Chapter 8: Force and Pressure
8.1 Force – A Push or a Pull
8.1.1 Action of Forces
- Force: In science, a push or a pull on an object is called a force.
- Actions: Actions like picking, opening, shutting, kicking, hitting, lifting, flicking, pushing, and pulling are often used to describe certain tasks. Each of these actions usually results in some kind of change in the state of motion of an object.
8.2 Forces are due to an Interaction
8.2.1 Interaction
- Interaction: At least two objects must interact for a force to come into play. Thus, an interaction of one object with another object results in a force between the two objects.
8.3 Exploring Forces
8.3.1 Net Force and Magnitude
- Forces in Same Direction: Forces applied on an object in the same direction add to one another.
- Forces in Opposite Directions: If two forces act in the opposite directions on an object, the net force acting on it is the difference between the two forces.
- Magnitude: The strength of a force is usually expressed by its magnitude. We also have to specify the direction in which a force acts.
8.4 A Force can Change the State of Motion
8.4.1 State of Motion
- State of Motion: The state of motion of an object is described by its speed and the direction of motion. The state of rest is considered to be the state of zero speed.
- Change in Speed and Direction: A force applied on an object may change its speed, its direction of motion, or both.
8.5 Force can Change the Shape of an Object
8.5.1 Change in Shape
- Deformation: The application of force on an object may change its shape (e.g., pressing dough or stretching a rubber band).
8.6 Contact Forces
8.6.1 Muscular Force
- Muscular Force: The force resulting due to the action of muscles in our body. It can only be applied when it is in contact with an object, making it a contact force.
8.6.2 Friction
- Friction: The force responsible for changing the state of motion of objects in contact, which always acts in a direction opposite to the direction of motion.
8.7 Non-contact Forces
8.7.1 Magnetic Force
- Magnetic Force: The force exerted by a magnet on another magnet or a piece of iron without physical contact.
8.7.2 Electrostatic Force
- Electrostatic Force: The force exerted by a charged body on another charged or uncharged body, which comes into play even when the bodies are not in contact.
8.7.3 Gravitational Force
- Gravity: An attractive force that pulls objects towards the Earth. It is a non-contact force and acts on all objects.
8.8 Pressure
8.8.1 Defining Pressure
- Pressure: The force acting on a unit area of a surface (Pressure = Force / Area). The smaller the area, the larger the pressure for the same force.
8.9 Pressure Exerted by Liquids and Gases
8.9.1 Fluid Pressure
- Liquid Pressure: Liquids exert pressure on the bottom and walls of their container, and this pressure increases with depth.
- Gas Pressure: Gases exert pressure on the inner walls of their container in all directions.
8.10 Atmospheric Pressure
8.10.1 Atmosphere and Pressure
- Atmosphere: The envelope of air surrounding the Earth.
- Atmospheric Pressure: The pressure exerted by the weight of atmospheric air on a unit area.