Chapter Force And Laws Of Motion Formula, Explanation, Types, Derivations

Force is a fundamental concept in physics that describes the interaction between objects that can cause changes in their motion. It's a vector quantity, meaning it has both magnitude and direction.

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The chapter “Force and Laws of Motion” delves into the fundamental principles that govern the behavior of objects in motion. These principles were first formulated by Sir Isaac Newton and have since become a cornerstone of physics. This chapter explores the relationships between force, motion, and inertia, unraveling the intricate interplay that shapes the way objects move in our world.

At its core, this chapter introduces the concept of force as a factor that influences an object’s state of motion. Whether it’s a gentle push, a powerful pull, or any interaction that causes a change in velocity, force is the key to understanding why objects move as they do.

Newton’s Three Laws of Motion take center stage in this chapter. These laws provide a framework for comprehending the intricate dynamics of objects under the influence of various forces. The first law introduces the idea of inertia, revealing why objects tend to stay at rest or in motion unless acted upon by an external force. The second law establishes a mathematical relationship between force, mass, and acceleration, showcasing the precise connection between the intensity of a force and the resulting motion. The third law, perhaps one of the most intriguing, uncovers the symmetry of action and reaction, highlighting the interconnectedness of forces in the universe.

What Is Force?

Force is a fundamental concept in physics that describes the interaction between objects that can cause changes in their motion. It’s a vector quantity, meaning it has both magnitude and direction. Force is what can accelerate, decelerate, or deform an object. The comprehensive theory of force is built upon the works of Sir Isaac Newton, particularly his second law of motion.

Newton’s Second Law of Motion:

Newton’s second law states that the force acting on an object is directly proportional to the rate of change of its momentum (mass times velocity). Mathematically, this law can be expressed as:

 F = ma 


–  F  represents the force applied to the object (in Newtons, N).

–  m  is the mass of the object (in kilograms, kg).

–  a  is the resulting acceleration of the object (in meters per second squared, m/s²).

This law indicates that the greater the force applied to an object, the greater its resulting acceleration will be. Additionally, if the mass of the object is increased, the same force will result in a smaller acceleration, and conversely, if the mass is reduced, the acceleration will be greater.

In essence, Newton’s second law provides a quantitative relationship between force, mass, and acceleration. It explains how the application of force can alter an object’s state of motion. This law has a wide range of applications and is crucial for understanding the behavior of objects in response to forces in various scenarios.

It’s important to note that forces can come from various sources, such as contact forces (like pushing or pulling) and non-contact forces (like gravitational or electromagnetic forces). These forces can also be balanced or unbalanced, leading to different effects on an object’s motion.

In summary, force is a central concept in physics that underpins the way objects move and interact with each other. It’s a key component of Newton’s laws of motion and provides a framework for understanding the dynamic behavior of the physical world.

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Laws Of Motion

Certainly, here are the three laws of motion formulated by Sir Isaac Newton, along with their details:

Newton’s First Law Of Motion (Law of Inertia): 

  Statement:  An object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.

–  Explanation:  This law highlights the concept of inertia, which is an object’s resistance to changes in its state of motion. An object at rest tends to stay at rest, and an object in motion tends to stay in motion with the same speed and direction, unless a force acts on it. In the absence of a net external force, an object’s velocity remains constant.

–  Example:  A book resting on a table will stay there until a force (like someone pushing it) causes it to move.

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Newton’s Second Law Of Motion 

–  Statement:  The rate of change of momentum of an object is directly proportional to the force applied, and this change takes place in the direction in which the force is applied.

–  Mathematical Expression:  F = ma , where  F  is the force applied, m  is the mass of the object, and a  is the resulting acceleration.

–  Explanation:  This law provides a quantitative relationship between force, mass, and acceleration. It indicates that the force applied to an object will cause it to accelerate, with the acceleration being directly proportional to the force and inversely proportional to the mass. This law essentially explains how forces cause changes in motion.

–  Example:  Pushing a car requires more force to accelerate it than pushing a bicycle with the same force to achieve the same acceleration.

Newton’s Third Law Of Motion (Action and Reaction) 

–  Statement:  For every action, there is an equal and opposite reaction.

–  Explanation:  This law emphasizes the reciprocity of forces. When object A exerts a force on object B, object B simultaneously exerts an equal and opposite force on object A. These forces are always acting on different objects. The law illustrates the symmetry of forces in interactions and is fundamental to understanding how objects interact with each other.

–  Example:  When you walk, your foot pushes backward on the ground (action), and the ground pushes your foot forward (reaction), propelling you forward.

Together, these three laws of motion provide a comprehensive framework for understanding the behavior of objects in response to forces. They describe how forces affect motion, how motion can be changed, and how interactions between objects involve pairs of forces. These laws have had a profound impact on physics and continue to play a crucial role in various scientific and engineering applications.

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What Is Frictional Force?

The physical phenomenon known as frictional force resists any relative motion or attempted motion between two surfaces that are in touch. Frictional force is what prevents motion when two things are in contact and one tries to move or slide over the other. The interactions between the tiny imperfections on the object’s surfaces give rise to this force.

There are two main types of frictional forces:

  • Static Friction: This type of friction prevents the initiation of motion between two surfaces that are in contact but not moving relative to each other. It’s the force that you need to overcome to start an object moving. Once the object starts moving, static friction is typically overcome by other forces, like kinetic friction.
  • Kinetic Friction: Also known as sliding or dynamic friction, this force opposes the relative motion between two surfaces that are already in motion with respect to each other. Kinetic friction is generally slightly weaker than static friction for most surfaces.

Calculating The Frictional Of Force

With the aid of the following formula, the maximum amount of friction force that a surface can exert on an item can be quickly determined:


Here are some meticulous methods to follow when estimating the frictional force that will assist you in doing so.

  • Find the Normal Force

When an object contacts another stable object, a known support force is applied to it. Most of the time, the normal force may be easily defined using the following formula:

N = mg

In this equation, m denotes the mass of the item, and g is the acceleration brought on by gravity. The strength of the normal surface decreases as an inclined surface grows more inclined, hence the formula becomes:

N = mg cos(θ)

θ represents the angle to the surface is inclined to. In a simple calculation, you would calculate the normal force of a 2-kg block of wood sitting on a surface as N = 2 kg × 9.8 N/kg = 19.6 N

  •  Finding the Right Co-efficient

Your choice of coefficient will rely on the nature of the object and the circumstances. Use the coefficient of static friction when an object isn’t moving over the surface; nevertheless, while it is moving, use the coefficient of sliding friction. 

The co-efficient is also impacted by the materials utilised. For instance, the coefficient for a block on a brick surface would be 0.6, whereas for a block of clean wood, it would be in the range of 0.25 to 0.5.

  • Calculating Frictional Force

As was mentioned, F = N is the formula for frictional force.

Let’s use the 2-kg block of wood that is resting on a table to be pushed from rest as an example. The static friction coefficient is taken into account in this situation. 0.5 is the static coefficient of wood.

With the information provided, we can get the normal force using the formula N = 2 kg 9.8 N/kg = 19.6 N. Knowing the static friction coefficient and normal force,we can use the formulas below to determine the frictional force:

F = 0.5 × 19.6 N = 9.8 N

Different Types of Frictional Force

The main types of frictional forces are:

  • Static Friction: When two surfaces come into touch but are stationary in relation to one another, friction of this kind develops. It prevents an object from starting to move when a force is applied. Once the force applied exceeds the maximum static friction, the object will start to move.
  • Kinetic (or Dynamic) Friction: Kinetic friction occurs when two surfaces are in relative motion. The force of kinetic friction is typically less than static friction.
  • Rolling Friction: This type of friction occurs when an object rolls over a surface. It is usually lower than sliding friction (kinetic friction) and is observed in situations where an object, like a wheel or a ball, is in rolling motion.
  • Fluid Friction (Drag): Fluid friction, also known as drag, is the resistance an object experiences when moving through a fluid (liquid or gas). It is influenced by factors such as the shape of the object, the viscosity of the fluid, and the speed of motion.
  • Lubricated Friction: This type of friction occurs when a lubricating substance, such as oil or grease, is used to reduce friction between two surfaces in contact. Lubricants help to minimize wear and heat generated by friction.
  • Internal Friction: Also known as deformational or molecular friction, this type of friction arises from the interactions between molecules within a solid material when it is deformed. It contributes to the material’s overall resistance to deformation.
  • Traction: Traction is a specialized form of friction that comes into play when one surface provides a gripping force on another surface, allowing for the transfer of motion. This is commonly seen in applications like tire-road interactions.
  • Air Resistance: Air resistance, also known as air friction or aerodynamic drag, is the frictional force experienced by an object moving through the air. It becomes significant at higher speeds and can affect the motion of objects like vehicles and projectiles.

Inclined Surface

Component of Weight Parallel to the Incline

= mg ⋅sin(θ)

  1. Sliding Down an Incline (Constant Acceleration): If an object is sliding down an inclined plane without any external force applied (other than friction), you can use Newton’s second law along the incline:

          Fnet =m ⋅ a = mg ⋅ sin(θ) − Ffriction


          Fnet = Net force along the incline

            a = Acceleration along the incline

  1. Impending Motion (Maximum Friction): When you’re calculating the maximum angle at which an object is on the verge of sliding down an incline, you set the friction force to its maximum value:

Ffriction, max = μ max ⋅ N

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The laws of motion formulated by Sir Isaac Newton have far-reaching applications in various scientific, engineering, and everyday contexts. Here are some notable applications:

  1. Transportation and Engineering: 

–  Automobile Design:  The laws of motion are fundamental to designing vehicles with appropriate acceleration, braking, and handling capabilities. Engineers use these laws to optimize safety features like airbags and seat belts.

–  Aerospace Engineering:  The laws of motion are crucial for designing rockets, spacecraft, and satellites. They help engineers calculate trajectories, velocities, and forces needed for space travel.

–  Bridge and Building Design:  Architects and engineers apply these laws to ensure structures can withstand forces due to gravity, wind, and other environmental factors.

  1. Sports and Recreation: 

–  Sports Equipment:  Understanding the laws of motion aids in designing sports equipment such as tennis rackets, golf clubs, and bicycles for optimal performance.

–  Athletic Performance:  Athletes and coaches use knowledge of motion to enhance performance through techniques like improving running form, maximizing jumping height, and optimizing ball trajectories.

  1. Planetary Motion and Astronomy: 

–  Celestial Mechanics:  The laws of motion explain the motion of planets, moons, and other celestial bodies in our solar system. They also form the basis of Kepler’s laws of planetary motion.

  1. Mechanics and Industry: 

–  Manufacturing Processes:  Engineers use the laws of motion to design efficient manufacturing processes, assembly lines, and robotics for various industries.

–  Quality Control:  These laws help ensure product quality by predicting how forces will affect materials and components under various conditions.

  1. Medicine and Biomechanics: 

–  Biomechanics:  Understanding the laws of motion is crucial for studying human movement, gait analysis, and designing prosthetics and orthotics.

–  Medical Imaging:  Techniques like MRI and CT scans rely on knowledge of motion to generate accurate images of internal body structures.

  1. Environmental Science: 

–  Climate Science:  Knowledge of motion helps scientists model atmospheric circulation patterns, ocean currents, and other environmental processes.

–  Natural Disasters:  Understanding forces and motion contributes to predicting and mitigating natural disasters like earthquakes, tsunamis, and landslides.

  1. Everyday Life: 

–  Driving and Traffic:  Drivers use knowledge of motion to navigate safely, anticipate other vehicles’ movements, and apply brakes effectively.

–  Household Objects:  The laws of motion influence the design and use of everyday objects like doors, windows, chairs, and appliances.

  1. Research and Innovation: 

–  Scientific Research:  Researchers employ the laws of motion to study particle physics, quantum mechanics, and other advanced areas of physics.

–  Innovation:  These laws inspire new technologies, materials, and inventions that shape modern life.

In essence, the laws of motion have universal applicability, influencing virtually every field of science, engineering, and technology. They provide a foundation for understanding the physical world and drive advancements that improve our lives in countless ways.

Force And Laws Of Motion Formula FAQs FAQs

What are Newton's Laws? 

Newton's Laws are rules that explain how things move when forces act on them.

What's Inertia?

Inertia is when stuff wants to keep doing what it's already doing—staying still or moving at a steady speed.

Why Do Things Accelerate?

Things speed up or slow down when forces push or pull on them. Big pushes make big changes, and heavy stuff changes less than light stuff.

How Do Objects Push Back?  

When you push something, it pushes you back with the same force, but in the opposite direction. It's like a secret push-pull deal.

Why Do Rockets Go Up?  

Rockets go up because they shoot stuff downward really fast. The stuff going down (action) makes the rocket go up (reaction).

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