P8 - Forces in Balance

• Scalar quantities


  Only contain magnitude
• Vector quantities

  Contain magnitude and direction
• Contact forces:

  Friction, air resistance, tension
• Non-contact forces

  Gravitational, electrostatic, magnetic
• Resultant force:

  All the forces acting on an object
• What can resultant force change?

  Objects speed, shape or direction
• What if the resultant force is 0?

  Object stay at rest or moves at constant velocity depending on initial velocity of object.
• Parallelogram of force:

  Used to find resultant force when forces don't act in the same line.

  
• How to find resultant force with parallelogram?

  1)Work out scale

  2)Copy out one force to scale at the same angle

  3)Repeat with other force

  4)Draw resultant force from point where forces meet and where they act on the object

  5)Measure size of arrow and use scale to find magnitude of force
• Work done (J)?

  Energy transfer as a result of a force acting on an object. Force x distance
• What happens when work is done against friction?

  Energy is transferred to the thermal energy store of the surroundings.
• Displacement?

  Vector quantity and always straight line.
• Newton's First Law

  An object remains in the same state of motion unless a resultant force acts on it.
• Newton's Second Law

  An object will accelerate in response to a resultant force.

  Acceleration ∝ resultant force

  Acceleration ∝ 1/mass.

  Force = Mass x Acceleration
• Newton's Third Law

  Whenever 2 objects interact they exert equal and opposite forces against each other.
• Inertial mass

  How difficult it is to change the velocity of an object. m = F/a
• Inertia

  The tendency of an object to continue in its current state.
• Moment?

  A turning effect of a force. Moment = Force x distance
• Moments on a balanced object?

  Total clockwise moments = total anticlockwise moments
• Lever

  A simple machine consisting of a pivot, effort and load
• Pivot

  A point around which something can rotate or turn
• Effort

  Force used to move a load over a distance
• Load

  The overall force that is exerted, usually by a mass or object, on a surface
• Arrangement of lever components

  • Effort - pivot - load
  • Pivot - load - effort
  • Pivot - effort - load
• Effort - pivot - load

  • see-saw, crowbar, scissors
• Pivot - load - effort

  • wheelbarrow, nutcracker
• Pivot - effort - load

  • tweezers, cooking tongs
• Simple levers and rotation

  • A solid beam laid across a pivot, as effort is applied to rotate one end about the pivot, the opposite end is also rotated about the pivot in the same direction
• Force multiplier

  Something that increases the effect of a force
• The longer the lever, and the further the effort acts from the pivot, the greater the force on the load will be
• Gears

  Toothed wheels used with other gears to turn axles at different speeds
• Axle

  A bar, rod or shaft which passes through a wheel or gear. The wheel or gear will rotate around the axle.
• Rotation and transmission of forces by gears

  1. One gear turns, the other gear must also turn

  2. Where the gears meet, the teeth must both move in the same direction

  3. The forces acting on the teeth are identical for both gears, but their moments are different
• If the driven gear is made larger

  It will rotate more slowly but with a greater moment
• If the driven gear is made smaller

  It will rotate more quickly but with a smaller moment
• Calculating moment of larger gear

  1. Find force on teeth of smaller gear

  2. Use force and radius to calculate moment of larger gear
• Turning a gear that has double the radius doubles the turning effect - it is a 2x force multiplier