ICSE Class 6 Physics Chapter 03 Force and Pressure

Force and Pressure

Force and pressure are two words we use often without quite knowing what they mean to a scientist. To a scientist, they are clearly defined physical quantities that can be measured.

Force

When we look around us, we see objects in different conditions. Some, like a desk or a parked car, are stationary. They are said to be in a state of rest. Others, like the vehicles moving on a road, are in a state of motion. A body's state of rest or motion cannot change on its own. For example, a chair does not move unless you pull or push it. Similarly, to stop a cart rolling down a slope, you need to pull or push it.

We use the word force to describe a push or a pull. We say that we are applying a force when we push or pull something. When a force acts on a body, it can change the state of rest or of motion of the body. It can also do other things. Let us study the effects of force in greater detail.

Effects of Force

Force can cause motion

A body at rest may start moving when a force is applied. You see this happening when you push a chair, pull open a drawer, or pick up a box. Quite often, when we apply a force on a body, it moves in the direction of the force. For example, in Figure 3.1, a child is pushing a toy engine to the left, and the engine is moving in that direction. In diagrams, we show a force by an arrow. The arrowhead points in the direction of the force.

Force can increase speed

The speed of a body increases when a force acts on it in the direction in which it is moving. For example, if you are on a swing and your friend pushes the swing in the direction in which it is moving, the swing will move faster. If you are cycling, and the wind pushes you from behind, your speed will increase. Similarly, if you kick a ball in the direction in which it is rolling, its speed will increase.

Force can decrease speed

Suppose a body is moving in a particular direction, and you apply a force on it in the opposite direction, the speed of the body will decrease. Thus, if you are on a swing and your friend pulls it in a direction opposite to its motion, the swing will slow down. It may even stop moving altogether. We often apply a force to stop a moving body. Fielders and goalkeepers, for example, apply force to stop the ball. Brakes push against wheels to stop their motion.

Force can change the direction of motion

Sometimes a force acting on a moving body changes its direction of motion. For example, when a ball bounces back from the floor, and you hit it with your hand, the force you apply changes the direction of its motion from upward to downward. And the direction in which a kite is flying changes because of the force exerted on it by the wind.

Force can change shape and size

When your head pushes down on a soft pillow, the shape of the pillow changes. When you press or pull dough, its shape and size change. When you pull a rubber band, its length increases. When an archer pulls the string of a bow, the shapes of its wooden frame and string change. Thus, a force can change the shape or size of a body.

We have discussed the kinds of changes that a force can produce when it acts on a body. Looking at it in another way, these changes occur because a force is acting on a body. We can now define force. Force is something that can bring about a change in the state of rest or of motion of a body or a change in its shape or size.

Unit of Force

Force is a physical quantity that can be measured. The SI unit of force is the newton, whose symbol is N. One newton is approximately the force you would apply to lift an object of mass 100 g. To lift 1 kg you would have to apply a force of about 10 N.

Types of Forces

When you push a box with your hands, you apply force on the box. In this case, your hands are in contact with the box. Now consider a leaf falling from a tree. It moves downwards because it is pulled by the earth. The earth exerts a force on the leaf though the leaf is not in contact with the earth. In general, we divide forces into two major types - contact forces and action-at-a-distance forces.

Contact Forces

Forces that two bodies in contact apply on each other are called contact forces. We apply contact forces when we push, pull or lift things around us. The forces we use when we push a switch, open a window, squeeze a lemon and kick a football are contact forces because our hands or feet are in contact with the object. Similarly, contact forces act when a bullock pulls a cart, and an elephant breaks the branch of a tree with its trunk. When we sit on a chair or lie in bed, the weight of the body presses down on the chair or bed. This too is a contact force.

Sometimes we apply a contact force on something without touching it directly. For example, you can attach a string to a toy car and pull the string to make the car move. You apply a force on the string and the string applies a force on the car. This type of contact force applied by a string, wire or thin rod is called tension. Tension acts along the rope from which a bucket hangs in a well, or the rope that is pulled by two teams in a tug of war.

Friction

Flick a coin across a table. You will notice that the coin gradually slows down and stops. Since a force is necessary to change the state of motion of a body, some force must be acting on the coin while it is moving. Also, since the speed of the coin decreases, this force must be acting in a direction opposite to the motion of the coin. This force is called the force of friction, or simply, friction. It acts where the surfaces of the coin and table are in contact. It is the same force which stops a rolling ball or your bicycle once you stop pedalling.

Consider a book placed on a table (Figure 3.5a). There is no force of friction between the two as long as the book is lying at rest.

But if you push the book gently, it will have a tendency to move, and friction will act to prevent it from moving (Figure 3.5b). If you push the book harder, the book will begin to slide (Figure 3.5c). Friction will keep trying to stop the book from moving, or oppose its motion. If you want the book to keep moving, you will have to continue pushing. Friction acts only when something has a tendency to move or is actually moving. The direction of friction is opposite to that of the motion or the tendency of motion. The force that opposes the motion or tendency of motion between two surfaces in contact is called the force of friction.

Friction depends on the surfaces in contact. The magnitude of the force of friction between two surfaces depends largely on the nature of the surfaces in contact.

Place a book in an inclined position on the floor, as shown in Figure 3.6. Place a small ball or marble at the top and let it roll down. Note how far the ball rolls on the floor. Repeat the activity on a carpet, on the ground outside your house, and so on. Note how far the ball moves in each case. You will find that on a rough surface the ball stops after a short distance. On a smooth surface it moves much farther.

The ball moves much farther over a smooth surface because the force of friction, which opposes its motion, is smaller for smooth surfaces. For the same reason, it is easier to move an object over a smooth surface than to move it over a rough surface.

Friction is useful

Friction can be useful as well as harmful. Let us first see how friction plays a helpful role in our lives.

1. When you walk or run, you push the ground backwards with your foot. The tendency of the foot is to move in that direction. The force of friction opposes this tendency and acts in the opposite direction, i.e., in the direction in which you want to move. This helps you move forward. If the force of friction between your foot and the floor is small, your foot may continue to move backwards. In that case you may slip and fall. This is what happens when we slip on highly polished floors, ice, or oil or water spilt on the floor.

2. The friction between the wheels of a vehicle and the surface of the road allows the wheels to roll forward. When friction is reduced, the wheels slide instead of rolling, and the vehicle skids. This is why cars skid on wet, muddy, oily or ice-covered roads.

3. Brakes are applied to stop a vehicle. The force of friction between the brakeshoes and the wheels slows down the wheels.

4. Friction allows us to grip an object (for example, a pencil) between our fingers. The object has a tendency to fall. The force of friction opposes this tendency. It is difficult to grip a smooth object, such as an ice cube, because of low friction.

5. Pencils write because of friction. The lead of a pencil is made of graphite (a form of carbon). The friction between the lead of a pencil and paper causes small particles of graphite to break off and get deposited on the paper. Can you figure out why it is difficult to write on smooth surfaces, such as glass and polythene?

6. Friction is used to polish surfaces. You might have seen a carpenter use sandpaper to polish wood. The large force of friction produced when this rough paper is rubbed on wood wears down the tiny bumps on the surface.

7. When two surfaces rub against each other, the friction between them produces heat. This has some uses. You rub your hands together when they feel cold. Primitive man lit fires by rubbing stone or wood. To ignite a match, we rub it against a rough surface. The heat produced by friction causes the chemicals on its head to catch fire.

Teacher's Note

Friction is essential for everyday activities like walking, writing, and gripping objects, yet it also causes wear and tear on machinery and increases energy consumption in vehicles.

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