pressure & pressure in fluids

• Pressure is force per unit area. Pressure determines the effect of a force on a surface.
• if the force acts on a small area, the pressure is increased.
• Liquids and gases are fluids
• The pressure in fluids causes a force normal to a surface. A force that is normal to a surface acts at right angles (90°) to it.
• pressure in fluids = force normal to that surface / area of that surface
• The pressure in a liquid is different at different depths. Pressure increases as the depth increases. 
• he pressure in a liquid is due to the weight of the column of water above
• Since the particles in a liquid are tightly packed, this pressure acts in all directions
• The pressure caused by a column of liquid can be calculated using the equation:

  pressure = height of column × density of the liquid × gravitational field strength
• An object that is partly, or completely, submerged experiences a greater pressure on its bottom surface than on its top surface. This causes a resultant force upwards. This force is called upthrust
• The atmosphere is the layer of air around the Earth.
• Key features of the atmosphere:

  • it is thin compared to the size of the Earth
  • it becomes less dense as the altitude increases
• Air molecules colliding with a surface cause atmospheric pressure.
• Atmospheric pressure decreases as the height of a surface above ground level increases. This is because, as the altitude increases:

  • the number of air molecules decreases
  • the weight of the air decreases
  • there is less air above a surface
• If the air pressure is too low, humans cannot take in oxygen quickly enough to meet their bodies’ needs.
• as the volume of a gas sample is decreased, the same number of gas molecules collide with walls more frequently, resulting in greater pressure