P11 - Forces and Pressure

• Pressure in solids

  The amount of force that an object exerts on another object
• Pressure equation for solids

  Pressure = Force / Area
• Increasing the area reduces pressure, e.g. wide caterpillar wheels on a tank
• Pressure unit

  Pascals (Pa), equal to Newtons per square metre (N/m^2)
• Pressure in liquids

  Pressure increases with depth, due to the weight of the liquid above
• Pressure in liquids equation

  Pressure = Height x Density x Gravity
• Pressure increases with depth in liquids, e.g. felt in ears when diving
• Scuba divers need to descend and ascend slowly to adjust to pressure changes
• Atmospheric pressure

  Pressure decreases as altitude increases, due to lower air density
• Atmospheric pressure changes can have major impacts, e.g. on Everest climbers
• Upthrust

  The upward force exerted by a liquid on an object immersed in it, equal to the weight of the displaced liquid
• Determining if a boat will sink or float

  Compare the weight of the boat to the upthrust force
• Upthrust can be measured by weighing an object in air and then in water
• In a closed system, objects are not thought to be affected by external forces or other objects outside the system.