PHY AS Lvl: Dynamics + Forces, Vectors, Moments

• Mass


  Quantity of matter
• Weight


  Force on an object caused by a gravitational field acting on its mass
• Weight


  • It is a vector quantity
  • Measured in newtons
• Normal force of reaction


  The force acting against an object attracted to the Earth, when an object is placed on another object
• Newton's laws


  • 1st law
  • 2nd law
  • 3rd law
• 1st law


  1. If object is at rest, it will remain at rest

  2. If object is in state of uniform motion, it will remain so unless acted on by a resultant force
• 2nd law


  Resultant force = mass x acceleration (F = ma)
• When two bodies interact, the forces they exert on each other are opposite in direction, acting along the same line and equal in size
• Newton's law of gravitation


  Any two point masses attract each other with a force that is proportional to the product of their masses and inversely proportional to the square of their separation
• Centre of gravity


  The point where the entire weight of an object appears to act
• Contact forces


  Forces that push or pull an object by making direct contact with it, e.g. lifting, force of car engine
• Weight


  The force of gravity acting on an object
• Weight is less on the moon than on Earth
• Friction


  The force that arises when two surfaces rub over one another, always acts along the surface and opposite to the direction of motion
• Drag


  The force that arises when an object moves through a fluid, acts opposite to the direction of motion
• Upthrust


  The upwards force experienced by an object in a fluid, caused by the pressure of the fluid acting on the object
• If upthrust > object weight


  The object will float up to the surface
• Contact force


  The force that pushes against an object touching it, always acts at right angles to the surface that produces it
• Tension force


  The force in a rope or string when stretched, tries to shorten the string
• An object at rest will stay at rest, and a moving object will continue to move at a steady speed in a straight line, unless a force acts on it
• Inertia


  The tendency of a moving object to carry on moving, and of a stationary object with large mass to be difficult to start moving
• A vehicle will accelerate so long as thrust exceeds air resistance, and when thrust equals air resistance, the resultant force on the vehicle is zero
• Balanced forces


  Forces on an object are balanced when the resultant force is zero, the object will remain at rest or at constant velocity
• Free fall


  The motion of an object falling under the force of gravity alone, with no air resistance
• A skydiver's initial velocity is 0 m/s, as they fall air resistance increases and acceleration decreases, until they reach terminal velocity
• Terminal velocity


  The maximum velocity reached by an object falling under gravity, when the upward air resistance force equals the downward weight force
• Drag in water causes a decrease in speed and less time needed for an object to reach terminal velocity
• Newton's 3rd law


  When two bodies interact, the forces they exert on each other are equal in size and opposite in direction
• Types of force interactions

  • Gravitational forces
  • Electrostatic forces
  • Contact forces
  • Tension forces
  • Magnetic forces
• SI base units


  • Length (m)
  • Mass (kg)
  • Time (s)
  • Electric current (A)
  • Temperature (K)
  • Amount of substance (mol)
  • Luminous intensity (cd)
• Components of a vector

  Magnitudes of a vector quantity in two perpendicular directions
• Resolving a vector


  1. Splitting a vector into two vectors at right angles

  2. The perpendicular components are independent of one another
• Free-body force diagram


  Diagram showing all forces acting on an object (but not the forces it exerts on other objects)
• Momentum is velocity that has mass (p=mv), and is inversely proportional to velocity
• Finding the centre of gravity of an irregular object


  Hang the object from different points and trace the paths of plumb lines to find the intersection point, which is the centre of gravity
• Moment of a force


  The product of the force and the perpendicular distance from the line of action of the force to the point
• Principle of moments


  If the sum of the clockwise moments equals the sum of the anticlockwise moments, the object is in equilibrium
• Conditions for equilibrium


  • Sum of forces = 0
  • Sum of moments = 0