ICSE Class 6 Physics Chapter 2 Force

Chapter 2: Force

Syllabus

1. Force: The effects of a force (starting or stopping motion, changing direction, changing speed, changing the shape/size of a body) - simple everyday examples of these.

2. Types of forces: Contact forces, friction, forces that act at a distance - gravitational, magnetic - examples of these in action.

3. Force is measured in newtons (N). The resultant of forces acting along the same line, in similar or opposite directions.

State Of Rest

When we look around, we observe that many objects do not appear to move, rather they are considered to be at rest. A book lying on the table will not change its position if it is not disturbed by anyone and will be considered to be in the state of rest. Similarly, a bench fixed in a garden is at rest as there is no change in the position of the bench with respect to its surroundings, such as trees, houses etc.

A body is said to be at rest if it does not change its position with time with respect to its surroundings.

State Of Motion

We see many things around us moving from one place to another like a flying bird, a moving bus, a boy playing football, the oscillating pendulum of a wall clock, etc. All these things are said to be in motion.

A moving car changes its position with time with respect to a tree by the side of a road. So, the car is said to be in motion.

Similarly, a flying bird is also said to be in motion as it changes its position with time with respect to a tree.

Even when we are at rest, blood is moving through our arteries and veins, air is flowing in and out of our nostrils and our heart is beating.

We must note here that rest and motion are relative terms. Imagine yourself sitting inside a moving bus. On looking outside the bus, you will observe that you are moving. Now look towards the roof of the bus, you will feel that you are in a position of rest with respect to the roof.

Therefore, an object is in the state of motion with respect to a set of objects, but may be in the state of rest with respect to the other set of objects (moving with the same speed and in the same direction). It is the observer and the surroundings that decide whether a given object is at rest or in the state of motion. Thus, we can say:

A body is said to be in the state of motion if it changes its position with time with respect to its surroundings.

To change the state of rest to motion or the state of motion to rest, a physical cause is needed. The cause which can change these states is called force.

Force

In our everyday life, we push or pull objects. To open a door, we either push or pull it. To draw a bucket from a well, we pull the rope. We kick, throw or hit a ball, i.e., we are pushing it. We pull or push chairs, tables, etc. This pull or push is known as force. The direction in which we apply the force is called the direction of force. It is true that a pull or push may or may not produce a fruitful result but it definitely tries to produce a change in the state of the body. For example, an almirah when pushed may not move by a single person but the same almirah can be pushed by two or three persons.

Therefore, we can say that force is a physical cause that changes or tries to change the state of rest or the state of motion of an object.

Force As A Push

A body is considered to be pushed when we apply a force on it and the direction of motion is away from us.

Examples:

(i) To open a door, we push it.

(ii) To switch On or Off a T.V., we press (push) its switch.

(iii) To remove a pile of rubbles (i.e., rough broken pieces of stone), it is pushed by a bulldozer.

(iv) To move a baby cart, it is pushed as shown in Fig. 2.2 (a).

(v) If a car does not start, we push it to move as shown in Fig. 2.2 (b).

Force As A Pull

A body is considered to be pulled when we apply a force and the body moves towards us.

Examples:

(i) We pull a door to open it.

(ii) To level the soil in a lawn, a roller is pulled by the gardener.

(iii) To move a cart, pull is exerted by a horse or a bull.

(iv) To move a luggage trolley, pull is exerted by the coolie.

(v) A rickshaw puller exerts pull on the rickshaw to move it.

From the above discussed examples, we can infer that at least two objects must interact for a force to come into play. Thus, an interaction of one object with another object results in a force between the two objects.

Stretch And Squeeze

The terms stretch and squeeze also means force. Let us define these terms.

A body is considered to be stretched if a force applied on it increases its length, i.e., the increase in length is in the direction of the applied force. For example, when we apply a force on a rubber band or a spring, its length increases. The force and increase in length are in the same direction.

A body is considered to be squeezed, if a force applied on it decreases its length or changes its dimension. For example, when we squeeze a tube to take cream out of it, its shape changes.

Effects Of Force

We cannot see a force. However, we can see the effects of force, i.e., what a force does.

1. A force can change the shape or size of an object. For example, the shape of a sponge changes on pressing, the size of a spring changes on pulling and the shape and size of an iron strip can change on hammering.

Examples:

(a) When an inflated balloon is pressed between the two hands, its shape and size changes.

(b) The shape and size of a ball of dough changes when it is rolled to make a chapati.

(c) The shape of tooth paste tube changes on squeezing.

(d) When a bag of wheat flour is pressed its shape changes. In fact, it can be given any shape.

2. A force can make a stationary object move. When you push a stationary car or when you kick a stationary football, you apply force to make them move.

3. A force can change the speed of a moving object. When we give a push to a moving swing, it moves faster. When we want to slow it down, we apply force in the opposite direction of motion. Such a push or pull is called force.

4. A force can change the direction of a moving object. When a player kicks a moving football, he changes the direction of the ball. Similarly, a batsman changes the direction of the ball by hitting it towards all sides of the field. In this process, the speed of the ball also changes.

Example: When you pass alongside a bus or truck emitting smoke, you wave the smoke away by moving your hand. Your moving hand exerts force on the smoke and pushes it away.

5. A force can stop a moving object. By applying force on a moving body, it can be made to stop. For example, if a person is riding slowly on a bicycle and we pull the bicycle from behind, it stops. Similarly, a moving bicycle, bus or train is stopped by applying the brakes. The brakes provide the stopping force to the vehicle.

Normal Reaction Force

When a body is kept on a table top, the body's weight acts vertically downwards. As a reaction of the table, a force is exerted by the surface of the table on the body in vertically upward direction. This force is called normal reaction force.

Tension Force Applied On String

When an object is suspended by a string, the weight of the body acts downwards. As a result a force acts vertically upwards on the body by the string. This is known as the tension force.

Thus, force is a cause (a push, pull, stretch or squeeze) which changes or tends to change the state of rest or the state of uniform motion, or direction of motion, or the shape and size of a body.

Types Of Forces

Force can be divided into two types - contact force and non-contact force (force at a distance).

Contact Force

When the force applied on an object is in direct contact i.e., the object with which we direct contact, then we call it a contact force. Pulling a chair or pushing a chair is an example of contact force and is also known as muscular force.

Muscular Force

The force applied by the muscles of a human or animal body is called muscular force.

We use our muscular force during walking, running, kicking and lifting certain objects.

Animals exert muscular force to do heavy works such as pulling a cart, ploughing, carrying heavy loads etc.

When two objects collide with each other like a bus and a car, it is a contact force and we also call it a collision force.

Mechanical force and frictional force are two other examples of contact force.

Mechanical Force

When a body moves with the help of a machine, the force exerted by the machine is called the mechanical force. Actually, a machine does not exert force by itself, but it needs energy from other sources to exert a force. For example, the engine of a car exerts a mechanical force to move it. A railway engine exerts a mechanical force to pull the train. Similarly, the force exerted by wind to move a sailboat and to rotate the windmill is the mechanical force.

Frictional Force

It is our common experience that when we stop pedalling a cycle, it gradually slows down and ultimately stops after travelling a certain distance. Friction between water and the boat also stops the boat, once you stop rowing. Similarly, if we roll a ball on a plain ground, it moves some distance and then stops. Since force can stop a moving body, so we can say that there must be a force acting on the cycle or the ball which stops them.

The force which slows down the motion of a moving body in contact with the other body, is called the frictional force or the force of friction.

This force always opposes the motion. Figure 2.11 shows that while pushing a sofa, the force of friction opposes the motion and it becomes difficult for us to push it.

Reducing or increasing friction - We need to oil our bicycle regularly. Oiling reduces the frictional force between its parts and makes it easier to ride the bicycle. Bicycles and other machines have ball bearings. Their function is to reduce the frictional force and thus to reduce the wear and tear of the machines.

When a car gets stuck in mud, its wheels turn without moving forward. At such times, a wooden plank is placed under the wheels so that friction increases. This helps the wheels to come out of the mud. In short, frictional force can be increased or reduced to suit a purpose.

Non-Contact Force

Non-contact force is also called the force acting at a distance. When we apply a force on an object without actual contact, it is the force acting at a distance. Magnetic force, electrostatic force and gravitational force are the examples of non-contact forces.

Magnetic Force

When we bring a magnet close to iron nails, they get attracted towards the magnet. Clearly, the magnet has applied a force on the nails, although there is no direct contact between them.

This force exerted by the magnet is called the magnetic force.

A magnet also exerts a force on another magnet. This can be a force of attraction or repulsion. It depends on the ends (poles) of a magnet that are facing each other. All magnets have a north-seeking pole (N) on one end and a south-seeking pole (S) on the other end. When like poles face each other, the magnets repel (push) each other (Fig. 2.14). When unlike poles face each other, the magnets attract (pull) each other (Fig. 2.15).

The forces of attraction and repulsion can be shown by bringing two magnets closer together.

Scientists have designed a train which gets lifted up above the track and travels with great speed in air. The maglev train is a recent application of the magnet. Its speed is 500-580 kmph. It makes use of the principle of magnetic repulsion between like poles of a magnet. Magnetic repulsion between the track or the guideway and the bottom of the train lifts the train off the ground. Thus, the train actually floats in air. A linear motor is used to propel the train forward.

Maglev stands for magnetic levitation. The maglev train floats in air!

Electrostatic Force

The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.

If you rub a plastic comb on your dry hair and bring small pieces of paper close to it, the comb will attract the pieces of paper. This is because when we rub the comb with dry hair, an electric charge is produced on the surface of the comb which is responsible for applying a force on the pieces of paper (Fig. 2.16).

This force exerted on the pieces of paper is known as electrostatic force.

Gravitational Force

The force of attraction between any two objects possessing mass is called gravitational force which exists everywhere in the universe. The magnitude of gravitational force between any two bodies depends upon their masses and the distance between them.

For example, the force acting between any two books, between a book and a table, between earth and the moon etc., is gravitational force.

Leaves and fruits fall towards the ground, water begins to flow towards the ground as soon as we open a tap, water in rivers flows downstream due to the force of gravity.

Leave a stone from your hand. It goes towards the surface of the earth. When we throw a ball, it attains certain height, and then comes back to the surface of the earth (Fig. 2.17). Why does this happen? The earth has a characteristic property of its own to pull every object towards it. This pull is known as the force of gravity or gravitational force.

Here, it is to be noted that the force of gravity (unlike magnetic force) attracts but does not repel.

When we discuss gravitational force, we must discuss the term weight. The force of gravity on an object is actually the weight of that object. Weight is the force with which a body gets attracted towards the centre of the earth.

In the previous chapter, we have learnt that the mass of an object is measured by the physical balance and its S.I. unit is kg.

You should remember that the weight of an object is measured by a spring balance and its S.I. unit is newton.

Mass is measured in kg while weight is measured in newton.

Unit Of Force

The S.I. unit of force is newton. The symbol for newton is N. This unit is named after the English scientist Sir Isaac Newton who did a lot of research work on force.

1 newton is that much force which produces an acceleration of 1 m/s in a body of mass 1 kg.

One kgf is equal to nearly 10 newton, i.e., 1 kgf = 10 N

Thus, we can say, a force of 10 newton is needed to lift a mass of 1 kg.

A force is represented by an arrow. The length of the arrow represents the magnitude and the head of the arrow represents the direction of the force.

Teacher's Note

In daily life, we observe that pushing a shopping cart requires more force as it gets heavier with groceries. This demonstrates how force directly affects an object's ability to accelerate and move.

Measurement Of Force (Weight)

Force can be measured with the help of a spring balance. There are two types of spring balances - extension spring balance and compression spring balance (Fig. 2.18). In an extension spring balance, the spring stretches when a force acts on it. The greater the force, the more the spring stretches. In a compression spring balance, the spring gets compressed when a force acts on it.

Working of an extension spring balance - The spring balance consists of a special spring which is enclosed in a metal case with a slit cut at its centre along its length. The top end of the spring is attached to the body of the case and the lower end of the spring is attached to a steel strip where a hook is fixed at its end. A small pointer is attached at the junction of the steel strip and the spring which passes out of slit. On the side of the slit, there are markings on a scale in gf or kgf depending on the strength of spring (Fig. 2.19).

The body whose weight is to be measured is attached to the hook. The spring gets stretched due to the gravitational pull. The weight of the body is read where the pointer finally stops on the scale.

Action And Reaction

When you kick a football, does it exert a force on your boot? If you throw a cricket ball against a wall, does the ball exert a force on the wall or the wall exerts a force on the ball? Such experiments indicate that forces do not appear alone, but they come in force pairs. If you hit your head against a brick wall (action), the wall will retaliate (reaction). Sir Isaac Newton discovered this fact hundreds of years ago and said that the two forces, action and reaction, are always equal in magnitude, but opposite in direction. If body (A) applies a force on body (B) in one direction, body (B) will apply exactly the same force on the body (A) but in opposite direction.

Net Effect Of Force

To know the net result of forces applied, we must know the magnitude and direction of the force. The magnitude of a force tells us the how large or small a force is whereas the direction gives the negative or positive effects of the force as follows:

1. If two or more forces act in the same direction, they add up to give a resultant force in the same direction.

For example, in Fig 2.20, two horses pull the cart in the same direction, each by a force of 50 N. The net force on the cart is 50 N + 50 N = 100 N.

2. If two forces act in opposite directions, the net force is equal to the difference of the two forces, in the direction of the greater force.

For example, in Fig. 2.21, one person pulls the cart by a force of 50 N towards left and the other person pulls the cart by a force of 60 N towards right. The net force on the cart is 60 N - 50 N = 10 N towards the right.

Thus, if the two forces acting on a body are equal in magnitude, but opposite in direction, the net force on the body is zero (Fig. 2.22).

Teacher's Note

When you try to move a heavy box with a friend pushing from the opposite direction, you experience how forces combine to create a net effect - if both push equally opposite, the box won't move at all.

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