Forces

• Newton's second law


  The net force is equal to the mass times the acceleration for a constant mass
• The net force is equal to F = ma
• Net force


  The sum of all the forces acting on an object
• Net force examples


  • 1 newton acting downwards
  • 1 newton acting downwards and 2 newtons acting upwards (net force = 1 newton upwards)
  • 1 newton acting downwards and 2 newtons acting upwards (net force = 1 newton upwards)
• Newton


  The force required to accelerate a mass of 1 kilogram by 1 meter per second squared
• Weight


  The force due to gravity, equal to mg
• Normal force


  The equal and opposite force to gravity when an object is supported
• Upthrust


  The buoyant force experienced by an object submerged in a fluid
• Tension


  The force exerted by a rope or cable pulling an object
• Friction


  The force opposing motion
• Free body diagram


  • A diagram showing all the forces acting on an object, with the forces in the correct proportion
• Solving a problem with a helicopter lifting a mass at constant velocity


  Net force = 0 since velocity is constant

  2. Tension = weight (mg) = 980 N
• Solving a problem with a force increase and acceleration

  Net force = Tension - Weight

  2. Net force = ma

  3. Tension = ma + Weight = 1081 N
• Drag force


  The force opposing the motion of an object through a fluid, proportional to velocity squared
• Investigating motion of an object in a fluid

  Fill a tube with viscous fluid

  2. Drop a ball bearing and record time to reach marked intervals

  3. Plot speed vs time graph to find terminal velocity
• Analyzing forces on an object on an inclined plane

  Split weight into components parallel and perpendicular to the plane

  2. Parallel component = mg sin(theta)

  3. Perpendicular component = mg cos(theta)
• Splitting weight into parallel and perpendicular components

  1. Find angle theta

  2. Parallel component = mg sin theta

  3. Perpendicular component = mg cos theta
• Center of gravity


  Point at which the entire weight of an object seems to act
• Determining center of gravity

  1. Pin object

  2. Hang plumb line vertically downwards

  3. Rotate object and pin at different point

  4. Intersection of lines is center of gravity
• Moment of force


  Product of the force and perpendicular distance from the pivot point to the line of action of the force
• Moment = Force x Perpendicular distance
• Principle of moments: Sum of anticlockwise moments = Sum of clockwise moments
• Force couple


  Two equal and opposite forces acting in the same plane
• Torque


  Net moment, equal to magnitude of one force x distance between forces
• Triangle of forces: Used to determine forces when object is in equilibrium
• Density


  Mass per unit volume, in kg/m^3
• Pressure


  Force per unit area, in N/m^2 or Pascals
• Archimedes' principle


  Upthrust on submerged object = Weight of displaced fluid
• Newton's first law:

  An object will remain at rest or at constant motion unless acted upon by a resultant force.
• Newton's third law: for every action there is an equal and opposite reaction.
• In today's video we're going to be revising the topic of forces
• Newton's second law


  The net force is equal to the mass times the acceleration for a constant mass
• Net force


  The sum of all the forces acting on an object
• The net force is equal to F = ma
• Newton


  The force required to accelerate a mass of one kilogram by one meter per second squared
• Types of forces


  • Weight
  • Normal force
  • Upthrust
  • Tension
  • Friction
• Free body diagram


  • Shows the forces acting on an object, with the forces in the correct proportion
• Constant velocity


  No resultant force, so no acceleration
• Solving a problem with a helicopter lifting a mass at constant velocity


  1. Find the tension in the cable

  2. Tension = mg

  3. Tension = ma + mg
• Drag force


  Force that opposes the motion of an object through a fluid