Dynamics

• Balanced Forces:

  • When an object is at rest or moving at a uniform speed, there is no acceleration.
  • It means there is no resultant force acting on the object, but does not mean there are no forces acting on the object.
  • i.e. if resultant force = 0N, Fr = 0N -> object is at rest i.e. v =0m/s or object moves with a constant velocity
• Newton's First Law of Motion: Every object will continue in its state of rest or uniform motion in a straight line unless a resultant force acts on it to change its state of rest or motion.
• Unbalanced forces:

  • When two or more forces act on an object, the overall effect on the object is equivalent to a single (resultant) force acting on it which can be determined by vector addition.
  • If the resultant force on a body is not equal to zero, the forces acting on it are unbalanced and there will be a change in a body's initial velocity. i.e. it experiences an acceleration in the direction of the resultant force.
• Unbalanced forces:

  • The acceleration of an object is equal to the ratio of the resultant force acting on it and its mass. It is also directly proportional to the resultant force i.e. resultant force increases, acceleration increases and is inversely proportional to its mass i.e. mass increases, acceleration decreases
• Newton's Second Law of Motion:

  • When a resultant force acts on an object of a constant mass, the object will accelerate in the direction of the resultant force.
  • Equation: F (resultant force, N) = m (mass, kg) x a (acceleration, ms^-2)
• Newton's Third Law of Motion:

  • If body A exerts a force F^AB on body B, then body B will exert an equal and opposite force F^BA on body A
  • Forces always occur in pairs made up of an action and a reaction forces equal in magnitude, act in opposite directions and on different bodies.
• Free body diagrams:

  • a sketch that illustrates the type and direction of all the forces acting on a particular object
  • an important tool for analysing forces and solving mechanics problems in Physics

  
• Friction and its effects

  • the contact force that opposes or tends to oppose motion between surfaces in contact which is the result of irregularities of the surfaces
  • is a resistive force because it acts in the opposite direction to motion

  
• Reducing negative effects (Using wheels):

  • Being circular in shape, wheels greatly reduce the friction between the object and the floor
  • A smaller force can be applied to move the object around
• Reducing negative effects (Using ball bearings):

  • Ball bearings are spherical shape
  • They are used to reduce friction between moving parts of machines, cars and in-line skates
  • The ball bearings are placed between moving parts so that the ball bearings can roll around. This prevents the moving parts from rubbing against each other
• Reducing negative effects (Using lubricants and polished surfaces):

  • Applying a layer of lubricant, such as oil or grease, between surfaces in contact can greatly reduce friction
  • Lubricants are frequently used between the moving parts of an engine to reduce wear and tear. This helps prolong the life of the engine
  • Polishing a surface removes surface irregularities. This can also reduce friction between surfaces in contact.
• Reducing negative effects (Using air cushion):

  • The friction between two surfaces is reduced when there is a thin layer of cushion between the two surfaces
• Enhancing positive effects (Using threads):

  • Friction is important to the motion of vehicles
  • Without friction, a vehicle cannot move as its types will just spin at the same spot
  • Friction enables the tyres to grip the road surface and roll without slipping
• Enhancing positive effects (Using parachute): Air resistance is a type of friction in air. A skydiver in mid-air varies air resistance to change his speed.

  • To speed up, he reduces air resistance by reducing the surface area in contact with the air (head-first position).
  • To slow down, he increases air resistance by increasing the surface area in contact with the air (spread-eagle position)
• Enhancing positive effects (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.