Chapter 8 – Force and Pressure

Chapter 8 – Force and Pressure

 

1. Force

• Force  is a push or pull that causes or attempts to change the state or shape of motion of an object.
  • Examples: Pushing or pulling a door, kicking a football, lifting an object.
•  Effects of Force: The force can make the following changes:
  • To bring a stationary object into motion.
  • To increase or slow down the speed of a moving object.
  • To change the direction of a moving object.
  • To change the shape of something.

2. Types of Forces

Forces are mainly divided into two categories:

1. Contact Forces:

• • Forces that occur when two objects are in direct contact with each other.
  •  Muscular Force:
    • It is the force that is felt due to the action of our muscles.
    • Examples: cycling, lifting objects, pushing.
  •  Frictional Force:
    • It is the force that opposes the relative motion  between two contact surfaces.
    • It always seems to be opposite to the direction of motion.
    • Examples: Stopping of a ball rolling on the floor, walking, car stopping when braking applied.

 

1. Non-Contact Forces:

• Forces that occur when objects are not in contact with each other.
• Magnetic Force :
  • It is the force exerted by the magnet on magnetic objects (e.g. iron, nickel, cobalt).
  • Equal poles repel  each other and opposite poles attract.

 

 

3. Pressure

• Pressure is the force per unit area.
•  Formula: Pressure = Force/Pressure area
  • This means that the pressure increases if the force increases or the area decreases.
  • And the pressure decreases if the force decreases or the area increases.
•  Unit:  The  SI unit of pressure is Pascal (Pa).
•  Examples:
  • The pointed end of the nail moves easily into the wall because the smaller area generates more pressure.
  • The straps of the bag are made wider so that there is less pressure on the shoulder.
  • The knife has a sharp edge so that the vegetable is easily cut even with low force.

4. Pressure Exerted by Liquids and Gases
5. Pressure Exerted by Liquids:

• • Liquids also exert pressure on the walls of the container.
  • The pressure increases with increasing depth.
  • The pressure is felt evenly in all directions (at a certain depth).
  • Example: Water comes out when you make a hole in a bottle. Water travels far through the hole at the bottom because the pressure is high there.

1. Pressure Exerted by Gases:

• The gases also exert pressure on the walls of the container.
• Example:  The balloon swells up when it is inflated because air exerts pressure on the walls of the balloon.

5. Atmospheric Pressure

• There is a huge layer of air around us called  the atmosphere.
• The force exerted by the air in the atmosphere on the earth is  called atmospheric pressure.
•  Experiencing Atmospheric Pressure:
  • When we use a straw to drink water, we pull the air inside the straw, which reduces the pressure inside. The high atmospheric pressure outside pushes the water up into the straw.
  • When a rubber suction cup is pressed against a smooth surface, it sticks because the air inside the cup is escaped and the atmospheric pressure outside keeps it on the surface.
•  Change in Pressure with Height:
  •  As we go higher,  the atmospheric pressure decreases. This is why climbers have difficulty breathing at high altitudes or may bleed from their noses.