards

  : swimming : feet exert a force on the pool wall by pushing against it. the wall exerts a reaction forces on the boy´s feet and this force push the boy forward

  • equal magnitude but opposite direction
  • action and reaction are=

  1. equal in magnitude
  2. opposite in direction
  3. different in bodies

  • each action and reaction pair of forces must be of the same nature
• free-body diagrams

  • using arrows to represent the forces acting on individual objects
  • drawing a free-body diagram of an object helps us identify and visualise the forces and their effects on the object
  • used as visual aids to help solve problems about forces
• friction and its effects

  • positive effects of friction

  > walk without slipping

  > moving objects are able to slow down when needed

  • negative effects of friction

  > cars are less efficient by up to 20%

  > moving parts in engines, motor and machines suffer wear and tear
• friction

  • contact force that opposes or tends to oppose motion between surfaces in contact
  • results of irregularities of the surfaces
  • objects moving through a liquid or a gas experiences friction
  • resistive force cuz it acts in the opposite direction of motion
• enhancing positive effects of friction despite the negative effects of friction (friction can be useful too!)

  • using treads

  > friction is important to motor vehicles

  > without friction, a vehicle cannot stop moving as its tyres will just spin at the same spot

  > friction also enables the tyres to grip the road surface and roll without slipping
• enhancing positive effects of friction despite the negative effects of friction (friction can be useful too!)

  • using parachute

  >air resistance is a type of friction in air

  > to speed up, reduce air resistance by reducing the surface area in contact with the air

  > to slow down, increase the air resistance by increasing the surface area in contact with the air
• enhancing positive effects of friction despite the negative effects of friction (friction can be useful too!)

  • using chalk

  >rock climbers need to have a firm grip on the rock surface with their hands and feet

  > they usually use chalk powder on their hands to absorb perspiration and improve their grip