Hello, and welcome to today's physics lesson! Today, we are going to explore Chapter Three: Force and Pressure. By the end of this lesson, you will understand what makes objects turn and rotate, how force spreads over surfaces, and why liquids and gases push in every direction around us. Let us begin our journey into the fascinating world of force and pressure.
Let us start with something you already know. A force is simply a push or a pull. When you push a door open, pull a drawer, or kick a football, you are applying force. But force can do much more than just move things from one place to another. It can also make things turn or rotate. This is what we call the turning effect of force.
Imagine pushing a heavy door. If you push near the hinges, the door barely moves. But if you push at the handle, far from the hinges, the door swings open easily. Why is that? The answer lies in two things: how hard you push, and where you push.
The turning effect depends on the magnitude of the force, and on the perpendicular distance from the pivot point. The pivot point is the fixed point about which the object turns, like the hinges of a door. The greater this perpendicular distance, the easier it is to turn the object, even with less force.
This brings us to a very important concept: the moment of force, also known as torque.
The moment of force is defined as the product of the magnitude of the force and the perpendicular distance of the force from the pivoted point.
In symbols, we write this as: moment of force equals force multiplied by perpendicular distance.
Or, moment of force = F × d, where F is the force in N, and d is the perpendicular distance in m. The S.I. unit of moment of force is N m.
Here is a quick example to make this clear. Imagine Reena needs to open a door that is 1.2 m wide. She applies a force of 1.5 N at the handle. The moment of force she creates is 1.5 N multiplied by 1.2 m, which equals 1.8 N m. Now, if she tries to open the same door by pushing at the middle, only 0.6 m from the hinges, she would need to apply 3 N of force to create the same turning effect. This shows that when distance decreases, force must increase to maintain the same moment.
By convention, if a force tends to turn an object anticlockwise, we call it a positive moment. If it tends to turn clockwise, we call it a negative moment. You can change the direction of rotation either by changing where you apply the force, or by changing the direction of the force itself.
Now let us move to another important idea: thrust and pressure. When a force acts perpendicular to a surface, we give it a special name: thrust. Thrust is simply the total force pushing against a surface at right angles to it. When you stand on the floor, your weight creates a thrust pushing down on the ground.
But here is something interesting. The same thrust can have very different effects depending on how it is spread out. Stand on loose sand, and your feet sink deep. Lie down on the same sand, and you barely sink at all. Your weight, your thrust, is the same in both cases. What changes is the area over which that thrust acts.
This leads us to the concept of pressure.
Pressure is defined as thrust per unit area.
In formula terms: pressure equals thrust divided by area.
Or, P = F/A, where P is pressure in Pa, F is thrust or force in N, and A is the area in m². The S.I. unit of pressure is newton per square metre, written as N/m² or N m⁻², also called pascal with symbol Pa, named after the scientist Blaise Pascal. One pascal equals one newton acting on one square metre.
Let us see this in action. A sharp pin pierces wood easily, but a blunt nail with the same force does not. Why? The pin's tip has a tiny area, so the pressure is enormous. The nail's flat end spreads the same force over a larger area, so the pressure is much less. This is why cutting tools have sharp edges, and why walking on pointed heels is difficult on soft ground.
Conversely, increasing area reduces pressure. Heavy trucks have six to eight tyres instead of four, spreading their weight over more ground. Camels have broad feet to walk on sand without sinking. Skiers use long flat skis to glide over snow. Buildings have wide foundations to distribute their weight safely. All these examples use the same principle: larger area means less pressure for the same thrust.
Now let us explore pressure in liquids. Like solids, liquids have weight and therefore exert pressure. But liquids behave differently in one crucial way: they exert pressure in all directions, not just downward.
Picture a container of water. The water pushes down on the bottom, yes, but it also pushes sideways against the walls, and even upward against anything submerged in it. You can feel this upward push when you try to push an empty mug upside down into a bucket of water.
Two factors affect liquid pressure. First, pressure increases with the height of the liquid column above the point. Deeper water means greater pressure. This is why dam walls are thicker at the bottom, to withstand the greater pressure there. Second, pressure increases with the density of the liquid. A dense liquid like mercury exerts more pressure than water at the same depth.
Here is a remarkable fact: at any given depth, a liquid exerts the same pressure in all directions.
This is known as Pascal's law, which states that pressure exerted by a liquid at a depth is same in all directions. Whether you measure up, down, or sideways at the same depth, the pressure is identical.
Finally, let us turn to gases and atmospheric pressure. Gases, like liquids, exert pressure in all directions. Our Earth is surrounded by a vast envelope of air called the atmosphere, extending about 200 kilometres above the surface. This air has weight, and that weight creates pressure on everything beneath it.
At sea level, the atmospheric pressure is enormous: approximately 1.013 × 10⁵ Pa, or about 100,000 Pa, which equals 1 atm. This equals the pressure exerted by a column of mercury 76 cm or 760 mm high in a barometer. We call this standard pressure one atmosphere, written as 1 atm.
Remarkably, we do not feel this crushing pressure because the fluids inside our bodies, particularly our blood, exert a matching outward pressure. It is like a perfect balance between outside and inside forces.
Atmospheric pressure explains many everyday phenomena. When you suck through a straw, you do not pull liquid up; you reduce air pressure inside the straw, and atmospheric pressure pushes the drink up into your mouth. A rubber sucker sticks to walls because atmospheric pressure pushes it firmly from the outside. Fountain pens fill with ink, syringes draw medicine, and wells yield water, all thanks to atmospheric pressure doing the work.
As you climb higher, atmospheric pressure decreases because there is less air above you pressing down. This is why mountaineers need special suits at high altitudes, and why your ears pop when you ascend quickly in an aeroplane. The decrease in external pressure can even cause nosebleeds, as the pressure inside your body temporarily exceeds the reduced atmospheric pressure.
Let us quickly recap the key ideas from today's lesson.
First, the moment of force, or torque, measures the turning effect of a force. It equals force multiplied by perpendicular distance from the pivot, and its unit is the newton metre, written as N m. Note that we do not write this unit as joule, even though both involve newtons and metres.
Second, thrust is the perpendicular force on a surface, while pressure is thrust divided by area. Pressure increases when area decreases or when thrust increases. The unit of pressure is the newton per square metre, written as N/m² or N m⁻², also called pascal with symbol Pa.
Third, liquids exert pressure in all directions, with pressure increasing with depth and with liquid density. Fourth, gases also exert pressure in all directions, and our atmosphere creates substantial pressure of about 10⁵ Pa at sea level that decreases with altitude.
Fifth, understanding these principles helps explain countless everyday situations, from opening doors to designing buildings, from using tools to walking on sand.
Physics is all around you, in every push, every turn, and every step you take. Keep observing, keep questioning, and keep discovering how the world works. Until next time, stay curious and keep learning!