P8 - Forces in balance

• Scalar


  • quantities that only have a magnitude
• Examples of scalars


  • distance
  • speed
• Vectors


  • quantities that have direction and magnitude
• Examples of vectors


  • velocity
  • force
• Force


  • a push or a pull on an object
  • caused by an interaction with another object
  • vector quantities
• Contact forces


  • occur when two objects are touching each other
• Examples of contact forces

  • friction
  • air resistance
  • tension
  • normal contact force
• Non-contact forces


  • act at a distance
  • act without the two objects touching
• Examples of non-contact forces


  • gravitational forces
  • electrostatic forces
  • magnetic forces
• When an object exerts a force on another object, it will experience an equal and opposite force
• Resultant forces


  • the sum of the mangnitudes of the forces if they act in the same direction
  • the difference between the magnitudes of the forces if they act in opposite directions
• If the resultant force on an object is 0, the forces are said to be balanced
• If the forces do not act along the same line, the resultant of the two forces can be found by making a scale drawing using a ruler and protractor
• Resultant forces


  If two or more forces act on an object along the same line, their effect if they were replaced with a single resultant force
• Gravity


  • The force of gravity close to the Earth is due to the planet's gravitational field strength
• Weight


  • the force acting on an object due to gravity
• Weight


  • considered to act at the object's centre of mass
  • can be measured using a calibrated spring-balance (netonmeter)
• weight (N) = mass (Kg) x gravitational field strength (N/Kg)
• Weight and mass


  • weight and mass are directly proportional to each other
  • if the mass of an object doubles, the weight of the object doubles
• Drawing forces

  • Free body diagreams use arrows to show the forces acting on a single object
  • A dot represent the object with arrows drawn around it
  • the length of the arrow represents the magnitude
  • the direction of the arrow shows the direction of the force
• Scale drawings


  • used to find the resultant of the forces that are not acting along the same line
  • the forces are drawn end to end
  • the resultant is drawn between the two ends, forming a triangle
• Deformation


  • A change in the shape of an object
  • caused by stretching, compressing, bending or twisting
  • More than one force has to act on a stationary object to deform it
  • otherwise, the object would just move
• Elastic deformation


  • the object can go back to its original shape and size when the forces are removed
• Inelastic deformation


  The object does not go back to its original shape or size when the forces are removed
• force applied (N) = spring constant (N/m) x extension (m)
• Gradient of a graph of force against extension

  gradient = spring constant
• Limit of proportionality


  • the extension of an elastic object is directly proportional to the force
  • as long as the limit of proportionality is not exceeded
• Elastic potential energy


  • a force that stretches or compresses an object does work on it
  • this causes energy to be transferred to the object's elastic potential store
• Resolving forces


  • a single force can always be resolved into two component forces at right angles to each other
  • the two component forces added together give the same effect as the single force
• Moments


  • A force or system of forces that can cause an object to rotate
  • The turning effect of a force is called the moment of the force
  • If an object is balanced, the sum of the clockwise moments equals the sum of the anticlockwise moments
• moment of a force (Nm) = force (N) x distance (m)
• Levers and gears


  • can be used to increase the moment of a force
  • making it easier to lift or rotate an object
• Lever


  Allows a large moment of force to be produced by allowing force to be applied further from the pivot