Force and Pressure - Class 8 Notes & Olympiad Questions

Force

Origin of Force: Forces arise from the interaction between objects. It takes at least two objects to generate a force. When you push a shopping cart or grip a steering wheel, you're interacting with those objects, and this interaction results in the manifestation of force. It's crucial to understand that force doesn't appear out of thin air; it's a consequence of objects interacting with each other.

Magnitude and Direction: Force is characterised by two primary attributes: magnitude and direction. The magnitude represents the strength of the force, akin to how hard you're pushing or pulling. This magnitude can be measured in units called newtons (N) or dynes. The standard unit of force in the International System of Units (SI) is the newton (N), while in the CGS system, the unit of force is known as the dyne. It's worth noting that 1 newton is equivalent to 10⁵ dynes.
Direction, on the other hand, indicates the path along which the force is applied. By specifying both magnitude and direction, we can fully describe the force acting on an object.

Combining Forces: When multiple forces act on an object, they can either reinforce or counteract each other. If two forces are in the same direction, their magnitudes are added to yield the net force. Conversely, if forces oppose each other, their magnitudes are subtracted. This net force determines the object's overall motion. Equilibrium is achieved when opposing forces cancel each other out, resulting in zero net force.

Force and Motion: Force can set objects in motion. When you give a car a gentle push, you're imparting a force that initiates its movement. Moreover, force can also alter the speed and direction of an object in motion. Picture yourself riding a bicycle and pedaling harder to gain speed. The force you apply increases the bike's speed.

Effects of Force

Force is a powerful concept that has a significant impact on the motion and behaviour of objects. Some of these effects are:

1. Initiating Motion: When a force is applied to a stationary object, it can set it in motion. Similarly, if an object is already in motion, a force can change its speed or bring it to a stop. For instance:

a) Pushing bicycle pedals to start riding.
b) Kicking a soccer ball to make it move.
c) Applying brakes to a car to slow it down.

2. Changing Direction: Force can also alter the direction of an object's motion. An object moving in a straight line can be made to turn or change its path when a force is applied. Examples include:

a) Changing the direction of a moving skateboard by pushing it sideways.
b) A baseball player uses a bat to change the direction of a pitched ball.

3. Altering Speed: Force can also modify the speed of an object that is already in motion. Depending on the direction and strength of the force, the object's speed can increase, decrease, or remain constant. Examples include:

a) Accelerating a bicycle by pedalling harder.
b) Slowing down a moving car by applying the brakes.

4. Modifying Shape: Force can also lead to changes in the shape of objects. It can compress, stretch, or deform materials, often resulting in observable changes in their appearance. Examples include:

a) Squeezing a stress ball to compress it.
b) Stretching a rubber band to increase its length.

Types of Forces

Forces are categorized into two main types based on how they act on objects: contact forces and non-contact forces.

Contact Force

Contact forces are those forces that come into play only when two objects are physically in contact with each other. These forces require direct interaction between the objects for their effects to be observed.
Muscular force and frictional force are types of contact force.

1. Muscular Force

Muscular force, also known as muscular strength, refers to the force exerted by our muscles when they contract and generate movement. It is the force that allows us to perform various physical activities, such as lifting, pushing, pulling, throwing, and moving objects. Muscular force is a crucial aspect of our ability to interact with and manipulate our environment.

Key Points:

a) Muscle Contraction: Muscular force is generated through the contraction of muscles. Muscles are composed of fibres that can contract and relax, producing the necessary force for movement.
b) Voluntary Control: Muscular force is under our voluntary control. We can choose to engage specific muscles to perform actions like picking up a book, kicking a ball, or lifting weights.
c) Direction and Magnitude: Muscular force can be applied in various directions and can have different magnitudes depending on the effort exerted by the muscles.
d) Importance: Muscular force is essential for performing everyday tasks, participating in sports, and maintaining overall physical health. It enables us to carry out a wide range of movements and activities efficiently.
e) Activities and Examples:

1. Lifting a backpack: When you lift a heavy backpack, your muscles contract to generate the force needed to raise the weight.
2. Throwing a ball: Muscular force is applied to your arm muscles to propel the ball forward.
3. Pushing a door open: Muscular force is exerted by your arms and shoulders to open a door.
4. Running: Muscular force in your leg muscles propels you forward while running.

2. Frictional Force

The frictional force is a type of contact force that arises between two surfaces in contact when there is relative motion or an attempt of motion between them. This force occurs due to the interaction between the microscopic irregularities present on the surfaces. Frictional force opposes the motion of objects and always acts in a direction opposite to the intended or actual motion.

Key Points:

a) Direction of Force: Frictional force always acts in the direction opposite to the direction of motion or the intended motion. It acts to slow down or prevent motion.
b) Types of Friction:

1. Static Friction: This type of friction occurs when two surfaces are in contact but not moving relative to each other. It prevents the initiation of motion. For example, a stationary book on a table experiences static friction.
2. Kinetic (Sliding) Friction: Kinetic friction occurs when two surfaces are in relative motion. It opposes the motion of sliding objects. For instance, a sliding box experiences kinetic friction on the floor.

c) Factors Affecting Friction:

1. Surface Texture: Rough surfaces have higher friction compared to smoother surfaces.
2. Force: The force pressing the surfaces together affects the strength of friction.
3. Type of Material: Different materials exhibit varying frictional characteristics.

d) Applications and Examples:

1. Walking: Friction between shoes and the ground prevents slipping and allows us to walk.
2. Brakes in Vehicles: Frictional force between brake pads and the wheels slows down or stops a vehicle.
3. Gripping: Friction between our hands and objects allows us to grip and hold things securely.

e) Reducing Friction: In some cases, reducing friction is desirable. Lubricants like oil, grease, or wax are used to minimize friction between surfaces. For instance, oiling the parts of a machine reduces friction and wear.
f) Importance: Frictional force is crucial for stability, control, and safety. Without friction, walking would be challenging, driving vehicles would be hazardous, and objects could not be held firmly.
g) Drawback: While friction is essential, it also leads to energy loss in the form of heat. This is evident when rubbing hands together generates warmth due to friction.

Non-contact Force

Non-contact forces are types of forces that can exert an influence on objects even when those objects are not in direct physical contact. Unlike contact forces, which require objects to touch each other, non-contact forces can act across empty space. These forces reveal the hidden connections and interactions that exist between objects, even when they are separated by distances.
For instance, non-contact forces include gravitational force, electrostatic force, and magnetic force.

1. Gravitational Force

Gravitational force or the force of gravity is the natural attractive force that exists between all objects with mass. This force is responsible for the phenomenon of weight and is what keeps objects grounded on the Earth's surface.

Key Points:

a) Universal Attraction: Gravitational force acts universally on all objects with mass. Every object, regardless of its size or composition, experiences this force.
b) Earth's Attractive Force: Gravitational force is prominently observed as the Earth's pull on objects located on or above its surface. This force is what keeps us grounded on the planet.
c) Attractive Nature: Gravitational force is attractive, meaning it draws objects closer together. It's why objects fall when released from a height.
d) Dependence on Mass: The strength of the gravitational force is influenced by the masses of the objects involved. Objects with larger masses exert a stronger gravitational pull.
e) Influence of Distance: Gravitational force weakens as the distance between two objects increases.
f) Weight: The weight of an object is the force of gravity acting on it. It is the force with which an object is attracted toward the centre of the Earth or any other massive body.
g) Examples:

1. Falling Objects: When an object is dropped, it falls toward the Earth's surface due to the force of gravity.
2. Planetary Orbits: The gravitational pull between the Sun and planets maintains their orbits in our solar system.
3. Weight on Different Planets: The weight of an object would differ on various planets due to variations in their gravitational fields.
4. Astronomical Phenomena: Gravitational force plays a critical role in phenomena like black holes, neutron stars, and galaxy formation.

2. Electrostatic Force

The electrostatic force, also known as static electricity, is a fundamental force that arises between electrically charged particles. This force is responsible for interactions between objects that have acquired an electric charge. Electrostatic force plays a significant role in the behaviour of particles at the atomic and molecular levels, as well as in our daily lives.

Key Points:

a) Origin of Electrostatic Force: Electrostatic force originates from the presence of electric charges on objects. These charges can either be positive or negative, and they interact with each other based on their charges and distances.
b) Attractive and Repulsive Forces: Like charges (positive-positive or negative-negative) repel each other, while opposite charges (positive-negative) attract each other.
c) Magnitude of Electrostatic Force: The magnitude of the electrostatic force between charged particles depends on the amount of charge they possess and the distance between them. Larger charges or shorter distances result in stronger electrostatic forces.
d) Charge Transfer: Electrostatic force can cause charge transfer between objects. When two objects come into contact particles responsible for electrostatic force can move from one object to the other, leading to a redistribution of charges and the creation of attractive or repulsive forces.
e) Applications and Examples:

1. Static Electricity: The most common example of electrostatic force is static electricity, where objects acquire charges through friction, conduction, or induction. For instance, rubbing a balloon against your hair can create a charge imbalance, causing the balloon to stick to your hair.
2. Lightning: The buildup of charges between clouds or between clouds and the ground leads to a sudden discharge in the form of lightning.

f) Drawbacks: Uncontrolled electrostatic discharge can lead to damage to electronic devices, fires, and explosions in sensitive environments.

3. Magnetic Force

Magnetic force is observed when magnets interact with each other or with objects possessing magnetic properties. This force can lead to attraction or repulsion between the objects.

Key Points:

a) Interactions between Magnetic Materials: Magnetic force is observed when magnets interact with each other or with objects possessing magnetic properties. This force can lead to attraction or repulsion between the objects.
b) Influence of Magnetic Fields: Magnetic fields are regions where magnetic force is exerted. Objects within these fields experience the effects of magnetic force. The strength of the force depends on the magnetic properties of the materials involved.
c) Attraction and Repulsion: Every magnet has a north pole and a south pole. Similar poles repel each other, while opposite poles attract. This behaviour is a fundamental characteristic of magnetic force.
d) Magnetic Materials: Not all materials exhibit magnetic properties. Substances like iron, nickel, cobalt, and some alloys are strongly magnetic and can interact significantly with magnetic fields.
e) Applications and examples:

1. Compasses: Magnetic force plays a crucial role in compasses, guiding travellers and navigators in determining directions.
2. Electric Motors: Many devices, including electric motors, rely on magnetic force to convert electrical energy into mechanical motion, powering various machines and appliances.
3. Magnetic Storage: Magnetic force is utilized in storage devices like hard drives, where information is stored as magnetic patterns on disks.
4. Everyday Uses: Magnets are widely used in everyday items like refrigerator magnets, magnetic closures, and magnetic toys.

Pressure

Pressure refers to the force exerted per unit area on a surface. It is a measure of how much force is distributed over a given area. Pressure plays a significant role in various aspects of our daily lives and is a fundamental concept in physics and engineering.

Force and Area: Pressure is the result of applying force on a surface. It is calculated by dividing the force applied perpendicular to the surface by the area over which the force is distributed.
Mathematically,

Pressure is determined by the force applied perpendicular to a specific area. When force is distributed over a smaller area, it results in higher pressure compared to the same force spread over a larger area. This concept is evident in everyday situations – for instance, a sharp knife is more effective in cutting due to the concentration of pressure on a smaller surface.

SI Unit of Pressure: The standard unit of pressure in the International System of Units (SI) is the Pascal (Pa), which is equal to one Newton per square meter (N/m²).

Effects of Pressure

Pressure is a fundamental concept that has several important effects on objects and materials in our daily lives. The effects of pressure are observed in various situations and play a crucial role in understanding and manipulating our surroundings.

Some of the key effects of pressure are:

1. Compression and Expansion: Pressure can cause materials to compress or reduce in volume. For example, when you squeeze a sponge, its volume decreases due to the applied pressure. On the other hand, pressure can also cause materials to expand. For instance, when air is pumped into a balloon, the pressure inside causes the balloon to expand.

2. Fluid Behavior: Pressure influences the behaviour of fluids (liquids and gases). In a fluid, pressure is transmitted equally in all directions. This is why liquids and gases can exert pressure on the walls of their containers.

3. Sensation of Pressure: We can feel pressure when objects exert force on our bodies. For example, sitting on a chair or walking on the ground involves feeling pressure from the objects supporting us.

4. Changes in State: Pressure can cause substances to change their state. For instance, increasing pressure on a gas can lead to its condensation into a liquid state. Conversely, reducing pressure can cause liquids to vaporise into a gaseous state.

5. Atmospheric Pressure: The Earth's atmosphere exerts pressure on all objects at the surface. As you go higher in altitude, atmospheric pressure decreases. This is why the air is thinner at high altitudes.

Pressure Exerted by Fluids

The pressure exerted by fluids, which include liquids and gases, is a fascinating concept that involves the force applied by these substances on the surfaces they come in contact with.

Fluid Pressure: Fluid pressure refers to the force that fluids apply on the surfaces they are in contact with. This force is distributed over the area of contact.

Pressure Exerted by Liquids: When liquids are contained within a vessel, they exert pressure on the walls and bottom of the container. This pressure is a result of the weight of the liquid column above the point of interest.

Pressure in Gases: Gases also exert pressure on surfaces they encounter. This pressure is due to the constant and random motion of gas molecules colliding with the surfaces.

Relationship between Pressure and Depth: In liquids, pressure increases with depth. The deeper the point, the greater the weight of the liquid column above, leading to higher pressure. This principle is exemplified in situations like diving into a pool, where the pressure increases as you go deeper.

Fluid Pressure and Direction: Fluid pressure acts in all directions, transmitting force evenly throughout the fluid. This characteristic of fluid pressure ensures that it influences objects and surfaces from various angles.

Applications:

a) Fluid pressure is integral to various engineering applications. Hydraulic systems, brake systems in vehicles, and lifting equipment utilize fluid pressure to perform tasks efficiently.
b) Blood pressure measurement in medicine is based on the pressure exerted by blood flowing within blood vessels.

Atmospheric Pressure

a) Atmospheric pressure is an intriguing natural phenomenon that has a profound impact on our environment and everyday life.
b) Atmospheric pressure refers to the force exerted by the weight of the Earth's atmosphere on objects and surfaces within it. It is the pressure exerted by the air molecules in the atmosphere due to their collective weight.

Some key aspects of atmospheric pressure are:

a) Surrounding Influence: Atmospheric pressure envelops us from all directions, acting on our bodies and on objects at ground level and beyond. We are constantly under the influence of this pressure without even noticing it.
b) Balancing Act: The human body and other objects maintain equilibrium by exerting internal pressure that matches the external atmospheric pressure. This equilibrium is crucial for our well-being and the functioning of various systems.
c) Decrease with Altitude: As you ascend higher above the Earth's surface, atmospheric pressure decreases. This is why climbers and aviators often experience changes in pressure, which can lead to altitude-related effects.
d) Units of Measurement: Atmospheric pressure is measured using units like atmospheres (atm), millimetres of mercury (mmHg), and pascals (Pa). One standard atmosphere is approximately equal to the pressure exerted by a column of mercury that's 760 millimetres high at sea level.
e) Barometric Pressure: Barometric pressure is a term used interchangeably with atmospheric pressure. It refers to the pressure exerted by the Earth's atmosphere at a specific location and time.
f) Measuring Instruments: A barometer is an instrument designed to measure atmospheric pressure. A mercury barometer, for instance, uses the height of a column of mercury in a vacuum to indicate atmospheric pressure.
g) Weather Influence: Changes in atmospheric pressure are often associated with weather patterns. Rising atmospheric pressure generally indicates fair weather, while falling pressure can signify the approach of stormy conditions.
h) Effects on Nature: Atmospheric pressure influences weather patterns, ocean currents, and the behaviour of gases. It's a fundamental factor in the Earth's atmospheric dynamics.
i) Health and Altitude: Altitude sickness, experienced at high elevations, is attributed to lower atmospheric pressure and decreased oxygen levels. Proper acclimatization is essential to avoid altitude-related health issues.