Force and pressure

• Force


  A push or a pull on an object
• At least two objects must interact for a force to come into play
• Forces applied on an object in the same direction

  Add to one another
• If two forces act in opposite directions on an object


  The net force acting on it is the difference between the two forces
• Force


  • Its strength is usually expressed by its magnitude
  • It has a direction in which it acts
• More than one force may be acting on an object, but the effect on the object is due to the net force acting on it
• How a force can change the state of motion

  1. Force applied in the direction of motion increases speed

  2. Force applied opposite to the direction of motion decreases speed

  3. Force can change the direction of motion
• A change in either the speed of an object, or its direction of motion, or both, is described as a change in its state of motion
• How a force can change the shape of an object


  Applying force can change the shape of an object
• Application of a force does not always result in a change in the state of motion of the object
• None of the actions (change in speed, direction, or shape) can take place without the action of a force
• Force


  • May change the direction of motion of an object
  • May bring about a change in the shape of an object
  • May cause some or all of these effects
• State of motion


  • Described by an object's speed and direction of motion
  • State of rest is considered to be the state of zero speed
  • An object may be at rest or in motion
• Application of force does not always result in a change in the state of motion of the object
• Contact force


  Force resulting from the action of muscles, where the body has to be in contact with the object
• Friction


  The force responsible for changing the state of motion of objects, acts in the opposite direction to the motion
• Non-contact force


  Force exerted by one object on another without being in contact, e.g. magnetic force, electrostatic force, gravitational force
• Magnetic force


  Force exerted by a magnet on another magnet or magnetic object, even without contact
• Electrostatic force


  Force exerted by a charged body on another charged or uncharged body, even without contact
• Gravitational force


  Attractive force exerted by every object in the universe on every other object
• Pressure


  Force acting on a unit area of a surface
• Liquids and gases also exert pressure, which depends on the area on which the force acts
• Pressure exerted by liquids

  1. Water column height increases

  2. Rubber sheet bulges out more
• Pressure exerted by a liquid depends on the height of the liquid column
• Observing the bulge in the rubber sheet and the height of the water column in the pipe

  1. Observe the bulge

  2. Observe the height of the water column

  3. Repeat the process a few more times
• Bulge in the rubber sheet


  Relates to the height of the water column in the pipe
• Investigating pressure exerted by water at the bottom of a container

  1. Fix a cylindrical glass tube near the bottom of a plastic bottle

  2. Cover the mouth of the glass tube with a thin rubber sheet

  3. Fill the bottle up to half with water

  4. Observe what happens
• The rubber sheet has been fixed on the side of the container and not at the bottom
• Investigating pressure exerted by liquids

  1. Take an empty plastic bottle or cylindrical container

  2. Drill four holes all around near the bottom of the bottle

  3. Fill the bottle with water

  4. Observe what happens
• The water coming out of the holes falls at the same distance from the bottle
• Air exerts pressure on the inner walls of an inflated balloon or tube
• Atmospheric pressure


  The pressure exerted by the air around us
• Demonstrating atmospheric pressure


  1. Take a good quality rubber sucker

  2. Press it hard on a smooth plane surface

  3. Try to pull it off the surface
• The weight of air in a column of the height of the atmosphere and area 10 cm × 10 cm is as large as 1000 kg
• The reason we are not crushed under this weight is that the pressure inside our bodies is also equal to the atmospheric pressure and cancels the pressure from outside
• Otto von Guericke's demonstration of atmospheric pressure


  • Joined two metallic hemispheres and pumped air out of them
  • Employed eight horses on each hemisphere to pull them apart