Revision: Force and Pressure Physics ICSE ICSE Class 8 CISCE

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.

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

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 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.

The physical quantity is ‘torque’.

Torque may be defined as the turning effect produced by a force on a rigid body about a point, pivot, or fulcrum. It is measured by the product of force and the perpendicular distance of the pivot from the line of action of force.

The forces which act on a body without the help of any connector, are called non-contact forces or forces of distance. Gravitational force, Mechanical force, etc., are non-contact forces.

The moment of a force (or torque) is equal to the product of the magnitude of the force and the perpendicular distance of the line of action of the force from the axis of rotation. 

The turning effect produced by a force on a rigid body about a point, pivot or fulcrum is called the moment of force or torque. It is measured by the product of force and the perpendicular distance of the pivot from the line of action of force.

Moment of a force = Force × perpendicular distance of the pivot from the force.

The turning effect of force acting on a body about an axis is called the moment of force.

Force is a physical cause that changes or may tend to change the state of rest or the state of motion of an object. 

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 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.

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

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

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.

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

or

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

Moment of force = Force × perpendicular distance from the point (axis) of rotation

τ =  F × d

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). 

• A force applied to a pivoted body causes rotation (not linear motion) about the axis passing through the pivot point.
• The turning effect of a force (torque) depends on two factors:
  The magnitude of the force
  The perpendicular distance from the axis of rotation
• Maximum torque is produced when the force is applied at the maximum perpendicular distance from the axis.
• The S.I. unit of moment of force is newton-metre (N·m), and the C.G.S. unit is dyne-centimetre.
• Anticlockwise moments are taken as positive and directed outwards from the axis, while clockwise moments are negative and directed inwards.

• 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.