Unit 4

• Friction


  A force which tries to stop things sliding past one another
• Friction only acts on moving objects and it cannot make objects move
• Friction


  • It is useful because it helps objects grip on a surface
  • It can be a problem as it makes objects wear out and get hot
• Parachute


  Uses air resistance to work. It is very light in weight and has a very big surface area. It catches a lot of air when it falls down, creating a lot of air resistance or drag
• The direction of air resistance is opposite to the direction of motion of an object moving through the air
• Air resistance


  The force caused by air pushing against moving objects
• Friction between a car's tyres and the road makes the tyres wear out
• Lubricants


  Slippery substances that help things move smoothly and reduce friction
• Balanced forces


  When two forces acting on an object are equal and opposite
• Unbalanced forces


  When two or more forces acting in opposite directions on an object are not the same size
• Force diagrams


  Show all the pushes and pulls acting on an object
• Net force


  When two opposite forces are not balanced, there is a net force
• Work


  The amount of energy transferred to an object to make it move
• Forces can make things move, speed up moving objects, and change an object's direction of movement
• The direction of gravitational force is downward
• Forces act in pairs and in different directions
• Friction is greater between rough surfaces than between smooth surfaces
• Friction is greater between small surfaces than between large surfaces