WBBSE Notes For Class 8 School Science Chapter 1 Physical Environment Force And Pressure

Chapter 1 Physical Environment Force And Pressure

Concept Of Force

No one has seen a force. However, we always see or feel the effect of a force. It can only be explained by describing what happens when a force is applied to an object.

It is our common experience that an object at rest can move when force is applied to it. Pushing, pulling, and hitting objects are the ways of bringing objects in motion.

We open or close a door by applying a pull or a push. We kick a football to set it in motion. Application of force can change the direction of a moving body and accelerate or decelerate its velocity.

In a cricket match, when a moving cricket ball is hit by a bat, then the direction of the ball changes, and it goes in another direction.

When a hockey player hits a moving ball, the speed of the ball increases. When we pedal the bicycle faster, then the speed of the bicycle increases.

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If we apply brakes to the moving then the speed of the bicycle increases aluminum metal, it undergoes a change in shape and an aluminum sheet is formed.

We require force to lift an object. Suppose, a book is lying on the table. Some force is required to lift this book from the table. The heavier the book, the greater the force needed to lift it.

Class 8 Science Force And Pressure Notes WBBSE

A weightlifter uses enormous force to lift the weights. Even when we are standing on our feet, force plays the most important role in our muscles and bones. Likewise, we can find several events in our everyday life where force plays an important role.

Famous scientist Sir Isaac Newton put forward three laws of motion that elaborate the concept of force and motion.

Newton’s Laws of Motion

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

2. Second Law: The rate of change of momentum is directly proportional to the impressed force and takes place in the direction in which the force acts.

3. Third Law: To every action, there is an equal and opposite reaction. Newton’s first law gives us an idea about the inertia and definition of force. bicycle, then the speed of the bicycle decreases.

Even the shape of an object can change when we apply force to it. Force is required to expand or compress a spring.

When we hammer a piece of aluminum metal, it undergoes a change in shape and an aluminum sheet is formed. We require force to lift an object.

Suppose, a book is lying on the table. Some force is required to lift this book from the table. The heavier the book, the greater the force needed to lift it.

A weightlifter uses enormous force to lift the weights. Even when we are standing on our feet, force plays the most important role in our muscles and bones.

Likewise, we can find several events in our everyday life where force plays an important role. Famous scientist Sir Isaac Newton put forward three laws of motion that elaborate the concept of force and motion.

Newton’s first law clearly states that no object can move unless force is applied to it. A body at rest does not take any initiative to move all by itself.

Class 8 Science Force And Pressure Notes WBBSE

An external force is required for this purpose. Similarly, a moving body can change its velocity (either the direction or magnitude of velocity or both) only when an external force is applied to it.

To Understand Clearly What We Mean By The Term ‘Force’, Let Us Consider The Following Situations :

1. It is a matter of common experience that if a book is lying on a table, it continues to be lying on the table at the same position forever until somebody comes and displaces it to some other position. For moving it, one has to either lift it, push it, or pull it.

It shows that to bring a body into motion from its state of rest, some external agency or influence has to act on the body.

An isolated body (i.e., a body that is free from external influences) will maintain its state of rest forever. This fact demonstrates the inertia of rest. It is an intrinsic property of the material.

2. Let us now consider a ball rolling on a rough surface. We observe that the speed of the ball gradually decreases and finally the ball stops.

If the surface on which the ball is made to roll is made smooth, we see that it covers a much longer distance before coming to rest.

One can well imagine that the smoother the surface, the longer and longer will be the distance covered by the ball before it stops.

In an ideal situation where the surface is perfectly smooth, the ball would continue moving forever in the same direction with a constant speed. This fact demonstrates the inertia of motion.

Now the question arises—why does the ball stop after moving some distance on a rough surface? What does the rough surface do to the motion of the ball? The roughness of the surface provides an external influence called friction (or force of friction) which decreases the speed of the ball.

3. In the game of football, a player can change the direction of the moving ball by kicking it. The kick applied by the foot is the external influence that changes the direction of the moving ball.

This external influence is called force which is necessary to change the state of rest or speed or the direction of motion of a body.

So from the first law, we realize that whether force is operating on an object, we have to observe the change of velocity of the object.

If the object remains static or its velocity remains unchanged, we can conclude that either no force is operating on the object or the aggregate of various forces operating on the object is zero.

In other words, we can say that if no net force acts on a body, its acceleration is zero. Net force implies the resultant or aggregate of various forces operating on an object.

Balanced And Unbalanced Forces

If the resultant force of all the forces acting on a body is zero then the forces are called balanced forces.

For example, take a string. If two persons pull the string by equal and opposite forces, then it will not move at all since the two forces are equal in magnitude and opposite in direction.

Class 8 Science Force And Pressure Notes WBBSE

Such forces are known as balanced forces. However, balanced forces may change the size and shape of the body.

For example, when we press a rubber ball with both hands, we apply equal and opposite forces on the ball, and the shape of the ball is changed though it remains static in position.

If the resultant force of all the forces acting on a body is not zero then the forces are called unbalanced forces.

Unbalanced forces can change the state of rest or of uniform motion or the direction of motion of a body. Thus, unbalanced forces produce an acceleration in the body.

Naturally balanced forces do not cause any change in the state of rest or of uniform motion of a body along a straight line. Thus, balanced forces produce zero acceleration.

For example, in a tug of war, two teams pull the rope in opposite directions. If the rope moves in any direction, then the forces applied by both teams constitute unbalanced forces.

WBBSE solutions for 8 Chapter-1 Physical environment Sec-1 Forces And pressure balnced and unbalnced forces

The resultant or net force makes the rope move in its direction with a certain acceleration. From the above, we can draw the following conclusions:

  1. Balanced forces cannot change the speed or direction of motion or state of the rest of a body.
  2. To change the speed or direction of motion or state of the rest of a body, an unbalanced external force must act on the body.

WBBSE solutions for 8 Chapter-1 Physical environment Sec-1 Forces And pressure balanced2

Measurement and Unit of Force

1. Unit of Momentum

Let a cricket ball and a loaded truck move equally fast toward you. It would be possible to stop the cricket ball but you just cannot think of stopping the truck even though both the ball and the truck move equally fast.

On the other hand, it is convenient to stop a slow-moving cricket ball as compared to a fast-moving one. This happens due to the quantity of motion contained in the body.

The quantity of motion contained in a body in turn depends on its mass and velocity. This defines another important physical quantity called momentum.

Class 8 Science Force And Pressure Notes WBBSE

The momentum of an object is the product of its mass and velocity.
Momentum = mass x velocity

CGS unit of momentum is gram-centimeter per second [g.cm/s]

SI unit of momentum is kilogram-meter per second [kg.m/s]

When an object of mass 1kg is moving with a velocity of 1 meter per second then its momentum is 1 kg.m/s.

2. Measurement of Force

To measure the force on a particular object, we have to measure the acceleration of the object which is produced by the impact of the force.

From Newton’s law, it can be derived that force acting on an object is the product of its mass and its acceleration.

Force = mass of the object x acceleration or, F = m x a

The higher the force applied to an object, the greater will be its acceleration.

The higher the mass of the body, the lesser will be its acceleration for equal force.

The SI unit of force is Newton and the CGS unit of force is done.

The force that produces an acceleration of 1 meter per (second)2 when it acts on a mass of 1 kilogram is 1 Newton.

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Similarly, the force that produces an acceleration of 1 centimeter per (second)2 when it acts on a mass of 1 gram is 1 dyne. It can be shown that 1 Newton =105 dyne as below :

1 Newton = 1kg×1m/s²
=1000g×100cm/s²
= 105 dyne

Chapter 1 Physical Environment Force And Pressure Application of Newton’s Third Law

An interaction between two objects results in a force between the two objects. At least two objects must interact with each other for a force to come into play and show its effect.

Suppose a man is standing in front of a wall. Since there is no interaction between the man and the wall, no force acts between them.

Now, suppose the man kicks the wall. This activity generates an interaction between the man and the wall. This interaction gives birth to a pair of forces called to action and reaction.

From Newton’s third law, one gets the idea that when one body exerts a 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 a reaction.

So every force of action is always associated with an equal but opposite force of reaction. This means that no force can exist singly in nature.

Class 8 Force And Pressure Notes WBBSE

In other words, we can say that forces always appear in pairs in nature and they act on different bodies.

As the man kicks the wall, he applies a force called the action on the wall. The wall also applies a reaction force on the foot of the man.

This reaction force causes a feeling of pain in the foot. Action and reaction are the same in magnitude.

The harder the kick, the greater would be the pain experienced by the man. Let us study some more examples.

1. For example, when a cricket ball is hit with a bat, the bat exerts force on the ball. So the cricket ball is the point of application of the action force.

The ball also exerts equal reaction force on the bat. So cricket bat is the point of application of reaction force.

2. When a person is standing on the floor, the person is exerting a force on the floor. According to Newton’s third law, the floor is also exerting an equal amount of force on the person.

If the former is called action, the latter is the reaction force. But both the weight and the hand are static. So, there is no question of acceleration. But one can “feel” the force

3. 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.

Class 8 Force And Pressure Notes WBBSE

exerted by the weight on the hand. This is because the earth is pulling the object downwards. This pull is called “gravity”.

If the earth is “pulling” the object downwards with a force, known as gravity, then according to Newton’s law, it must cause acceleration.

However, the absence of any acceleration suggests that there is a force equal in magnitude to the gravitational force acting in the opposite direction through the hand.

WBBSE solutions for 8 Chapter-1 Physical environment Sec-1 Forces And pressuremeasurment and unit of force

Chapter 1 Physical Environment Force And Pressure Magnitude of Force

Generally, force produces acceleration on the object on which it is acting. So, the magnitude of the force is measured by measuring the acceleration of the body and the mass of the body.

But even in the absence of any acceleration, a force can still operate. For example, when we are holding a weight in our hand, it is exerting a force on our hand.

We can feel it but we find no acceleration. This force arises because the weight is being attracted by the earth towards its center.

This force is called the force of gravity and it is equal to the weight of a body on earth. The force of gravity causes all the objects to fall towards the earth.

Class 8 Force And Pressure Notes WBBSE

Had we not held the weight by our hand, it would have fallen on the ground due to the force of gravity. Consider another situation.

To lift a body of mass 1 kg upwards from the surface of the earth, the minimum amount of force (or effort) needed should be equal to the force of the earth’s gravity acting on it downwards (i.e., towards the center of the earth).

It means the force to be applied in the upward direction must be able to overcome the weight of 1kg mass This type of force can be measured with the help of a spring balance.

A spring balance utilizes the fact that a spring is elongated due to the application of force. A spring balance consists of a spring, attached to the ceiling at one end, and at the other end, there is a hook used to hang a weight.

If a weight is hung from this end of the spring, then the spring will be elongated. There will be no acceleration to the weight, but the elongation of the spring occurs due to the force applied by the weight hanging from one end of the spring.

If a scale is placed by the side of the spring and if a pointer is placed by the side of the spring as shown then it can be utilized to “measure” the force applied by various objects of different masses hung onto the spring.

When no weight is hung on the spring, the position of the pointer on the scale is marked “0”. If a mass of exactly 1 kg is hung on the spring, the spring is elongated and the position of the pointer is marked on the scale as “1 kg”.

In this way weights of different masses can be hung onto the spring and using the position of the pointer attached to the spring, the instrument can be “calibrated” This can now be used to measure the magnitude of force with which the earth is pulling a weight.

WBBSE solutions for 8 Chapter-1 Physical environment Sec-1 Forces And pressure Fricition and its measurment

 

 

 

 

 

 

 

 

The force with which the earth is pulling 1 kg of mass downwards is equal to the force applied to the spring.

The magnitude of this force is equal to the weight of the mass attached to the spring and it is given by, F = m x g where m = mass of the body and g = acceleration due to gravity

F = 1kg x 9.8 m/s² = 9.8N.

Class 8 Force And Pressure Notes WBBSE

When a body is released from a certain height, it falls freely towards the surface of the earth under the action of gravitational force with a constant acceleration called acceleration due to gravity (g).

The more the mass of the object more will be the elongation of the spring and the more will be the force with which the earth will pull it downwards.

For example, the magnitude of this force for a 2 kg mass will be (2 kg x 9.8 m/s²) = 19.6 N. direction in which an object moves (or tends to move).

In this section, a brief discussion will be made regarding the measurement of frictional forces. Let us go through the following illustrations one after another.

Chapter 1 Physical Environment Force And Pressure Friction and its Measurement

 

1. Frictional Force

The force which always opposes the motion of one object over another object in contact with it is called friction.

Friction occurs between the two surfaces which are in contact with each other. Friction always acts in a direction opposite to the

1. Suppose a box, kept on the top of a table, is being pulled by a spring balance. Since a spring balance is attached, hence the force with which the box is being pulled can be measured.

If the spring balance records 1 kg, then the box is being pulled with a force of 9.8 Newton. Despite applying force, if the box does not move from its initial position, then we can conclude that the surface of the table is exerting a frictional force of equal magnitude on the box.

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So the applied force (used to pull the box) is balanced by an opposite frictional force.

If the box is now gradually pulled with increasingly greater force, then we find that at some definite force, the box just begins to move.

Once the motion of the box starts, the same force also produces some acceleration. But then the magnitude of the frictional force cannot be determined from the reading on the spring balance.

The frictional force operating between the two surfaces at rest with respect to each other is called the force of static friction.

‘g’ is a constant for the earth and it has a value of 9.8 m/s2 For a mass of 1kg (i.e., m = 1kg), the magnitude of the force is therefore,

WBBSE solutions for 8 Chapter-1 Physical environment Sec-1 Forces And pressure Friction

In the above experiment, if the force with which the box is being pulled is gradually increased, the static frictional force also proportionally increases till the maximum or limiting force of static friction is attained, which is the minimum force necessary to move the box.

2. Let us consider a rectangular block remaining static on the surface of a table. With the help of a spring balance, the magnitude of the maximum force of static friction is measured.

Then, some additional weight is placed on the block. Again with the help of a spring balance, the magnitude of the maximum force of static friction is determined.

It Is found that the magnitude of the maximum force of static friction is greater in the case of the latter.

When some additional weight is placed on the block, the weight of the block is now effectively increased and consequently, the downward force (action) on the table is increased.

According to Newton’s third law, the table is also exerting an upward force (reaction), equal in magnitude, on the block. This reaction force is called the normal reaction (R).

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We know that every object has a rough surface, though the surface may appear to be smooth to the naked eye.

These microscopic tiny hills and grooves on the surfaces of objects are called ‘irregularities of surfaces’.

When we try to move one object over another object (i.e., the surface of another object), the irregularities present on their surfaces get locked with one another to oppose the motion. This results in friction.

The more the weight of the object to be moved, the greater the normal reaction (R) of the underlying surface and consequently, the greater the locking of the irregularities of the two surfaces in contact.

This explains why greater force is needed to move the block with an additional weight placed on it over the same surface of the table.

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3. Let us consider another situation where a rectangular wooden block is placed on the surface of a table in such a way that the side with the largest area is in contact with the table.

Then the magnitude of the maximum force of static friction is measured. Next, the wooden block is placed on the table in such a way that the side with the minimum area remains in contact with the table.

The magnitude of the maximum force of static friction is again measured with the help of a spring balance. It will be found that in both cases the magnitude of the static frictional force is the same.

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So it can be concluded that static frictional force is approximately independent of the area of contact between the two bodies.

WBBSE solutions for 8 Chapter-1 Physical environment Sec-1 Forces And pressure Pointer

4. Friction depends on the nature of the surface of the two bodies in contact with each other. This can be proved by keeping a wooden block on two different tables – one with a smooth surface and the other with a rough surface.

In both cases, the maximum static frictional forces are measured with the help of a spring balance. In the former case, the magnitude of this force is found to be smaller.

So The Above Experimental Observations Can Be Summarized As Follows:

1. The more the action-reaction force that operates vertically between the two surfaces in contact, the more is the magnitude of the static frictional force.

2. The magnitude of the static frictional force is approximately independent of the area of contact between the two bodies.

3. It depends on the nature of surfaces in contact with each other.

4. It is our common experience that if a block is pushed along a long, horizontal table with some initial velocity, the block, after some time, eventually comes to rest.

We can infer that the surface of the table is exerting some frictional force. In fact, whenever the surface of one body is sliding over the surface of another body, each exacts a force of a fraction on the other.

WBBSE Notes Of Force And Pressure Class 8

5. This force on each body is a way in the direction oppos tie to its motion relative to the other Dody. The friction force present when one object moves slowly (or slides) over the surface of another object is known as sliding friction.

Thus, Iding friction comes [Units of volume are liter, cubic centimeter (cc), cubic meter (m³), gallon, etc. So, the CGS unit of density is gram per cubic centimeter (g/cc), kilogram per liter (kg/lit), etc.

SI unit of density is kilogram per cubic meter (kg/m³).

In the above experiment, the density of concentrated saline water was more than that of the ordinary water and hence the former was found to be heavier than the latter.

Into play when an object is sliding over another object. Sliding friction is smaller than static friction. Thus it is easier to keep an object moving which is “ready in motion than to move the same object from the rest position.

Since the interlocking of the two surfaces is less when an object has already started moving, therefore, the sliding friction is smaller than the static friction.

6. When an object (like a wheel or a ball) rolls over the surface of another object, the resistance to its motion is called rolling friction.

Rolling friction is much less than sliding friction. Thus it is easier to ‘roll’ than to ‘slide’ an object over another object.

Heavy machines can be easily moved from one place to another by placing round logs of wood under them. The logs act as rollers that turn around like wheels to move the machine forward. Heavy luggage is fitted with rollers to pull them easily.

2. Disadvantages of Friction

When an object moves over a surface, for example, a car is moving on the road, it feels frictional forces that always try to reduce its velocity.

So to maintain a steady velocity, additional expenditure of energy is required. Some machine parts experience frictional forces during their operation.

As a result machine parts suffer wear and tear, and they have to be replaced from time to time. Thus maintenance cost of the machine increases.

Friction produces heat which may damage the machines and the tires of vehicles.

WBBSE Notes Of Force And Pressure Class 8

3. Advantages of Friction

Though it seems that frictional force only creates problems in our everyday life, it actually has several advantages.

We can stand and walk or run easily on a surface due to the presence of frictional forces. An automobile can smoothly run on the road due to this force.

If the roads were very smooth, the frictional force would have reduced considerably, and then controlling the movement of the automobile would have been difficult.

We can hold various things in our hands due to the existence of frictional forces which exists between the object and our hand.

Force And Pressure Notes Class 8 WBBSE

In absence of frictional forces, we could not have gripped any object properly.
We can light a match stick using frictional force.

To overcome the resistance offered by the frictional force we have to exert force to move the match stick over the matchbox, a part of which produces heat that lights the match stick.

Chapter 1 Physical Environment Force And Pressure Density of Liquid

Let us now discuss the density of the liquid. Let us take two bottles, both of which can contain equal volumes of liquid.

One bottle is completely filled up with water and the other bottle is completely filled up with concentrated saline water.

Each of the bottles was hung from the hook of a spring balance. It was found that concentrated saline water is heavier than normal water, although the volume was the same in both cases.

Definition of Density

This indicates that the same volume of concentrated saline water is heavier than the same volume of normal water. At this point, we introduce a term called “density” which is the mass of a unit volume of a substance.

In other words, the quantity of matter (mass) contained in a unit volume of a substance is called the density of that substance.

Density = Mass/Volume

Force And Pressure Notes Class 8 WBBSE

Unity of Density

Units of volume are liter, cubic centimeter (cc), cubic meter (m³), gallon, etc. So, the CGS unit of density is gram per cubic centimeter (g/cc), kilogram per liter (kg/lit), etc.

SI unit of density is kilogram per cubic meter (kg/m³). In the above experiment, the density of concentrated saline water was more than that of the ordinary water and hence the former was found to be heavier than the latter.

 

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