Revision: Force and Pressure Science SSLC (English Medium) Class 8 Tamil Nadu Board of Secondary Education

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.

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.

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.

The thrust (Normal Force) exerted by a liquid at rest on unit area of the surface in contact is called pressure.

P = \[\frac {F_⊥_​​}{A}\]

• SI Unit: pascal (Pa) = 1 Nm⁻²
  Dimensions: [ML⁻¹T⁻²]

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

The difference between the hydrostatic pressure (P) and the atmospheric pressure (P₀), measured by a manometer, is called gauge pressure: P − P₀ = ρgh

The ratio of the density of a given fluid to the density of pure water at 4°C is called relative density. It is a unitless quantity.

The weight per unit volume of a substance is called specific weight or weight density: W = mg/V

• SI Unit: N/m³

The mass per unit volume of a substance is called density: ρ = M/V

• SI Unit: kg/m³, Dimensions: [ML⁻³T⁰] 

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.

The force exerted by the air column on unit cross-sectional area at sea level (= 1.01 × 10⁵ Pa = 1.01 bar) is called atmospheric 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.

RELATIVE DENSITY: “is the ratio of the density of a substance to the density of water at 4° C.”
Or
RELATIVE DENSITY “is the ratio of the mass of the substance to the mass of an equal volume of water at 4° C.”

UNIT OF RELATIVE DENSITY: No units since it is a pure ratio.

Density of a substance is defined as “Mass per Unit volume”.

Density [d]=`"mass  of the substance"/"volume of the substance"`

d=`m/v`

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 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.

SI unit of pressure is the pascal (Pa) or Nm⁻²
One Pascal: When a force of one newton acts normally on an area of one square metre (1 m²) then the pressure acting on the surface acting on the surface is called one Pascal.

Surface tension is defined as the force per unit length acting at right angles to an imaginary line drawn on the free surface of the liquid. 

When a liquid is in contact with a solid, the angle between the tangent drawn to the free surface of the liquid and the surface of solid at the point of contact measured inside the liquid is called the angle of contact.

Surface tension is defined as the force acting on a unit length of an imaginary line drawn on the free surface of the liquid, the direction of the force being perpendicular to the line so drawn and acting parallel to the surface.

The angle between tangents drawn at the point of contact to the liquid surface and the solid surface inside the liquid is called the angle of contact for a pair of solid and liquid. It is denoted by θ.

The maximum constant velocity acquired by a body while falling freely through a viscous medium is called the terminal velocity V_T.

The property of a fluid by virtue of which it opposes the relative motion between its different layers, with the force that comes into play, is called viscosity; and that force is called the viscous force.

where η is the coefficient of viscosity.

The rate of change of velocity (dv) with distance (dx) measured from a stationary layer is called velocity gradient.

∴ Velocity gradient = `(dv)/dx`

The coefficient of viscosity of a liquid is defined as the viscous force acting tangentially per unit area of a liquid layer having a unit velocity gradient in a direction perpendicular to the direction of flow of the liquid.

The minimum force required to keep the body moving over a surface such that it moves equal distances in equal intervals of time is called the force of sliding friction.

The minimum force required to roll a body on a surface is called the force of rolling friction.

A push or pull acting upon an object as a result of its interaction with another object is called force. Its SI unit is the newton (N).

The maximum force exerted by a surface on a body so long as it remains stationary is called the force of static friction.

"Friction between two bodies in contact when one body is rolling over the other, is called rolling friction."

OR

The resistive force or rolling resistance that occurs when an object rolls across a surface and slows down the motion of a rolling ball/wheel, which is the weakest form of friction compared to static/sliding friction, is called Rolling Friction.

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

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​}

Statement: Pascal's Law states that when pressure is applied to a confined (enclosed) fluid, it is transmitted undiminished and equally in all directions throughout the fluid and to the walls of its container.

Mathematical Expression:

Key Points:

• Pressure changes by the same value at every point inside an incompressible, confined liquid.
• Used in hydraulic machines where a small force on a small area produces a large force on a large area.
• Applications: Hydraulic lift, hydraulic brake, hydraulic press, hydraulic jack.

• 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.

• Hydraulic Press — Two cylinders (C & D) filled with liquid; small force applied on piston P₁ (smaller area A₁) is converted into a very large upward force on piston P₂ (larger area A₂), since A₂ > A₁.
• Hydraulic Lift — Works on Pascal's Law to lift or support heavy objects such as cars and trucks using liquid pressure.
• Hydraulic Brakes — Small force on the brake pedal is instantly transmitted equally through brake fluid to all cylinders, producing a large thrust on the wheels to stop the vehicle.

• A highly soluble impurity increases surface tension, while a partially soluble impurity (e.g., detergent) decreases it; a waterproofing agent increases it.
• Surface tension decreases with increase in temperature, given by T = T₀(1 − αθ), where T₀​ is surface tension at 0°C and α is the temperature coefficient.
• When a soap bubble is charged (positively or negatively), force acts outward on the surface, increasing its radius — thus electrification always decreases surface tension.