# Shree Jain Vidyalaya School Class 8 Notes

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### PHYSICAL ENVIRONMENT- FORCE AND PRESSURE

INDEX

• INTRODUCTION
• NEWTON’S LAW OF MOTION

INTRODUCTION

We have seen that no work can be done without applying force. So we apply force to mobilize a static object. We also apply force to change the direction of a moving body and accelerate or decelerate its velocity. We apply force even to change the shape of an object. Force plays an active role in our bones or muscles therefore we can stand on our feet.

Famous scientist sir ISSAC NEWTON put forward three laws of motion which elaborates the concept of force and motion

NEWTON’S LAW OF MOTION

First law: everybody continues in its state of rest or uniform motion along a straight line unless it is compelled to change that state by force impressed on it.

Second law: The more force applied on the object the more is the acceleration or change of velocity of the object per second. If the force is doubled, then the acceleration will also be doubled.

Third law: To every action there is an equal and opposite reaction

MEASUREMENT AND UNIT OF FORCE

##### Shree Jain Vidyalaya School Class 8 Notes

Unit of momentum: Momentum of an object is the product of its mass and velocity

MOMENTUM = MASS X VELOCITY

MEASUREMENT OF FORCE: The force which produces an acceleration of 1 centimeter per second2 when it acts on a mass of 1 gram is 1 dyne.

FORCE = MASS OF THE OBJECT X ACCELERATION

Or, F = m x a [Where F = force, m = mass and a = acceleration]

### APPLICATION OF NEWTON’S THIRD LAW

From third law one gets the idea that when one body exerts force on the other, the force is called action. At the same time, the second body also exerts an equal force on the first body but in the opposite direction. This force is called reaction.

For example: When someone is holding a weight of 1 kg in his/ her hand, then the weight is applying a force of approximately 9.8 Newton on his / her hand. But both the weight and the hand are static. So, there is no question of acceleration. But one can ‘feel’ the force exerted by the weight on the hand. This is because the earth is pulling the object downwards. This pull is called gravity.

### MAGNITUDE OF FORCE:

Generally, force produce acceleration on the object on which it is acting. So, the magnitude of force is measured by measuring the acceleration of the body and the mass of the body. But in the absence of any acceleration a force can still operate. This type of force can be measured with the help of spring balance.

Spring Balance: The spring balance is made on the basis of the expansion of the spring due to application of force.

### FRICTION AND ITS MEASUREMENT

Frictional Force: Friction is another kind of force about which you have already received some preliminary ideas in the preceding years. A brief discussion will be made regarding the measurement of frictional forces.

There are two types of friction:

• Forces of static friction
• Forces of sliding friction

Forces of static friction: The frictional forces operating between the two surfaces at rest with respect to each other are called forces of static friction

Forces of sliding friction: The frictional forces operating between surfaces in relative motion are called forces of kinetic friction or forces of sliding friction

• Oppose motion.
• Produce heat in moving parts of machines/engines.
• Produce noise in machines /engines.
• Makes engines to consume more fuel, in other words it reduces the efficiency of engines.
• Cause wear and tear in moving parts of machines/engines

Advantages of friction: The force of friction acts in the opposite direction that the object is moving, so friction slows down the motion of moving objects. Friction also produces unnecessary heat. For example, car engines become hotter because of friction.

### DENSITY AND PRESSURE OF WATER

Density: The same volume of concentrated saline water is heavier than same volume of normal water. At this point, we introduce a term is called ‘density’ which is the mass of unit volume of a substance. In other words the quantity of matter (mass) contained in unit volume of a substance is called the density of that substances.

###### Shree Jain Vidyalaya School Class 8 Notes

Unit of Density: CGS unit of density is gram per cubic centimeter (g/ cc) kilogram per litre (kg/ lit) etc. SI unit of density is kilogram per cubic metre (Kg/ m3)

### PRESSURE OF LIQUID

Pressure is the force per unit area, applied to the surface of the object on which it is placed.

Pressure = Force / Area

SI unit of pressure is Newton per square metre (N /m2)

AIR PRESSURE

We have just discussed the pressure at any point within a liquid. But is the pressure at that point due only to the pressure exerted by the liquid on that point? The answer is no. the air also exerts pressure on the surface of the liquid.

BUOYANCY AND IMMERSION

When an object is immersed in a fluid, the upward force on the bottom of an object is greater than the downward force on the top of the object. If the buoyant force is less than the object's weight, the object will sink. If the buoyant force equals the object's weight, the object will remain suspended at that depth.

### ARCHIMEDES PRINCIPLE

When a solid object is immersed in a liquid (partially or completely), it experiences an upward buoyant force, which is equal to the weight of the liquid displaced by the immersed part of the object.

These notes are very important for the examination of Shree Jain Vidyalaya School.
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