Chapter 2 — Force and Pressure: Motion

What is a Force?

A force is a push or a pull that can change the state of motion of an object or change its shape. Forces act at specific points on objects and always have both size (magnitude) and direction. The direction tells us which way the force acts and the magnitude tells us how strong the force is. In school problems and diagrams, forces are shown by arrows: the arrow points in the direction of the force and the length of the arrow shows how large the force is compared to other forces.

Forces can make objects start moving, stop moving, speed up, slow down or change direction. For example, when you push a parked bicycle it starts moving; when you apply brakes it slows down. Forces also cause deformation: squeezing a sponge makes it change shape and stretching a rubber band makes it longer. Some forces only act when objects touch, while others act even when objects are apart — you will learn both kinds in the next topic.

Measuring forces requires standard units. The SI unit of force is the newton (N). Simple instruments such as a spring balance measure force by how much they stretch; more advanced tools exist but the basic idea is the same. In many situations you will need to consider all forces acting on an object together. The combined effect of these is called the resultant force. If the resultant is zero the object does not change its motion; if it is non-zero the object accelerates in the direction of the resultant. Drawing clear diagrams with labelled arrows helps to add forces and predict what will happen to objects in everyday situations.

Learning about force is important because it explains many familiar actions: walking involves frictional forces, doors move when we push or pull them, and machines use forces to lift heavy loads. Practising how to represent, add and compare forces gives students a strong foundation for studying motion and mechanics later in higher classes.

Contact and Non-contact Forces

Forces are classified by how they act between bodies. Contact forces require physical touching of the bodies. When two surfaces press, slide or collide, contact forces appear. Examples include pushes and pulls, normal reaction from a surface, friction between moving surfaces, and tension along a rope. These contact forces usually depend on the nature of surfaces and how hard they are pressed together.

Non-contact forces act even when the bodies are not in contact; they operate through a distance. The most familiar non-contact force at this stage is gravity: the Earth pulls objects toward its centre, causing them to fall. Magnetic forces act between magnets and magnetic materials without touch; you see this when a magnet attracts a paper clip through air. Electrostatic forces appear between charged objects and can attract or repel at a distance.

Understanding the difference matters when planning demonstrations and experiments. A contact force like friction can be reduced by lubrication or increased by roughening surfaces, while a non-contact force like gravity cannot be stopped by a barrier—mass always experiences gravitational pull. Many real situations involve both kinds of forces together. For example, when you slide a magnet over a table carrying a metal sheet beneath, magnetic (non-contact) and frictional (contact) forces both act.

In diagrams, contact forces are drawn at points of contact and labelled (for example, 'normal reaction' at the surface). Non-contact forces are drawn between objects even when there is a gap. Thinking about whether a force needs contact helps students choose the right explanation for an observed effect and decide how to reduce or enhance a force in practical problems.

Balanced and Unbalanced Forces

When several forces act on an object, their combined effect determines whether the object's motion changes. If the vector sum of all forces acting on a body is zero, the forces are said to be balanced. Balanced forces keep an object at rest or moving at constant velocity. For example, a book lying still on a table experiences gravity downwards and an equal normal force upwards; these two forces balance and the book stays at rest.

Unbalanced forces occur when the sum of forces is not zero. An unbalanced force produces acceleration — the object speeds up, slows down or changes direction depending on the resultant's direction. For example, if you push a toy car harder in one direction than friction opposes it, the resultant force makes the car accelerate in that direction. In real life, many problems are one-dimensional (left-right or up-down) and we add forces algebraically, giving positive or negative signs depending on direction.

Free-body diagrams help to visualise and decide whether forces are balanced. In these diagrams, draw the object as a dot or box and represent each force by an arrow starting at that dot, labelled with its name and size if known. For classroom practice, choose a positive direction and add forces using signs: if the sum is zero, motion does not change; if non-zero, the object will accelerate. Being able to identify balanced versus unbalanced forces is essential to predict everyday outcomes like whether a stationary trolley will start moving when a push is applied or whether a moving ball will slow down because of friction.

Also note that balanced forces can act on moving objects — constant speed motion has balanced forces. Observing situations around you and sketching free-body diagrams will make this idea clear and prepare you for calculations in later classes.

Effects of Forces on Motion

Forces change motion. When a net force acts, the velocity of an object changes; this change can be in magnitude (speed) or direction or both. If the net force is along the direction of motion, the object speeds up; if opposite, it slows down. If the force acts at an angle, the direction will change and the path will curve. Constant balanced forces keep motion steady, while unbalanced forces produce acceleration.

Different factors affect how a force changes motion. The same force acting on a lighter object produces a larger change in motion than when acting on a heavier object; this is why it is easier to push an empty cart than a loaded one. Also, the duration of the applied force matters: a short, strong push may produce the same change in motion as a long, weaker push. Forces can also deform objects: stretching, bending or compressing materials depending on how the force is applied and the object's properties.

Real-life examples show these effects: brakes produce a backward force on a bicycle to stop it; an air resistance force slows down falling leaves more than heavy stones; a football changing direction after being kicked by a player experiences a sideways force at the moment of contact. Understanding how forces influence motion helps in safety design too, for example adding crumple zones in cars lengthens the time taken to stop during a crash and reduces the force on passengers.

In class we study simple cases and observe these effects using toy cars, balls and springs. Measuring how motion changes with different applied forces and masses helps build intuition. Drawing motion diagrams that show position or velocity at successive times clarifies how forces cause acceleration, and prepares students for the mathematical treatment of motion in higher classes.

Friction: Causes and Types

Friction is a force that opposes relative motion between surfaces in contact. It arises because surfaces, even when they look smooth, have microscopic roughness and because adhesive forces act at contact points. When the surfaces are pressed together, these tiny bumps and adhesive forces resist sliding and produce friction. Friction acts along the surface and always opposite to the direction of motion or the tendency to move.

There are several kinds of friction. Static friction acts when an object tends to move but stationary friction holds it in place up to a maximum value. It adjusts to match applied small forces until that maximum is exceeded. Once motion begins, sliding friction (kinetic friction) acts, usually with a lower value than maximum static friction. Rolling friction occurs when objects roll and is typically much smaller than sliding friction; this is why wheels and rollers reduce the effort needed to move heavy objects. Fluid friction or drag acts when an object moves through air or water and depends on speed, shape and viscosity of the fluid.

Factors affecting friction include the nature of surfaces (smooth or rough), the normal force pressing them together, and the presence of lubricants. For many common surfaces, friction is approximately proportional to the normal force and can be written as F_friction = μN, where μ is the coefficient of friction. However, contact area often has little effect for rigid bodies because microscopic contact points determine friction. Friction produces heat and wear, which can be harmful, but it is also essential for walking, braking and holding objects. To reduce unwanted friction we use lubricants, ball bearings or polished surfaces; to increase friction we roughen surfaces or add treads to tyres.

Classroom experiments that compare pull forces needed on different surfaces, or show static vs sliding friction using inclined planes, help students understand these ideas and how friction influences real-world devices and safety measures.

Measuring Force: Spring Balance

A spring balance measures force by using a spring that stretches under load. Within the instrument a spring is attached to a hook; when a force pulls down on the hook, the spring extends. For springs that obey Hooke's law (within the elastic limit), the extension is directly proportional to the force applied. The scale on the balance converts the measured extension into a force reading, usually in newtons. This makes spring balances useful for measuring weights and small pulling forces in the classroom.

Using a spring balance correctly requires care. The balance should be held vertically and the reading taken at eye level to avoid parallax error. Before placing an object on the hook, ensure the pointer reads zero; if not, use the calibration screw to adjust. Do not overload the balance or pull it beyond its elastic limit; this can permanently deform the spring and give inaccurate readings. For better accuracy, repeat a measurement several times and take the average. Be aware that the balance measures the force due to weight when the object hangs steadily; in accelerating situations the reading will differ.

Students often use spring balances to measure the force needed to pull a block across a surface, comparing values on different materials to study friction. The spring balance also illustrates Hooke's law: F = kx, where k is the spring constant and x is extension. Plotting force versus extension gives a straight line whose slope is k. Understanding this instrument connects the abstract idea of force with a physical measurement and introduces the importance of calibration, careful reading and error sources like friction in hooks or air currents affecting light loads.

Overall, the spring balance is a simple, practical tool that helps students relate numerical values to forces they can see and manipulate, preparing them for later, more precise instruments used in higher study.

Gravity and Weight

Gravity is the attractive force that masses exert on each other. On Earth we see this as the downward pull that keeps us on the ground and causes objects to fall when released. The strength of this pull near Earth's surface is usually described by the acceleration due to gravity, g, which has a value of about 9.8 m/s2 (often approximated as 10 m/s2 for simple calculations). A body's weight is the force with which it is pulled towards the Earth and equals the product of its mass and g.

Mass and weight are different: mass is a measure of how much matter an object contains and does not change with location; weight depends on local gravity and so changes slightly with height above Earth and changes greatly on other planets. For example, a 10 kg mass has the same mass on the Moon but weighs less there because the Moon's gravity is weaker. Scales and spring balances measure weight, which is a force, while mass is typically measured in kilograms using balances that compare masses.

Weight acts at the centre of gravity of an object, which is the point through which the total weight may be considered to act for many practical calculations. Engineers and architects use this concept for stability: if the centre of gravity is over the base of support, the object stays upright; if it moves beyond, the object tips. Classroom examples include weighing objects, comparing weights of identical masses in different locations, and observing that weight produces pressure on supporting surfaces. Understanding the difference between mass and weight is important when solving problems involving forces, motion and pressure and for interpreting instrument readings correctly.

Common calculations use the formula W = m × g to find weight from mass. For standard school problems using g = 10 m/s2 simplifies arithmetic and gives results close enough for conceptual learning. Observations such as lighter apparent weight in orbit and increased weight with added mass help students form a clear picture of gravitational force and its effects.

Pressure: Definition and Units

Pressure describes how much force acts on a particular area. It is defined as force per unit area and expresses how concentrated a force is at a surface. The mathematical relation is P = F/A where P is pressure, F is the perpendicular force on the surface and A is the area over which the force acts. Because pressure uses area, even the same force can produce different pressures when the contact area changes.

The SI unit of pressure is the pascal (Pa). One pascal equals one newton acting on one square metre (1 Pa = 1 N/m2). In practical school problems you may also see kilopascals (kPa) or newtons per square centimetre (N/cm2). Always convert units to the same system before calculating: for example, convert cm2 to m2 if using SI units for force in newtons. It is useful to remember that a small force over a small area gives a large pressure — this is why a pin pricks skin easily while the same force applied by a broad object might not.

Pressure acts perpendicular to the surface, and although pressure is a scalar (it has magnitude only), its effects appear in a direction normal to the surface. In cases where pressure varies over a surface, the total force is the sum (or integral) of pressure over each small area. For Class 7, problems mainly use uniform pressure and simple areas. Understanding pressure helps explain many devices and phenomena: why nails have sharp tips, why snowshoes prevent sinking, and why heels damage soft floors more than flat shoes.

Students should practise calculating pressure for different forces and areas and visualise how changing area or force changes pressure. Drawing diagrams with labelled areas and forces helps avoid mistakes and strengthens the link between the abstract formula and practical situations.

Pressure in Solids and Applications

When a force acts on a solid object through a contact area, it creates pressure on the surface receiving the force. This idea is widely used in designing tools and supports. Objects that need to penetrate materials, such as nails, pins and blades, have very small contact areas at their tips so they produce very large pressures from modest forces. Conversely, supports like tables and foundations spread the load over a larger area so the pressure on the ground is lower.

Engineers calculate pressures to ensure that structures do not fail. For example, building foundations are designed so the pressure on soil does not exceed what the soil can safely support. If the pressure is too high, the structure will sink or tilt. Tyres are designed to distribute a vehicle's weight over enough area so that the pressure on the road surface is within limits; tyre treads also help increase friction for grip. Footwear designers consider pressure under heels and soles to make walking comfortable and to reduce damage to floors.

In problems, you may be asked to compute the pressure when force and area are given or to find the area needed to keep pressure below a specified safe value. Units must be consistent; for instance, convert cm2 to m2 if using SI units. Practical classroom activities include comparing pressure under a heel and the sole of a shoe with the same person standing, or measuring pressure beneath supports using simple force meters and known contact areas. These activities show how the same weight can be safe or damaging depending on how it is spread out.

Remember the formula P = F/A and rearrangements (A = F/P). Understanding pressure in solids links the concept of force with everyday engineering and safety problems and helps students see why certain shapes and sizes are chosen in tools and structures.

Pressure in Liquids: Pascal's Law and Upthrust

Fluids transmit pressure in ways that lead to useful effects. Pascal's law states that when pressure is applied to an enclosed fluid, the change in pressure is transmitted equally to every part of the fluid and to the walls of the container. This means a small force applied to a small piston produces a pressure that acts throughout the fluid and can lift a larger piston with greater force. This principle is the basis of hydraulic systems such as car jacks and brakes.

Pressure in a liquid also depends on depth: the deeper you go, the greater the pressure because more liquid lies above and presses down. The pressure at a depth h in a liquid of density ρ is p = p0 + ρgh, where p0 is the pressure at the surface (often atmospheric pressure). This explains many designs: dams are thicker at the bottom where pressure is larger, and divers experience increasing pressure with depth. Importantly, liquid pressure acts equally in all directions at a point, and it acts perpendicular to any surface in contact with the fluid.

Upthrust, or buoyant force, is another consequence of liquid pressure. A body immersed in a fluid experiences greater pressure on its lower surface than on its upper surface, producing a net upward force. Archimedes' principle states that this upthrust is equal to the weight of the fluid displaced by the body. Whether an object floats or sinks depends on the comparison between its weight and the upthrust: if upthrust equals weight it floats, if upthrust is less it sinks. Understanding these ideas helps explain floating boats, submerged submarines, and how hydraulic lifts multiply force while conserving energy. Classroom demonstrations with connected syringes, water columns with holes, and floating objects make Pascal's law and upthrust easy to see and measure.

Atmospheric Pressure and its Effects

The air around Earth has weight and so exerts pressure on everything at the surface. This atmospheric pressure acts in all directions, on our bodies, on buildings and on fluids. At sea level the average atmospheric pressure is about 101325 Pa, commonly called one atmosphere. We usually do not feel this pressure because it acts evenly and our bodies are adapted to it, but it has many observable effects and practical consequences.

Atmospheric pressure decreases with height because there is less air above, so mountaineers and pilots experience lower pressure at high altitudes. Weather systems are linked to changes in atmospheric pressure; barometers measure these changes and help predict weather. Atmospheric pressure also affects boiling points: at lower pressure liquids boil at lower temperatures, which is why cooking times change at high altitudes. Devices like suction cups and vacuum pumps use differences in atmospheric pressure to work: a suction cup sticks because most air is pushed outside it while little or no air remains under the cup.

Other demonstrations include inverting a glass filled with water covered by a card: atmospheric pressure holds the card against the glass when the glass is upside down because the pressure outside is greater than the pressure of the trapped air below the card. Sealed containers can bulge or implode if external and internal pressures differ significantly. Practical knowledge of atmospheric pressure is important in aviation, weather forecasting and everyday devices; careful experiments and barometer observations in class help students see how pressure changes and what its effects are.

Hydraulic Machines and Applications of Pressure

Hydraulic machines use liquids to transmit and multiply force by exploiting the principles of fluid pressure and Pascal's law. A common setup has two pistons of different areas connected by an enclosed fluid. When a force is applied to the small piston, it creates a pressure in the fluid that is transmitted equally and produces an upward force on the larger piston. Because force = pressure × area, a small pressure acting on a large area creates a large output force. This makes hydraulic jacks and presses able to lift heavy loads with small input effort.

Designers select piston areas to get the desired multiplication. In the ideal case (no leaks or friction), F2 = (A2/A1) × F1, where F1 and F2 are the forces on pistons 1 and 2 and A1 and A2 their areas. Energy considerations show that if the large piston is lifted, it moves a smaller distance than the small piston moves down; input work equals output work (ignoring losses): F1 × d1 = F2 × d2. This means hydraulic systems trade force for distance. Car lifts, hydraulic brakes and industrial presses use these ideas to advantage.

Practical hydraulic systems include pipes, seals and valves. Safety requires strong materials and careful maintenance because leaks and broken seals reduce effectiveness and can cause sudden failure. Demonstrations using two syringes connected by tubing and filled with water allow students to see force multiplication in a simple, safe way. Other applications include hydraulic brakes where pedal force is transmitted to brake pads, and hydraulic systems in excavators that lift heavy arms. Understanding these machines connects the basic definition of pressure to powerful and widely used technologies in everyday life and industry.

Resultant Force and Simple Vector Addition

The resultant force of several forces acting on a body is a single force that has the same effect as all the original forces together. Because forces have direction and magnitude, they are vectors and must be added using vector rules. In Class 7 we mainly add forces in one dimension or at right angles using simple methods that build geometric intuition for later study.

Forces along the same straight line are added algebraically: choose a positive direction and give forces in that direction positive signs and those opposite negative signs. The resultant is the sum of these signed numbers. For perpendicular forces you can use graphical or Pythagoras methods: if two forces are at right angles with magnitudes F1 and F2, the magnitude of the resultant R is √(F1^2 + F2^2). The direction of R relative to one force can be found using tan θ = opposite/adjacent when required.

Graphical vector addition uses the head-to-tail method. Draw one force as an arrow to scale, then from its head draw the second force arrow to scale in its direction. The resultant is the arrow drawn from the tail of the first to the head of the last. This gives a visual picture of how forces combine and is useful when forces are not aligned along simple axes. For three or more forces repeat the head-to-tail process. In experiments, measuring directions and magnitudes carefully and using accurate scales for drawing helps students see how the computed resultant matches the graphical one. Practising these methods with toy-cars, weights and pulleys prepares students for more formal vector addition in later grades.

Floating and Sinking: Archimedes' Principle

Archimedes' principle explains the buoyant behaviour of objects in fluids. It states that a body wholly or partly immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced by the body. This upward force occurs because fluid pressure increases with depth, so the pressure on the lower surface of the object is larger than on the upper surface, producing a net upward force.

Whether an object floats or sinks depends on the comparison between its weight and the buoyant force. If the upthrust is equal to the object's weight, the object floats in equilibrium; if upthrust is less than the weight, it sinks. Density is a convenient way to predict behaviour: density is mass per unit volume. An object whose average density is less than the fluid will float because the volume of fluid displaced weighs more than the object; if the object's density is greater, it will sink. Ships, which are made of metal denser than water, float because they enclose air and their overall average density is less than water.

Calculations use the formula upthrust = ρ_fluid × V_displaced × g. For a floating object the volume displaced is such that ρ_fluid × V_displaced × g = weight of object. Submarines change buoyancy by filling or emptying ballast tanks with water to change average density and sink or rise. Classroom activities such as floating objects of different materials and shapes, or measuring displaced water in a beaker when an object floats, make Archimedes' principle tangible and show its wide applications in ship design, hydrometers and buoyancy control in marine crafts.

Practical Experiments and Safety

Hands-on experiments help students connect ideas about force and pressure with real observations. Typical classroom experiments include measuring weight with a spring balance, investigating friction by dragging blocks over different surfaces and recording the pull force, observing how water pressure increases with depth using bottles with holes, and demonstrating hydraulic action with connected syringes filled with water. These activities show how to make measurements, record results, and compare them with theoretical expectations.

Safe laboratory practice is essential. Wear protective eyewear where there is risk of splashes or flying bits, and handle glassware with care. Do not overload instruments such as spring balances beyond their marking. Keep floors dry and clean spills immediately to prevent slipping. When lifting heavy objects, use legs and not the back and ask for help. Ensure syringes and tubing in hydraulic demonstrations are securely connected to avoid leaks. For experiments involving magnets, be cautious with electronic devices nearby. Always follow teacher instructions and know the location of safety equipment like a first-aid kit.

When collecting data, take repeated readings and calculate averages to reduce random errors. Note possible systematic errors such as parallax when reading scales, friction in moving parts, and air bubbles in hydraulic setups. Discuss sources of error and how to improve the experiment. Recording observations carefully and drawing labelled diagrams of your apparatus will make reports clear. Good experimental skills and safety habits developed now will serve well in higher-level practical work and promote accurate and reliable scientific thinking.