Dynamics
Table of Contents
Introduction of Dynamics
What it is: Dynamics is all about figuring out how forces acting on something make it move. Imagine a push starting a swing – that push is the force, and the swinging motion is what dynamics helps us understand.
The Kinematics Connection: Kinematics describes the “what” of motion – how fast or slow something moves, its direction, etc. Dynamics takes that information and adds the “why” – how the forces acting on it cause that specific motion.
Engineering All Around: From soaring airplanes to sturdy bridges, dynamics plays a crucial role in many engineering fields:
 Mechanical Engineers: They use dynamics to design car safety features, like airbags and crumple zones, to absorb impact forces and protect passengers.
 Civil Engineers: They rely on dynamics to analyze forces on buildings and bridges caused by wind, earthquakes, or traffic, ensuring their safe construction.
 Aerospace Engineers: Dynamics helps them understand the forces acting on rockets during launch, allowing them to design efficient and safe trajectories.
RealWorld Examples: Dynamics isn’t just theory! It’s used in:
 Rocket Launches: Engineers use dynamics to calculate the thrust needed from a rocket engine to overcome gravity and propel the spacecraft into orbit.
 Car Safety: By understanding how forces impact a car during a crash, engineers design crumple zones to absorb energy and airbags to cushion passengers.
Fundamental Concepts
Force

Imagine a tugofwar! Force is any push or pull between objects that can make them start moving, stop moving, or change direction. It’s like an interaction that affects their motion.

Newton’s First Law (Law of Inertia): This law says that objects like to stay in their state of motion. If an object is at rest, it wants to stay at rest. If it’s moving, it wants to keep moving at the same speed and in the same direction. But something needs to change this state, and that’s where force comes in!
 Example: A bowling ball sitting on the lane won’t move by itself. You need to push it (apply force) to make it roll. Once rolling, it will keep going (inertia) until it hits something or you apply another force (like friction) to slow it down.
Types of Forces
There are many forces, but here are a few common ones:
Contact Force: This happens when two objects touch and push or pull on each other. Like kicking a soccer ball (your foot is the force) or crumpling a piece of paper (your hand is the force).
Normal Force: When you place an object on a surface, the surface pushes back up on the object to support its weight. This upward push is the normal force.
Tension Force: Imagine a rope pulling on something. The force pulling along the length of the rope is the tension force. Think of a tugofwar rope!
Weight: This is the force pulling down on an object due to gravity. It’s like Earth’s constant tug on everything! Your weight depends on your mass (how much stuff you are made of) and the strength of gravity (which can vary slightly depending on location).
Friction: This is the force that resists the movement of two surfaces touching each other. It can slow things down or even stop them from moving altogether. Like trying to slide a box across the floor.
Mass and Weight:

Mass: Think of mass as the amount of “stuff” in an object. It’s a measure of how much matter is there. A heavier object has more mass than a lighter one. Imagine two balls of playdough – the bigger one has more mass. Mass is measured in kilograms (kg).

Weight: Weight is different! It’s the force pulling down on an object due to gravity. So, a heavier object (more mass) will have a greater weight because there’s more “stuff” for gravity to pull on. But the mass of the object itself doesn’t change depending on gravity. An astronaut in space may feel weightless because there’s very little gravity pulling them down, but their mass stays the same! Weight is measured in Newtons (N).
Gravitational Constant (G)
 Gravity is like an invisible glue holding everything together. The gravitational constant (G) is a number that helps us calculate the force of gravity between two objects. It’s a tiny number, but it’s super important! The more mass an object has, or the closer two objects are, the greater the force of gravity between them.
Motion with Newton’s Laws:
Sir Isaac Newton laid the foundation for classical mechanics with his three Laws of Motion. These laws explain how objects move or stay still when forces act on them. Let’s break them down:
First Law: Law of Inertia
Imagine an object sitting on a table. It stays put, right? This is inertia, an object’s tendency to resist changes in its motion. The First Law says:
 An object at rest stays at rest unless a force makes it move. (Think sleeping soundly!)
 An object in motion keeps moving at the same speed and in a straight line unless a force changes its direction or speed. (Imagine a bowling ball rolling straight until it hits the pins!)
Second Law: Law of Acceleration
This law explains how forces make objects speed up, slow down, or change direction. It says:
 The acceleration of an object depends directly on the force acting on it and inversely on the object’s mass. (More force = bigger push, heavier object = harder to push)
Here’s the equation derived from this law:
Force (F) = Mass (m) x Acceleration (a)
 Force is measured in Newtons (N)
 Mass is measured in kilograms (kg)
 Acceleration is measured in meters per second squared (m/s²)
So, a bigger force (push or pull) will cause a greater acceleration (change in speed) for a lighter object. Conversely, a smaller force will have a lesser effect on a heavier object.
Third Law: Law of Interaction
This law talks about what happens when two objects interact. It says:
 Whenever one object exerts a force on another, the second object exerts an equal and opposite force back on the first. (Think pushing a wall – you feel the wall pushing back on you!)
Imagine a rocket blasting off. The hot gases shoot out the back (action), pushing the rocket forward with an equal and opposite force (reaction). This is how rockets propel themselves without needing anything to push against in space!
Free Body Diagrams: The Secret Weapon
A free body diagram is a simple sketch showing an object isolated from its surroundings, with all the forces acting on it represented by arrows. These forces can be pushes, pulls, gravity, friction, etc.
Why are they important?
 They help us visualize the forces acting on an object.
 By applying Newton’s Laws to these forces, we can predict how the object will move.
Friction
Friction is like a party popper for motion. It’s the force that acts between two surfaces whenever they touch and try to slide past each other. Imagine trying to walk on ice – that’s low friction, making it super slippery! Friction can be both helpful and bothersome, depending on the situation.
Types of Friction
There are two main types of friction to consider:

Static Friction: This is like a friend holding your hand to stop you from slipping. It acts between objects at rest, preventing them from moving. The force of static friction increases until it’s strong enough to keep things in place. For example, a rug stays on the floor because static friction holds it there.

Kinetic Friction: Once you overcome static friction and the object starts moving, kinetic friction takes over. It’s like that same friend slowing you down a bit as you walk. Kinetic friction is always a little weaker than static friction, which is why it’s easier to keep an object moving than to get it started. Imagine pushing a box – static friction stops it at first, but then kinetic friction slows it down as you push.
Coefficient of Friction
The slipperiness or grip between two surfaces is determined by something called the coefficient of friction (µ). It’s like a rating system for friction, with a higher number meaning more grip and a lower number meaning more slide. Imagine a carpet with a high coefficient of friction (µ) and a smooth tile floor with a low coefficient of friction.
Here’s how the coefficient of friction helps calculate the force of friction:
 Force of Friction = µ * Normal Force
 Normal Force: The pushing force between two surfaces, perpendicular to their touching area (like the force of gravity pushing an object on the floor).
So, the coefficient of friction, along with the normal force, tells you how strong the friction force will be. A higher coefficient of friction or a higher normal force will result in a stronger force of friction.
Conclusion
In conclusion, Dynamics has established itself as a cornerstone of mechanics, empowering us to delve deeper than the “what” of motion and explore the fundamental “why.” By unveiling the intricate relationship between forces and the resulting motion of objects, Dynamics serves as a powerful tool for engineers and scientists across various disciplines.
The journey through Dynamics has equipped us with the understanding of forces, their interactions (Newton’s laws), and the ability to analyze motion through free body diagrams. We explored the concepts of equilibrium, the dynamics of particles and rigid bodies, and the fundamental workenergy principle. Understanding friction, the everpresent force opposing motion, further enriched our understanding of realworld scenarios.
FAQ’s
These are two important branches of physics that deal with motion, but from different angles:

Kinematics: Focuses on describing how things move. It talks about things like position, speed, acceleration, and time, without considering the forces that cause the motion. Imagine a car speedometer – it tells you how fast you’re going (kinematics) but not why you’re speeding up or slowing down (dynamics).

Dynamics: Explains why things move the way they do. It brings forces into the picture, using concepts like Newton’s laws to understand how forces affect motion. This is where you calculate the force needed to accelerate a car or the brakes needed to stop it.
There isn’t one single formula for dynamics, but it relies heavily on Newton’s second law of motion:
Not quite! Kinetics is sometimes used interchangeably with dynamics, but it has a slightly narrower focus. Kinetics specifically deals with the motion of objects and the energy associated with that motion. Dynamics, on the other hand, incorporates forces to explain why that motion happens.
The title of “father of dynamics” is often attributed to Sir Isaac Newton. His laws of motion laid the groundwork for our understanding of how forces influence the motion of objects.
Related Links
MCQ’s
1. What is dynamics in physics?
 A) The study of motion
 B) The study of forces and their effects on motion
 C) The study of energy
 D) The study of heat transfer
Answer: B) The study of forces and their effects on motion
2. What is Newton’s first law of motion also known as?
 A) Law of inertia
 B) Law of acceleration
 C) Law of actionreaction
 D) Law of gravity
Answer: A) Law of inertia
3. According to Newton’s second law of motion, what is the relationship between force, mass, and acceleration?
 A) Force = Mass × Acceleration
 B) Force = Mass ÷ Acceleration
 C) Force = Mass + Acceleration
 D) Force = Mass – Acceleration
Answer: A) Force = Mass × Acceleration
4. What does Newton’s third law of motion state?
 A) Every action has an equal and opposite reaction
 B) An object at rest will remain at rest unless acted upon by an external force
 C) The acceleration of an object is directly proportional to the net force acting on it
 D) Force is equal to the rate of change of momentum
Answer: A) Every action has an equal and opposite reaction
5. Which of the following is an example of Newton’s third law of motion?
 A) A car accelerating on a straight road
 B) A person pushing against a wall
 C) A ball rolling down a hill
 D) A book sliding on a table due to a force
Answer: B) A person pushing against a wall
6. What is the SI unit of force?
 A) Joule
 B) Newton
 C) Watt
 D) Meter per second
Answer: B) Newton
7. What is the weight of an object?
 A) The mass of the object
 B) The force exerted on the object due to gravity
 C) The volume of the object
 D) The acceleration of the object
Answer: B) The force exerted on the object due to gravity
8. What is the net force acting on an object in equilibrium?
 A) Zero
 B) Equal to the weight of the object
 C) Equal to the mass of the object
 D) Infinity
Answer: A) Zero
9. What happens to the velocity of an object when the net force acting on it is zero?
 A) It increases
 B) It decreases
 C) It remains constant
 D) It becomes negative
Answer: C) It remains constant
10. What is frictional force?
 A) A force that opposes motion between two surfaces in contact
 B) A force that increases the velocity of an object
 C) A force that maintains the motion of an object
 D) A force that changes the direction of an object
Answer: A) A force that opposes motion between two surfaces in contact
11. Which of the following is NOT a type of friction?
 A) Static friction
 B) Kinetic friction
 C) Rolling friction
 D) Magnetic friction
Answer: D) Magnetic friction
12. What is the acceleration of an object in free fall near the surface of the Earth?
 A) Zero
 B) 9.8 meters per second squared
 C) 1 meter per second
 D) 2 meters per second squared
Answer: B) 9.8 meters per second squared
13. Which of the following is NOT a conservative force?
 A) Gravity
 B) Friction
 C) Elastic force
 D) Tension in a string
Answer: B) Friction
14. What is the work done by a force when the displacement is perpendicular to the direction of the force?
 A) Positive work
 B) Negative work
 C) Zero work
 D) Variable work
Answer: C) Zero work
15. What is the formula to calculate work done by a force?
 A) Work = Force × Time
 B) Work = Force × Displacement
 C) Work = Mass × Acceleration
 D) Work = Power ÷ Time
Answer: B) Work = Force × Displacement
16. What is the unit of work and energy?
 A) Newton
 B) Watt
 C) Joule
 D) Kilogram
Answer: C) Joule
17. What is the principle of conservation of energy?
 A) Energy can be created but not destroyed
 B) Energy cannot be created nor destroyed, only transformed from one form to another
 C) Energy is always lost as heat during any process
 D) Energy is directly proportional to mass and velocity
Answer: B) Energy cannot be created nor destroyed, only transformed from one form to another
18. What is the unit of power?
 A) Newton
 B) Watt
 C) Joule
 D) Kilogram
Answer: B) Watt
19. What is the formula to calculate power?
 A) Power = Force × Displacement
 B) Power = Work ÷ Time
 C) Power = Mass × Acceleration
 D) Power = Energy ÷ Time
Answer: B) Power = Work ÷ Time
20. Which of the following is a unit of momentum?
 A) Newton
 B) Watt
 C) Joule
 D) Kilogram meter per second
Answer: D) Kilogram meter per second