science wa3

• Types of Forces


  • Gravitational force
  • Friction
  • Tension
  • Normal contact force
  • Non-contact force
  • Contact force
• The SI unit of force is newton (N)
• Mass


  A measure of the amount of substance in a body (or object)
• Gravitational force


  The attractive force between masses
• Weight


  The gravitational force acting on a body
• The SI unit of mass is kg
• Weight calculation formula


  W = m g

  
• Differences between Mass and Weight

  • Mass is a measure of the amount of substance (matter) in a body
  • Weight is the gravitational force acting on a body
  • Mass has only magnitude, i.e. it is a scalar
  • Weight has both magnitude and direction, i.e. it is a vector
  • SI unit for mass is kilogram (kg)
  • SI unit for weight is newton (N)
  • Mass is constant regardless of the gravitational field strength
  • Weight varies according to the gravitational field strength
  • Mass is usually measured by an electronic balance or a beam balance
  • Weight is usually measured by an extension spring balance or a compression spring balance

  
• Weight is a gravitational force or force due to gravity acting on a body
• Friction occurs between two surfaces rubbing against each other
• Viscous force


  Friction in liquids and gases
• Air resistance


  Friction in air
• Tension


  Acts through a stretched rope, string or cable
• Normal contact force


  Exerted on a body by a surface in contact with it, perpendicular to the surface
• The normal contact force


  Has the same magnitude as the weight of the box
• The normal contact force acting on the floor would be downwards
• As the box was previously moving, it will continue to move at constant speed and direction
• The net force is now opposite in direction to the velocity of the box


  The box will decelerate
• Pressure


  The force acting perpendicularly on per unit area of contact
• Pressure depends on


  • Area of contact
  • Force
• The greater the force applied or the smaller the area of contact, the greater the pressure
• F is force in units of newtons (N)
• A is contact area in units of square metre (m2)
• The SI unit of pressure is newton per square metre (N/m2) or pascal (Pa)
• Pressure in a liquid


  Increases with the depth and density of the liquid and with gravitational field strength
• The atmospheric pressure at sea level is about 1.01 × 105 Pa = 101 kPa
• The natural pressure within our human body is also about 101 kPa
• Atmospheric pressure decreases with increasing altitude


  The lower pressure makes it more difficult for oxygen to enter our vascular systems
• This decreases the air pressure in the straw

  Atmospheric pressure acting on the surface of the drink in the container is now greater than the air pressure in the straw
• Atmospheric pressure pushes on the liquid surface


  Forcing the liquid to rise in the straw
• When a suction cup is pressed against the wall, most of the air is squeezed out from under the sucker
• This decreases the air pressure between the suction cup and the wall


  As atmospheric pressure is now greater than the pressure of the trapped air under the suction cap