Question

class 8 chapter 4 $\huge\underline\mathcal\red{Force}$ $\huge\underline\mathcal\blue{and}$ $\huge\underline\mathcal\cyan{Pressure}$

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Answer 1

Force

A pull or a push acting on a body which tends to change its state of rest or motion or the direction of motion, or its shape or size.

F = m × a

where,

F is force

m is magnitude

a is acceleration.

SI unit of force is newton, denoted by N.

CGS unit of force is dyne. It is smaller unit than newton.

1N = 1kgm/s²

Force is a vector quantity. This means that to completely specify force, we need to specify its magnitude as well as its direction. It is denoted by the letter F. The length of the line shows the magnitude of force while the head of the arrow shows the direction of force A force is measured using a spring balance..

Moment of force.

The moment of a force gives us the turning effect of force about a fixed point. Moment of force is defined as the product of the magnitude of force and perpendicular distance between the point of application of force and the pivoted point.

1. It depends on two factors.

Magnitude of applied force: The moment of force is directly proportional to the applied force. Thus larger the force applied, greater is the turning effect.

2. Perpendicular distance between point of application of the force and the pivot: The turning effect of force also depends directly on the perpendicular distance (or the shortest distance) between. the point of application of force and the pivot.

1kgf m = 9.8m

Pressure

The normal force acting on a unit area of a surface is called Pressure.

P = F/Awhere,

P is pressure

F is force

A is area of contact

The SI unit of pressure is pascal (Pa). One pascal is numerically equal to the force of one newton acting on an area of one square metre.

1Pa = 1 N m⁻²

The bigger unit used for measuring pressure is kilopascal (kPa).

Thrust

Thrust is defined as a force acting perpendicular (normal) to the surface of a body.

The SI unit of thrust is newton. It is denoted by N

Pressure Exerted by Liquids

Liquids exert pressure on the base and walls of the container. The pressure exerted by liquids on the walls of the container is called lateral pressure. Unlike a solid which exerts pressure only in the downward direction, a liquid exerts pressure on all sides of the container.

1. Characteristics:The pressure exerted by a liquid increases with the depth.
2. A liquid exerts pressure on the walls of the container it is stored in. This pressure is known as lateral pressure.
3. The pressure exerted by a liquid is independent of the size and shape of the container.
4. The pressure due to a liquid is directly proportional to its density. A liquid seeks its own level.

Pressure Exerted by Gasses

What happens when you blow air in a balloon? The balloon inflates and grows in size in all directions.

When you open the mouth of the inflated balloon, the air filled in the balloon escapes and the balloon

immediately deflates. This simple activity shows that just like liquids, gases also exert pressure. According to the kinetic theory of matter, the molecules in a gas are very loosely packed and are in continuous random motion. As these molecules strike the walls of the container, they exert some force and consequently exert pressure which is known as gas pressure. Gas pressure is defined as the force exerted by a gas per unit area on the walls of the containing vessel.

Atmospheric Pressure:

Atmospheric pressure is defined as the force exerted by atmosphere on a unit area of the surface of the earth. Atmospheric pressure at a given point is same in all directions. However it decreases with altitude.

The instrument used to measure the atmospheric pressure at a place is called a barometer.

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