Exploring Forces

Hello students, welcome to today's science lesson. I am so happy to be here with you to explore one of the most interesting and practical topics in science - forces. You know, forces are something we experience every single moment of our lives, even though we may not always notice them. By the end of this lesson, you will understand what forces are, how they work, and why they are so important in our daily lives. So let's begin our journey into the world of forces.

Before we start learning about forces formally, let me ask you some interesting questions. Have you ever wondered why it feels much harder to pedal your bicycle when you are going uphill compared to when you are riding on flat ground? Or think about this - why is it easier to slip on a wet surface than on a dry one? And here's another one - have you ever been on a swing and noticed that just after the swing reaches its highest point and begins to come down, you feel light, almost like you are floating? These are all questions related to forces, and by the end of this chapter, you will be able to answer all of them and much more.

Now let me tell you a story about two friends, Sonali and Ragini, who were cycling during their summer vacation. They pumped air into their bicycle tyres and set off on their journey. As they rode through the village, they felt the wind pushing against them. Sonali explained to Ragini that they were riding against the wind and needed to push their pedals harder to move faster. Then they went up a long path to a hilltop. Some parts of the road were rough where they found it hard to pedal, while other parts were smoother. When they were coming down the slope of the hill, they realized that their bicycles were moving down at a great speed even though they were not pedalling! Sonali yelled that it felt like something was pulling them downhill. What do you think that something was? That, my dear students, is the force of gravity acting on them. We will learn all about gravity very soon.

So now let's start with the most basic question - what exactly is a force?

Let us try to experience push and pull ourselves. Take a large cardboard box. Now try moving the box in as many different ways as you can think of. You can push it, pull it, lift it up and carry it, or drag it sideways. Did you move the box in any other way? In all the ways that you might have used to move the box, you had to apply a push or pull to the box. Generally, the push or pull applied on an object is called force in science. So the simplest definition of force is that it is a push or a pull.

Now let's think about what a force can do to the objects on which it is applied. We experience push or pull in our daily lives all the time, often without even realizing it. Let me give you some examples from Table 5.1 in your textbook. When your friend holds your moving bicycle from behind to stop it, that is a pull, and its effect is stopping or decreasing the speed of the bicycle. When you hit a moving ball with a bat, that is a push, and it changes the direction of the moving ball. When you press an inflated balloon, that is a push, and it causes a change in the shape of the balloon. Can you think of more examples? Opening a drawer is a pull, stretching a rubber band is a pull, a fielder stopping a ball is a push or pull, kicking a football is a push, applying brakes on a moving bicycle is a push, rolling a chapati is a push, and turning the steering handle of an autorickshaw is a pull. What effect can the application of force have on objects?

From all these examples, we can conclude that the force applied on an object may make an object move from rest if it was not moving before. It can change the speed of an object if it is already moving. It can change the direction of motion of an object. And it can bring about a change in the shape of an object. In fact, a force can cause some or all of these effects. Does this mean that whenever there is a change in speed or direction, or change in shape, a force is acting on the object? Yes, that's exactly right. None of these take place without the action of force.

Now here's an interesting question. Suppose an object is at rest. Does it mean that no force is acting on this object? Not necessarily. It means that the forces acting on the object are balancing one another. You will learn about balanced forces in higher grades, but for now, remember that an object at rest could still have forces acting on it as long as they cancel each other out.

Now let's think about this - are forces an interaction between two or more objects? When you push a table, your hand is one object applying force on another object - the table. Here, we say that your hand and the table are two objects interacting with each other. Think of all the actions we listed earlier. How many objects are involved in each of the actions? Do you notice that forces result only when two objects are interacting in some way or the other? From these examples, we can infer that at least two objects must interact for a force to come into play. A force is a push or pull on an object resulting from the object's interaction with another object. The SI unit of force is newton, written with a small 'n', and its symbol is N.

Let me pause here and make sure you understand this. When you pushed the table with your hand, did you feel a force on your hand too? The moment you stopped pushing, the force on your hand disappeared. Whenever two objects interact, each object experiences a force from the other. As soon as the interaction ceases, the two objects no longer experience the force. This is a very important point to remember - forces always come in pairs, and they exist only during the interaction between two objects.

Now let's learn about the different types of forces. There are two main categories of forces - contact forces and non-contact forces.

Let's first talk about contact forces. In many situations, we find that to apply a force on an object, physical contact is necessary between our body and the object. This contact can be direct, such as using our hands or other body parts, or indirect, such as using a stick or rope. Forces of this type which act only when there is physical contact between the objects are called contact forces.

The first type of contact force we will learn about is muscular force. When we perform any physical activity, such as walking, running, lifting, pushing, jumping, or stretching, the force is caused by the action of muscles in our body. The force resulting due to the action of muscles is known as muscular force. Muscular force occurs when muscles contract and elongate while doing any activity. Animals, birds, fish, and insects use muscular forces for movement and survival. Humans used the muscular force of some animals like bullocks and horses to carry out many tasks for a long time - for plowing fields, carrying loads, and transportation. Muscular force plays an important role in many functions inside our body too. This force helps us chew food and push it through the alimentary canal during the process of digestion. The expansion and contraction of our heart muscles allows the blood to circulate in our body - a process essential for survival.

Now let's learn about another very important contact force - friction. Have you ever noticed that a ball rolling on a flat ground stops on its own after some time? If you stop pedalling your bicycle on a flat road, it slows down and comes to a stop. If the road is rough, it stops sooner than on a smoother road. What causes the change in the speed of objects in such situations? We have learnt that a force is essential to change the speed of an object. However, in all these situations no force appears to be acting on the objects, yet their speed gradually decreases and they come to a stop after some time. Is it possible that some force is indeed acting on them? Which force is that?

Let us investigate this with an activity. Take an object with a flat base, such as an empty lunch box or a geometry box or a notebook, and place it on a table or floor. Gently push it and observe. Does it stop after travelling some distance? Is there a force acting on it which brings it to rest? Now repeat by pushing the object in the opposite direction. Does it stop again after travelling some distance?

On pushing, the object stops after sliding a certain distance. This must be due to a force acting between the surfaces of the sliding object and the table or floor which are in contact. This force must be acting on the object in a direction opposite to its direction of motion. This force is what brings the object to a stop. The force that comes into play when an object moves or tries to move over another surface is called the force of friction or simply friction. Friction always acts in a direction opposite to the direction in which the object is moving or trying to move. The force of friction is a contact force since it arises due to two surfaces in contact.

Now, why does friction arise? Friction arises due to the irregularities in the two surfaces in contact. Even surfaces which appear smooth have a large number of minute irregularities. When placed in contact, the irregularities of two surfaces lock into each other and oppose any effort to move one surface over the other. Does this mean that the force of friction will be greater if the surfaces are rough? Yes, exactly!

Let us explore this further. Try the activity again, but this time place the same object on different surfaces, such as glass, cloth, wood, ceramic tile, and sand. Does the object stop after travelling the same distance as before? Does the object stop at the same distance on all surfaces? For different surfaces, the object stops after moving different distances, so we can say that the force of friction depends upon the nature of the surfaces in contact. Friction is greater on rough surfaces.

Now, here's an interesting question. Does the force of friction act only if the objects are moving on solid surfaces? What about objects moving through liquids and gases? Air, water, and other liquids also exert force of friction on the objects moving through them. Hence the objects such as aeroplanes, ships, boats, and high-speed trains are designed with specific shapes to reduce the force of friction due to the air or water around them. This is why airplanes have streamlined shapes and ships have smooth hulls - to reduce friction and move faster.

Now, is it essential for an object applying force on another object to always be in contact with it? Let me think about this. When you use a magnet to attract a piece of iron, do you need to touch the iron with the magnet? No, you don't! So there must be forces whose effect can be experienced even if the objects are not in contact. These forces are called non-contact forces.

Let's learn about non-contact forces now. The first non-contact force we will learn about is magnetic force. Do you remember learning about magnets in the chapter 'Exploring Magnets' in your Grade 6 textbook? We learnt that a magnet attracts objects made of magnetic materials. When two magnets are brought close to each other, like poles, which means North-North or South-South, repel each other while unlike poles, which means North-South, attract each other. Attraction and repulsion between objects are also a form of push and pull, that is, a force. A magnet can exert force on another magnet or a magnetic material without being in contact with it.

Let us do an activity to understand this better. Take two ring magnets and a wooden stick. While holding the stick in a vertical position over a wooden table, insert one ring magnet onto the stick. Now insert the second ring magnet above it such that the like poles of the two magnets face each other. Does the second magnet stay floating above the first magnet? Try pushing the second magnet down gently. Do you feel a force on it? Now, reverse the poles of both the magnets. Does the second magnet still remain floating? We find that a magnet can exert force on another magnet without being in contact with it. The force exerted by a magnet on another magnet or a magnetic material is called magnetic force. Since a magnet can exert a force from a distance without being in contact, it is called a non-contact force.

Now let's learn about another non-contact force - electrostatic force. Let me show you a fascinating activity. Take a plastic scale or a plastic straw, a piece of polythene, and small pieces of paper. Rub the plastic scale or straw vigorously with polythene. Do not touch the rubbed part with your hand or any metal object. Now, bring it close to the small pieces of paper placed on a table, taking care not to touch the paper pieces. Do you notice something surprising? The paper pieces get pulled towards the plastic scale or straw and stick to it when it is brought close to paper pieces. Why does this happen?

When two objects of certain materials are rubbed together, electrical charges build up on their surfaces. These charges are called static charges as they do not move by themselves. The object that acquires static charges is said to be a charged object. A charged object attracts, that is, exerts a force on uncharged objects made of certain materials, such as small pieces of paper. This force comes into play even when the objects are not in contact.

Let us do another activity with objects made of different materials. Take two balloons, a length of thread, and a woollen cloth. Inflate two balloons and hang them in such a way that they do not touch each other. Rub both balloons with the woollen cloth and release them. Be careful not to touch the rubbed balloons with your fingers. What do we observe? We observe that the balloons move away from each other as if they are repelling each other. Now bring the woollen cloth used for rubbing the balloons close to one of the rubbed balloons. What happens? They move towards each other as if they are attracting each other.

What do we infer from these observations? We found that the two similarly charged balloons repel each other whereas a charged balloon and the woollen cloth attract each other. Does this indicate that the charge on the balloon is of a different kind from the charge on the woollen cloth? Since the balloons were charged in the same way, we can say that they have acquired similar charges. As the similarly charged balloons repelled each other, we can infer that similar or like charges repel each other. Both the rubbing object and the rubbed object get charged but they acquire opposite kind of charges. Their attraction shows that opposite kind or unlike charges attract each other. The two kinds of static charges are said to be positive and negative. The force exerted by a charged body on another charged body or an uncharged body is called electrostatic force. It is a non-contact force.

Now let's learn about the most important non-contact force - gravitational force. Take a ball and throw it vertically upwards. Does it come down? Now throw it again, but this time harder. Does it still fall back down to the ground? Think about different situations around you where any object thrown up in any direction finally falls or comes back to the ground or floor. Why do all the objects fall towards the Earth? Is there any force which acts on them? What exerts this force?

Since all the objects fall towards the Earth, it means the Earth attracts or pulls them. The force with which the Earth attracts objects towards itself is called the gravitational force. The gravitational force exerted by the Earth is also called force of gravity or simply gravity. Since the gravitational force acts without contact with the object it attracts, it is a non-contact force. Gravitational force is always an attractive force, unlike magnetic force or electrostatic force, which can either be attractive or repulsive.

You might have noticed that when an object is dropped from a height, it takes a straight vertical path downwards before touching the ground. When an object is thrown vertically upwards, the object moves up straight, slows down, stops momentarily at the top, and then takes a straight vertical path downwards. While going up, the speed of the object goes on decreasing till the object comes to a stop, its direction of motion changes, and while coming down the speed goes on increasing. We say that the object undergoes a vertical motion when it moves in a vertical direction under the influence of the gravitational force.

Now, does the Earth pull every object with equal force? Let's find out.

The force with which the Earth pulls an object towards itself is called the weight of the object. The weight measures how strongly an object is pulled by the Earth. Since the weight is a force, it is measured in the same unit as that of force. Therefore, SI unit of weight is also newton or N.

Let us now try to find out if the Earth pulls every object with equal force. Take a spring and a few objects of different masses, such as a pencil box, a tiffin box, and a small stone. Hang one end of the spring from a nail. From the other end, hang an object and observe the spring. Does the spring stretch? Now hang the other objects, one by one, and notice the stretch in the spring each time. Is the stretch caused by each object the same?

When an object is hung from a spring, the spring stretches due to the force applied on the object by the Earth. We find that the stretch caused in the spring is different for different objects. This indicates that the Earth pulls different objects with different forces, that is, the weight of different objects is different.

A spring balance is a simple device used to measure weight or force. It consists of a spring fixed at one end, with a hook attached at the other end. When we hang an object from the hook, the spring stretches, and the amount of stretching gives the weight of the object. There is a scale on the balance which is marked to show the weight in newton. Usually, there is also another scale to show the corresponding values of mass in gram. These values have been marked with the assumption that the spring balance is used on the Earth, with the Earth's gravitational force attracting the object.

Now let us learn to measure the weight using a spring balance. Look at the spring balance carefully. What is the maximum weight it can measure? The maximum weight it can measure is 10 N. Thus, this scale has a range of 0 to 10 N. Let us now try to find the smallest value of weight that can be measured by the spring balance. Look at the spring balance and note down the following. How much is the weight difference indicated between the two bigger marks? The weight difference indicated between 0 and 1 N or between 1 N and 2 N is 1 N. How many divisions, shown by smaller marks, are there between these two bigger marks? There are 5 divisions between these marks. How much weight does one small division indicate? One small division can read 1 divided by 5, which is 0.2 N. So, the smallest value that the spring balance can read is 0.2 N.

Now, here's an important distinction between mass and weight. The mass of an object is the amount of matter in an object and is measured in grams or kilograms. Its value remains the same at every place. Weight, on the other hand, is the gravitational force with which the Earth or another planet pulls an object. Since gravitational force can vary very slightly from place to place on the Earth and can be very different on different planets, weight can change, but mass does not.

Let me give you an interesting table to understand this better. On Earth, an object with a mass of 1 kg has a weight of about 10 N. On the Moon, the same object would have a weight of only about 1.6 N because the Moon's gravitational pull is weaker. On Mars, it would weigh about 3.8 N, on Venus about 9 N, and on Jupiter, which has very strong gravity, it would weigh about 25.4 N. But the mass remains 1 kg in all these places!

In everyday life, particularly for the goods we commonly use, we are more interested in the amount of matter in an object, its mass, rather than the force applied by the Earth upon it, its weight. However, while the units of mass are used, instead of the term mass, the term weight is typically used. For example, it is said that the weight of the wheat bag is 10 kg. But in scientific use, this is not correct, and it is important to use the correct terms with their correct units.

Now let's learn about floating and sinking. While taking out water from a bucket filled with water using a mug, do you notice that the mug feels lighter when it is inside water? Let us try to understand this. If we place some objects on water, some of them float, while others fall to the bottom. The gravitational force of the Earth is acting on all objects, then why don't all objects fall to the bottom?

Take an empty bottle with its lid closed tightly and a bucket full of water. Push the bottle in the water. Do you feel an upward push? Release the bottle. Does it bounce up? You would have felt an upward push and the bottle bounces back to the surface of the water. This indicates that water applies a force on the bottle in the upward direction. In fact, all liquids apply a similar force. The force applied by a liquid on an object in the upward direction is known as upthrust or buoyant force.

When an object is placed in a liquid, the gravitational force due to the Earth acts on it downwards. But a buoyant force is applied on it by the liquid in the upward direction. If the gravitational force is more than the buoyant force, the object sinks, but if the two forces are equal, the object floats. One of the factors on which the buoyant force depends is the density of the liquid. You will learn about density in a later chapter of your book.

Here's an interesting fact. Archimedes, a famous Greek scientist, discovered that when an object is fully or partially immersed in a liquid, it experiences an upward force which is equal to the weight of the liquid it displaces. This is known as Archimedes' Principle. If the weight of a liquid displaced by an object is smaller than the weight of the object, the object will sink in the liquid. If the weight of the liquid displaced is equal to the weight of the object, the object will float in the liquid.

And here's something really interesting - there are some rocks which can float on water! One such rock is Pumice, which is formed during volcanic eruptions. When lava with lots of gas and water vapour cools quickly, it traps tiny bubbles of gas inside. This creates a light, porous rock filled with air pockets which is less dense than water and floats on it.

Now let me summarize what we have learned in this chapter. A force is a push or pull on an object resulting from the object's interaction with another object. The SI unit of force is newton and its symbol is N. Forces can act with or without contact. Muscular force and frictional force are some examples of contact forces. Magnetic force, gravitational force, and electrostatic force are non-contact forces. Force can change an object's speed, direction of its motion, or both. Force can also change the shape of an object. The force which comes into play when an object moves or tries to move over another surface is called force of friction or simply friction. It acts in a direction opposite to the direction in which the object is moving or trying to move. The force exerted by a magnet on another magnet or a magnetic material is called magnetic force. The force exerted by a charged body on another charged body or uncharged body is called an electrostatic force. The force with which the Earth attracts objects towards itself is called the gravitational force. It is always an attractive force. The force with which the Earth pulls an object towards itself is called the weight of the object. The SI unit of weight is newton. The mass of an object remains unchanged whereas its weight may vary from place to place. When an object is placed in a liquid, the force applied by a liquid on an object in the upward direction is known as upthrust or buoyant force.

Now let's solve the exercises from your textbook. Let's start with the matching exercise in "Keep the curiosity alive".

Match items in Column A with the items in Column B.

Column A has types of force: (i) Muscular force, (ii) Magnetic force, (iii) Frictional force, (iv) Gravitational force, (v) Electrostatic force.

Column B has examples: (a) A cricket ball stopping on its own just before touching the boundary line, (b) A child lifting a school bag, (c) A fruit falling from a tree, (d) Balloon rubbed on woollen cloth attracting hair strands, (e) A compass needle pointing North.

Let me think through each one. Muscular force is the force exerted by our muscles. A child lifting a school bag uses muscular force. So (i) matches with (b). Magnetic force is the force exerted by a magnet. A compass needle pointing North is because of the magnetic force of the Earth. So (ii) matches with (e). Frictional force is the force that opposes motion. A cricket ball stopping on its own just before touching the boundary line is due to friction. So (iii) matches with (a). Gravitational force is the force with which the Earth attracts objects. A fruit falling from a tree is due to gravity. So (iv) matches with (c). Electrostatic force is the force exerted by charged objects. Balloon rubbed on woollen cloth attracting hair strands is due to electrostatic force. So (v) matches with (d).

Now let's answer the true or false questions.

(i) A force is always required to change the speed of motion of an object. This is TRUE because we learned that force can change the speed of an object.

(ii) Due to friction, the speed of the ball rolling on a flat ground increases. This is FALSE because friction actually decreases the speed of a rolling object and eventually stops it.

(iii) There is no force between two charged objects placed at a small distance apart. This is FALSE because charged objects exert electrostatic force on each other.

Now let's answer question 3. Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why? When two balloons are rubbed with a woollen cloth, they both acquire similar charges because they are charged in the same way. Since like charges repel each other, the two balloons will move away from each other or repel each other.

Now question 4. When you drop a coin in a glass of water, it sinks, but when you place a bigger wooden block in water, it floats. Explain. This happens because of the buoyant force. The coin is denser than water, so the weight of the coin is greater than the weight of water it displaces. Hence, the buoyant force is less than the weight of the coin, and it sinks. The wooden block, even though it is bigger, is less dense than water. The weight of water displaced by the wooden block is greater than or equal to the weight of the block itself. So the buoyant force is enough to support the block, and it floats.

Now question 5. If a ball is thrown upwards, it slows down, stops momentarily, and then falls back to the ground. Name the forces acting on the ball and specify their directions.

(i) During its upward motion: The gravitational force acts downwards, towards the Earth. This is the only force acting on the ball during its upward motion, and it causes the ball to slow down.

(ii) During its downward motion: Again, the gravitational force acts downwards, towards the Earth. This time, it causes the ball to speed up as it falls.

(iii) At its topmost position: At the exact topmost point, the ball momentarily stops before starting to fall. The gravitational force is still acting downwards. There is no other force acting on the ball except gravity.

Now question 6. A ball is released from the point P and moves along an inclined plane and then along a horizontal surface as shown in the figure. It comes to stop at the point A on the horizontal surface. Think of a way so that when the ball is released from the same point P, it stops (i) before the point A (ii) after crossing the point A.

To make the ball stop before point A, we need to increase the friction on the horizontal surface. We could make the surface rougher, for example, by spreading sand or a cloth on it. This would increase the frictional force and stop the ball earlier.

To make the ball stop after crossing point A, we need to reduce the friction on the horizontal surface. We could make the surface smoother, for example, by oiling it or using a polished surface. This would reduce the frictional force, and the ball would travel further before stopping.

Now question 7. Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain. We slip on smooth surfaces because the force of friction is very less on such surfaces. Smooth surfaces have fewer irregularities, so there is less interlocking between the surfaces, resulting in reduced friction. When friction is less, our feet cannot get a good grip on the surface, and we slip.

Now question 8. Is any force being applied to an object in a non-uniform motion? Yes, definitely! Non-uniform motion means the speed or direction of motion is changing. Since a force is required to change the speed or direction of an object, there must be some force acting on an object in non-uniform motion. This force could be friction, gravity, or any other force.

Now question 9. The weight of an object on the Moon becomes one-sixth of its weight on the Earth. What causes this change? Does the mass of the object also become one-sixth of its mass on the Earth? The weight changes because the gravitational force on the Moon is about one-sixth of that on the Earth. The Moon has much less mass than the Earth, so it exerts a weaker gravitational pull. However, the mass of the object does not change. Mass is the amount of matter in an object, and it remains the same everywhere in the universe. Only the weight changes because weight depends on the gravitational force.

Now question 10. Three objects 1, 2, and 3 of the same size and shape but made of different materials are placed in the water. They dip to different depths as shown in the figure. If the weights of the three objects are w1, w2, and w3, respectively, then which option is correct?

Looking at the figure, we can see that object 3 dips the least, meaning it floats most, object 2 dips more than object 3 but less than object 1, and object 1 dips the most, meaning it sinks the most. The more an object sinks, the heavier it is. So object 1 is the heaviest, object 2 is intermediate, and object 3 is the lightest. Therefore, w1 is greater than w2, and w2 is greater than w3. So the correct option is (ii) w1 > w2 > w3.

Now let me give you a complete summary of everything we have learned in this chapter.

In this chapter on Exploring Forces, we learned that a force is a push or pull on an object resulting from the object's interaction with another object. The SI unit of force is newton, denoted by N. We learned that forces can be contact forces, which require physical contact between objects, or non-contact forces, which can act from a distance without contact.

Contact forces include muscular force, which is the force exerted by our muscles, and friction, which is the force that opposes motion between surfaces in contact. Friction depends on the nature of the surfaces in contact and is greater on rough surfaces.

Non-contact forces include magnetic force, which is the force exerted by magnets; electrostatic force, which is the force exerted by charged objects; and gravitational force, which is the force with which the Earth attracts objects towards itself. Gravitational force is always attractive, while magnetic and electrostatic forces can be either attractive or repulsive.

We learned about weight, which is the force with which the Earth pulls an object. Weight is measured in newtons. We learned the difference between mass and weight - mass is the amount of matter in an object and remains constant, while weight can change depending on the gravitational pull.

We also learned about floating and sinking. When an object is placed in a liquid, it experiences an upward buoyant force or upthrust. If the weight of the object is greater than the buoyant force, it sinks. If the weight is equal to or less than the buoyant force, it floats.

We solved various exercises including matching types of forces with their examples, true and false questions, and numerical problems about forces, weight, and floating.

Now you should be able to answer all the questions from this chapter. Remember, forces are all around us, and understanding them helps us make sense of many everyday phenomena. Thank you for listening attentively. Keep exploring and stay curious!