Force and Pressure
Synopsis
- A force is a push or a pull. It can cause any one, two or all of the following when applied to a body:
(a) cause movement or stop motion of an object
(b) change its speed/direction of the motion of a body
(c) change its shape and size of a body
- Forces that are in contact with a body are muscular force, frictional force. Forces that are not in contact with a body and act from a distance are magnetic force, electrostatic force, gravitational force.
- Force is due to the interaction between two or more bodies.
- The strength of a force is expressed by its magnitude and is a vector quantity.
- Net force on an object can be zero if two (or more) forces acting on it in opposite directions are equal in magnitude.
- Frictional force is responsible for change of speed of an object. It tends to stop/slow down a moving object in the absence of external forces. Frictional force can be due to a surface, wind (drag) or water (ex. a moving boat stops in water if you stop rowing).
- Force acting on a unit area of a surface is called pressure. Pressure = Force/Area on which it acts.
Note: S.I. unit of force = newton
S.I. unit of pressure = newton \[{{(metre)}^{-2}}\] or pascal
- Pressure depends upon the area of contact.
- : A sharp needle causes more pain in the body as compared to a blunt object.
- Pressure exerted by water at the bottom of a container depends upon the height of the water column.
- Both liquids and gases exert pressure on the walls of a container in which they are stored.
- Liquids exert equal pressure at the same depth.
- Pressure exerted by the air surrounding the earth is called the atmospheric pressure. The reason we do not feel this pressure is due to the fact that the pressure inside our bodies (Our blood contains dissolved oxygen at a pressure, equal to or slightly more than the atmospheric pressure) is equal to the atmospheric pressure and hence, cancels the pressure from outside. Atmospheric pressure is measured in Pascals or kilopascals.
1 atmosphere (1 atm) = 101325 Pa or 101.325 k Pa.
Atmospheric pressure decreases with height (high altitude/mountains) and increases with depth (deep sea).
