Because force has both magnitude and direction, it is a vector quantity. The joule is the S.I. Unit of work or energy, and is equal to the work done by a force of one Newton acting through one metre. Work is a measure of the energy transfer that occurs when an object is moved over a distance by an external force at least part of which is applied in the direction of the displacement. If the force is constant, energy expended may be calculated by multiplying the length of the path by the force acting along the path. Power is the amount of work done or energy transferred, divided by the time interval in which this occurs.
Pressure is the perpendicular force per unit area, or the stress at a point within a confined fluid.
If a fluid is at rest, pressure is transmitted equally to all its parts and, at any one point, is the same in all directions. This fact was discovered by the French scientist Blaise Pascal . The S.I. Unit of pressure is the Pascal , equal to pressure of one Newton per square metre. Atmospheric pressure is the force produced by the weight of all the air above a particular spot, so the greater the altitude the less is the pressure. Atmospheric pressure at sea level is approximately 760 mm Hg, the height of a column of mercury that the atmosphere can support under standard conditions.