Forces

• Drag forces

  Forces that slow down an object moving through a fluid (liquid or gas)
• Friction

  Resistive force between two touching objects that grips objects and resists the motion of surfaces sliding against each other
• Friction and drag forces slow down moving objects or stop objects moving
• Factors affecting size of frictional force

  • Type of surface
  • Weight of object
• Drag forces


  Force slowing down an object moving through a fluid (liquid or gas)
• Resultant force on an object is zero


  Object moves with steady speed in same direction or remains stationary
• Parachute used as brake on dragster

  Air resistance depends on parachute area and car speed - biggest air resistance with large parachute on fast car
• What will happen to an object if the forces on it are balanced?
• What if they are unbalanced?
• Deform


  Change the shape of an object
• Pivot


  The point around which rotation happens
• Compress


  Squash an object
• Moment


  The turning effect of a force
• When you open a door, or tighten a nut using a spanner

  You are using a turning force
• Stretch


  Extend an object
• Force


  Measured in newtons (N)
• Extension


  The amount an object stretches
• Distance


  Measured in metres (m)
• Moment


  Measured in newton metres (Nm)
• Tension


  The force that stretches an object
• Incompressible


  Liquids are incompressible because the particles are touching each other and there is very little space between them so you cannot compress them
• Moment (Nm)


  force (N) x distance from pivot (m)
• Elastic limit


  The point where an object will not return to its original shape when the force is removed
• Stress


  When you stand on any surface you exert a force on it because of your weight. Your weight is spread out over the area of your foot. You are exerting a pressure on the ground, called stress. The stress acts in a direction that is at 90°, or normal, to the surface.
• Stress


  Stress is a measure of how much force is applied over a certain area.
• Upthrust

  The resultant force acting on a floating object due to more water molecules hitting the bottom than air molecules hitting the top
• The moment of a force is given by the equation: moment = force x distance
• Hooke's law

  The extension of an object is proportional to the force applied
• Linear


  A straight line relationship between two variables
• Water is a fluid. When you swim underwater the water exerts a pressure on you. The water molecules are pushing on each other and on surfaces. This liquid pressure acts in all directions.
• Solid materials are only compressed a very small amount when you apply a force to them
• Pressure in a liquid acts in all directions
• Equilibrium

  When an object is balanced, the sum of the clockwise moments is equal to the sum of the anticlockwise moments
• Reaction force


  The force from a solid surface that supports an object
• Law of moments


  An object is in equilibrium if the total anticlockwise moments equal the total clockwise moments
• If you double the force on a spring the extension doubles
• At some point the spring will not go back to its original length when you remove the force. This is the elastic limit
• Liquids pass on any pressure applied to them. We use this property of liquids to make hydraulic machines, like brakes in cars.
• If the extension doubles when you double the force then the object obeys Hooke's Law
• Centre of gravity


  The point through which all the weight of an object seems to act