forces

• scalar quantities have magnitude only
• vector quantities have magnitude and an associated direction
• scalar quantities : speed, mass, distance
• vector quantities : acceleration, velocity, force and displacement
• a vector quantity can be represented by an arrow.

  the size of the force represents the magnitude and the direction of the arrow represents the direction of the vector quantity
• a force is a push or pull that acts on an object due to its interaction with another object
• contact forces are forces where the objects are physically touching
• contact forces include : air resistance, friction, tension and the normal contact force
• non-contact forces are forces where the objects are physically separated
• non-contact forces include : gravitational force, electrostatic force and magnetic force
• weight is the force acting on an object due to gravity
• the force of gravity close to the Earth is due to the gravitational field around the Earth
• all matter has a gravitational field, and attracts all other matter
• the larger the mass of an object, the stronger the gravitational field around it, and the stronger the attraction
• the weight of an object depends on the gravitational field strength at the point where the object is
• the same personal at two different planets will have a different weight because will the mass of the person stays the same, the gravitational field strength changes and weight = mass x gravitational field strength
• weight = mass x gravitational field strength
• the weight of an object may be considered to act at a single point referred to as the objects 'centre of mass'.
• the weight and mass of an object are directly proportional
• weight can be measured using a calibrated spring-balance (a newton-metre)
• a resultant force is the single force that has the same effect as all the forces acting on an object.
• free-fall:

  1. when the skydiver first falls, there is little to no air resistance acting on the diver because the skydivers body hasn't collided with air particles yet to create air resistance. Their weight is the resultant force.
  2. as they fall, they accelerate, increasing their speed meaning they collide with more air particles, increasing air resistance
  3. as air resistance increases, the resultant force downwards decreases
  4. this leads to the skydivers acceleration decreasing
  5. and eventually the two forces reach equilibrium so resultant force is 0. The skydiver travels at terminal velocity.
• a single force can be resolved into two components acting at right angles to each other. The two component forces have the same effect together as the single, original force
• a force does work on an object when the force causes a displacement of the object
• work done is energy transferred by a force over a distance
• work done = force X distance moved
• one joule of work is done when a force of one newton causes a displacement of one metre (1 joule = 1 newton-metre)
• work done against the frictional forces acting on an object causes a rise in the temperature of an object.
• to stretch, bend or compress an object, more than one force has to be applied to it because if a single force is applied to an object it will just move in that direction
• if an object is pulled in opposite directions it will stretch
• its an object is fixed at one point and stretched, a force is still being applied by the fixed point
• inelastic deformation is when a stretched object is permanently deformed and will not return to its original shape when the load is removed
• elastic deformation is when a stretched object returns to its original shape when the load is removed
• Hooke's Law : the extension of an elastic object is directly proportional to the force applied, where spring constant is the constant, provided the limit of proportionality is not exceeded

  (force = spring constant X extension)
• a force that stretches or compresses a spring does work, and elastic potential energy is stored in the spring
• provided the spring is not inelastically deformed, the work done on a spring and the elastic potential energy stored are equal
• to calculate the work done to stretch or compress a spring up to the limit of proportionality:

  elastic potential energy = 1/2 x spring constant x extension x extension

  work done = elastic potential energy
• a force or system of forces may cause an object to rotate
• a moment is the turning effect of a force
• moment = force x distance from pivot