Chapter 1 Physical Environment Force And Pressure Long Answer Type Questions

**Question 1. State Newton’s laws of motion.**

**Answer:**

**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.

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.

**Read And Learn More WBBSE Solutions For Class 8 School Science Long Answer Type Questions**

**Question 2. When a weight is placed on our hand, it is exerting a force on the hand, but the weight does not undergo acceleration. Explain. **

**Answer:**

**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.

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

Similarly, the force which 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 =10^{5 }dyne as below :

1 Newton = 1kg×1m/s²

=1000g×100cm/s²

= 10^{5 }dyne

**Question 3. How do we measure force? Define momentum. **

**Answer:**

**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.

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

Similarly, the force which 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 =10^{5 }dyne as below :

1 Newton = 1kg×1m/s²

=1000g×100cm/s²

= 10^{5 }dyne

**Question 4. What are the effects of a force?**

**Answer:**

**Effects Of A Force:-**

A force of 100N acting on a certain mass for 4s, gives it a velocity of 20m/s. Find the mass of the body if the body was initially at rest. A force can produce the following effects:

- It can move a stationary object.
- It can stop a moving object.
- It can change the speed of a moving object.
- It can change the direction of a moving object.
- It can change the shape of an object. We know, acceleration
- Final velocity-initial velocity /time= 20-0 / 4

(The body was initially at rest) = 5m/s2. Now, force = mass (m) x acceleration mass (m)= F/a = 100/ 5 = 20 kg.

**Question 5. You are given a rubber ball and two types of liquid. How would you assess which liquid has greater density?**

**Answer:**

**Given:**

** I Got a rubber ball and two types of liquid,**

In what direction does the buoyant force on an object immersed in a liquid act?

We know that when an object floats in a liquid, the weight of the object and the weight of the displaced liquid are the same.

The more the density of a liquid, the less the volume of the displaced liquid. Then the liquid having greater density will produce greater buoyancy and consequently, the submerged volume of the ball will be lesser in that liquid.

The greater volume of the ball shall go inside the liquid having lesser density to generate upthrust same as the weight of the ball. The buoyant force acts on an object in a vertically upward direction.

**Question 6. A book can be moved by tilting the table a little as shown. In which direction would friction act in this case? Which force is responsible for the motion of the book? **

**Answer:**

**Given:**

** A book can be moved by tilting the table a little as shown.**

Give some practical examples of where we intend to increase friction for our benefit.

In this case force of gravity is responsible for the motion of the book. Since the book would move in the downward direction due to tilting under the effect of the force of gravity, the force of friction acts on it in the opposite direction – in the upward direction as shown.

- Friction is increased by the following methods:
- Vehicle tires are made with treads to prevent skidding
- Grooves are made on the soles of shoes
- Spikes are provided in the shoes of athletes
- Rough machine belts are employed in some mills to prevent the slipping of wheels.

**Question 7. An object just floats on water. What will happen to it if common salt is added to water? In which figure is the magnitude of buoyancy maximum?**

**Answer:**

**Given: An object just floats on water. **

The addition of common salt increases the density of water which in turn increases the buoyancy of water. Thus the submerged volume of the object decreases as salt is added to the water.

Buoyancy is greater in B since the body is completely immersed in liquid and the volume of displaced liquid is the same as the total volume of the body.

**Question 8. When a body floats in a liquid, what are the forces that act on the body? A metal ball is floating completely immersed anywhere inside a liquid. Explain it.**

**Answer:**

When a body floats in a liquid, what are the forces that act on the body? A metal ball is floating completely immersed anywhere inside a liquid.

- During floatation following forces act on a body:
- Its weight acts vertically downward
- The upthrust or buoyancy of displaced
- liquid acting vertically upward.

The density of the metal ball is the same as the density of the liquid. Then the weight of the ball is exactly equal to the buoyancy offered by the displaced volume of the liquid.

Hence the metal ball floats anywhere inside the liquid.

**Question 9. Write down the factors on which the pressure of a liquid depends at any point inside the liquid.**

**Answer:**

** Factors on which the pressure of a liquid depends at any point inside the liquid:-**

The factors on which the pressure at a point within a liquid depends are the density of the liquid – the more the density of the liquid, the more will be the more pressure exerted by the liquid at that point.

The depth of that point from the surface of the liquid more the depth of the point within a particular liquid, and the more will be pressure exerted by the liquid on that point.

the acceleration due to gravity(g) at that place-more the magnitude of g, the more will be the pressure exerted by the liquid at a point at a certain depth within a particular liquid.

**Question 10. Give a practical application of the fact that a liquid always seeks its own height.**

**Answer:**

Water supply system in a locality

**Pressure of Liquid**

When a solid block of 1 kg is kept on a table, the earth pulls it with a force of W = m x g = 1kg x 9.8m/s² = 9.8 Newton. The block is therefore applying pressure on the table on account of its weight.

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 meter (N/m²).

Let us consider a solid, wooden block of mass 5 kg, which is placed on a table. The total area of contact between the block and the table is 0.2 m².

We can calculate the pressure of the block exerted on the table.

Mass of the block = 5 kg

The magnitude of the force exerted by the block on the table

= 5 kg X 9.8 m/s² (D g = 9.8m/s²)

= 49 Newton

Area of contact of the block and the table =

0.2 m²

So, the pressure of the block exerted on the table = Force/Area

= 49 Newton / 0.2 m² = 245 N/m²

Since all liquids have weight, so when we pour a liquid into a vessel or tumbler, then the weight of the liquid pushes down on the base of the vessel producing pressure.

Therefore, to calculate the pressure exerted by a liquid on the base of a vessel using the formula, pressure =, we are to substitute ‘Force’ with the weight of the liquid and ‘Area’ by the area of the base of the vessel in which the liquid is placed.

Let us consider a tumbler filled with water of mass 10 kg. The area of the floor of the tumbler is 0.5 m². We can measure the pressure exerted by the water on the base of the tumbler.

Mass of the water in the tumbler = 10 kg Magnitude of the force exerted by water on the base of the tumbler

= 10 kg x 9.8 m/s²

= 98 Newton

Area of the base of the tumbler = 0.5 m²

So, Pressure exerted by water on the base of the tumbler

= Force / Area = 98 N / 0.5 m²

= 196 N/m²

**Question 11. Prove with an experiment that pressure at a point within a liquid depends on its depth from the surface of the liquid.**

**Answer:**

**Pressure of Liquid**

When a solid block of 1 kg is kept on a table, the earth pulls it with a force of W = m x g = 1kg x 9.8m/s² = 9.8 Newton. The block is therefore applying pressure on the table on account of its weight.

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 meter (N/m²).

Let us consider a solid, wooden block of mass 5 kg, which is placed on a table. The total area of contact between the block and the table is 0.2 m².

We can calculate the pressure of the block exerted on the table.

Mass of the block = 5 kg

The magnitude of the force exerted by the block on the table

= 5 kg X 9.8 m/s² (D g = 9.8m/s²)

= 49 Newton

Area of contact of the block and the table =

0.2 m²

So, the pressure of the block exerted on the table = Force/Area

= 49 Newton / 0.2 m² = 245 N/m²

Since all liquids have weight, so when we pour a liquid into a vessel or tumbler, then the weight of the liquid pushes down on the base of the vessel producing pressure.

Therefore, to calculate the pressure exerted by a liquid on the base of a vessel using the formula, pressure =, we are to substitute ‘Force’ with the weight of the liquid and ‘Area’ by the area of the base of the vessel in which the liquid is placed.

Let us consider a tumbler filled with water of mass 10 kg. The area of the floor of the tumbler is 0.5 m². We can measure the pressure exerted by the water on the base of the tumbler.

Mass of the water in the tumbler = 10 kg Magnitude of the force exerted by water on the base of the tumbler

= 10 kg x 9.8 m/s²

= 98 Newton

Area of the base of the tumbler = 0.5 m²

So, Pressure exerted by water on the base of the tumbler

= Force / Area = 98 N / 0.5 m²

= 196 N/m²

**Experiment**

A long jar with three outlets A, B, and C along its length is taken. The three outlets are closed with corks or with molten wax.

The jar is now filled with water or any other liquid. The three openings are then opened simultaneously. Liquid flows out through all the holes in the jets.

**Observation:** It is found that of the three liquid jets, the range of the lowest one is the longest as well as the strongest. As we go up, the range (i.e. the distance covered by the liquid jet) decreases.

**Inference:** Liquid is coming out from all three holes. This means the liquid exerts lateral pressure on the wall of the jar. The lateral pressure at any point within a liquid increases with the depth from the free surface of the liquid.

**Question 12. Prove with an experiment that the pressure at any particular point within a liquid is the same in any direction.**

**Answer:**

**Experiment**

**Apparatus needed:** A glass funnel with its mouth closed with a thin rubber sheet, a rubber tube, a glass tube that contains a drop of colored liquid, a scale, a beaker, and some liquid.

A glass funnel with its mouth closed with a thin rubber sheet is taken. A rubber tube is attached at the end of the stem of the funnel.

The other end of the rubber tube is connected to a glass tube that contains a drop of colored liquid (index). The glass tube is fixed horizontally on a stand.

A scale is also attached alongside the glass tube. Some liquid is taken in a beaker. The glass funnel (attached to the rubber tube) is immersed in the liquid and at a certain depth from the free surface of the liquid, the mouth of the funnel is made to face in different directions

**Observation:** The colored liquid drop in the horizontal glass tube remains at the same position, irrespective of the direction of the glass funnel immersed within the liquid (sideways or lateral, upward, downward, etc).

**Inference:** No change in the position of the liquid drop within the horizontal glass tube indicates that the pressure exerted by the liquid at a certain depth is equal in all directions.

- Pressure at a point inside a liquid depends on the density of the liquid [Experiment 1].
- Pressure at a point within a liquid increases with depth [Experiment 2].
- So long as the depth remains the same, the pressure exerted by a liquid is the same in any direction. In other words, liquid exerts pressure evenly in all directions at a given depth [Experiment 3].

Some more experiments can be carried out to understand other properties related to the All these observations can be summarized as the pressure of a liquid. follows:

**Question 13. Using the concept of buoyant force, explain the condition when an object will float on a liquid and when it will sink.**

**Answer:**

**Archimedes’ Principle**

Approximately two thousand years ago, famous Greek philosopher and scientist Archimedes proposed the relation between the apparent loss of weight of an object when immersed in liquid and the weight of the liquid displaced due to immersion of the object.

This relation is known as 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.

From the Experiment stated above we can also conclude that buoyant force is directly proportional to the volume of liquid displaced by the solid object.

When the object is completely immersed in water the weight loss is more because it displaces more water compared to when it is partially immersed.

Accordingly, the weight loss due to buoyant force is more in the case of complete immersion of the object in the water.

If the same object is now immersed completely in two different liquids having different densities, it is found that the weight loss is more in the case of liquid with higher density.

This is due to the fact that though the volume of liquid displaced by the object is the same in both cases, the mass of liquid displaced is more for the liquid Buoyancy is therefore absent in a vacuum since there is no medium to displace in a vacuum.

Principle, the buoyant force is more, which is evident from the higher weight loss in liquid with higher density.

For example, a piece of iron that sinks in water can float in mercury. Hence, we can summarize these observations as follows:

When a body is immersed in a liquid either completely or partly, it always experiences a buoyant force or upthrust.

The buoyant force is directly proportional to the volume of liquid displaced by the solid object.

The buoyant force is directly proportional to the density of liquid displaced by the solid object.

When a body floats in a liquid, the weight of the whole body acting vertically downward is completely balanced by the buoyant force produced by the displaced liquid.

The weight of a body immersed in a liquid is always less than its actual weight because of buoyancy or upthrust.

This weight of the body in the immersed condition is called the apparent weight which is always less than the true weight of the body.

Let us perform an experiment similar to that with higher density. So according to Archimedes shown earlier to verify Archimedes’ Principle.