Revision: Force and Pressure Science Secondary School (English Medium) (5 to 8) Class 8 CBSE

Force is defined as the rate of change of linear momentum of a body with respect to time.

The forces which are applied on a body through a connector, are called contact forces. Forces like Frictional force, Mechanical force, etc., are the forces of contact.

An inclined plane is usually a smooth, flat rigid surface inclined at an angle (θ) to the horizontal. It is used to raise heavy loads with a relatively small force. The longer the slope, the smaller is the effort needed.

1 dyne is that force which when acting on a body of mass 1 gram, produces an acceleration of 1 cm s^-2 in it.
1 dyne = 1 g × 1 cm s^-2.

The weight of the body is the force with which the earth attracts it towards the centre. It depends on acceleration due to gravity.

The gravitational force acting on an object is called the weight of the object.

A force is applied between two objects even if the two objects are not in contact; such a force is called a 'Non-contact force.'

OR

The forces experienced by a body without any physical contact with another object, e.g., gravitational force, electrostatic force, are called non-contact forces.

A force is seen to act through direct contact of the objects or via one more object. Such a force is called 'Contact force.'

OR

The forces experienced by a body due to physical contact with another object, e.g., frictional force, normal force, are called contact forces.

The force which produces compression is called thrust. Its S.I unit is the newton.

The phenomenon in which the liquid pressure at a point is independent of the quantity of liquid and depends only upon the depth of the point below the liquid surface is called hydrostatic paradox.

The gaseous envelope surrounding the earth is called the earth's atmosphere.

The pressure exerted by the atmosphere on the earth's surface is called atmospheric pressure.

The total pressure exerted by a fluid, which includes both the atmospheric pressure as well as any other additional pressure due to the fluid itself, is called absolute pressure.

The difference between the absolute pressure and the atmospheric pressure at a point in a liquid is called gauge pressure.

A low-pressure area is an area in the atmosphere where the pressure is lower than its surrounding areas. In this situation, the wind from the surroundings blows towards the center of low pressure.

High pressure is an area of the atmosphere where the barometric pressure is higher than its surrounding areas. In this case, the wind from the center of high pressure blows towards the surrounding low-pressure areas.

The pressure exerted by this mercury column is considered as the pressure of magnitude ‘one atmosphere’ (1 atm).

\[\overset{\rightarrow}{\operatorname*{F}}=\frac{d\overset{\rightarrow}{\operatorname*{p}}}{dt}=\frac{d(m\overset{\rightarrow}{\operatorname*{v}})}{dt}\]

or

\[\begin{array}
{rcl}\vec{F} & = & m\vec{a}
\end{array}\](if mass m is constant)

1 atm = 1.01 × 10⁵ Pa = 1.01 bar = 760 torr

P_gauge = P_absolute​ − P_{atmospheric​}

• Force is a Vector Quantity
• Unit of Force is Newton (symbol N) or kilogram-force (symbol kgf), where 1 kgf = g N if g is the acceleration due to gravity at that place (= 9·8 m s^-2 average value on the earth's surface). 

• Pressure exerted by a liquid column depends on height and density of the liquid column.
• It is independent of the shape of the containing vessel or total mass of the liquid.
• Atmospheric pressure is maximum at the surface of the earth and decreases as we move up into the atmosphere.