Friction and forces

• Forces


  Pushes or pulls
• Effects of forces


  • Change the shape or size of an object
  • Change the speed things are moving
  • Change the direction of a moving object
• The unit for measuring force is the newton (N)
• Friction


  A force caused by two things rubbing together
• Kinds of friction


  • Air resistance
  • Water resistance
• Upthrust


  Upwards forces from water or air
• Things float in water because of upthrust
• Contact forces


  Only act when two objects or materials are touching
• Examples of contact forces


  • Friction
  • Air resistance
  • Water resistance
  • Upthrust
• Non-contact forces


  Can have an effect without objects touching
• Non-contact forces


  • Magnetism
  • Gravity
  • Static electricity
  • Upthrust
• Mass


  The amount of substance in your body
• Your mass is measured in kilograms (kg)
• Weight


  A force caused by gravity pulling on your body
• The newton (N) is the scientific unit used to measure forces, and so it is also used as the unit for weight
• Wherever you take an object

  Its mass will not change but its weight depends on the force of gravity
• An object on the Moon would have a smaller weight than on Earth, because the Moon's gravity is not as strong as Earth's
• Elastic materials


  Will stretch with a force and then return to their original shape when the force is taken away
• Springs are elastic
• Extension of a spring

  The difference between its original length and its stretched length
• Hooke's Law


  The extension of a spring is proportional to the force on it
• If the spring is stretched too far, the extension stops being proportional to the force
• If it is stretched even further, it goes beyond its elastic limit
• The spring will no longer return to its original length when the force is removed
• Force meters have springs inside them
• Materials like Plasticine will stretch with a force but they will not return to their original shape afterwards
• Plasticine


  A plastic material
• Friction


  A contact force that can slow things down, wear things away, produce heat, and cause noise
• Effects of friction


  • Slows things down
  • Wears things away
  • Produces heat
  • Causes noise
• Increasing friction


  • Using rough surfaces
  • Using materials like rubber
• Reducing friction


  • Using smooth surfaces
  • Using lubrication
• Lubricants


  • Oil
  • Grease
• Pressure


  The amount of force pushing on a certain area
• For a certain area

  The bigger the force, the bigger the pressure
• For a certain force

  The bigger the area, the smaller the pressure
• Sharp knife


  • Small area giving a large pressure
• Snow shoes


  • Large area giving a small pressure
• Calculating pressure


  Pressure = force ÷ area
• Balanced forces


  Forces on an object that are the same size but work in opposite directions
• If forces are balanced

  • A stationary object stays stationary
  • A moving object continues to move at the same speed and in the same direction