What is Mechanics?

Mechanics is a part of physics that studies:

• How objects move (kinematics)
• Why they move (due to forces, which we call dynamics)
• What happens when they move (like gaining energy or doing work)

Kinematics – Describing Motion

Kinematics is the study of motion without thinking about the cause (like forces). Let’s look at some basic terms:

➤ Distance and Displacement

• Distance is how much ground an object covers. It is always positive.
• Displacement is the shortest straight-line path between starting and ending point. It can be positive, negative, or zero.

For example: If you walk 3 meters forward and then 3 meters back, your distance is 6 meters, but your displacement is 0.

➤ Speed and Velocity

• Speed = how fast something moves (e.g. 60 km/h). It has no direction.
• Velocity = speed with a direction (e.g. 60 km/h east).

If a car moves at 50 km/h north, its velocity is 50 km/h north.

➤ Acceleration

Acceleration means change in velocity. If a car speeds up or slows down, it's accelerating.

Equations of Motion

If an object is moving with constant acceleration, we use three main formulas:

1. v = u + at
2. s = ut + ½at²
3. v² = u² + 2as

Where:

• u = starting velocity
• v = final velocity
• a = acceleration
• s = displacement
• t = time

These formulas help us solve questions about moving cars, falling objects, etc.

Graphs in Motion

Physics uses graphs to show motion:

• Distance-Time graph: Slope = Speed
• Velocity-Time graph:
• Slope = Acceleration
• Area under graph = Distance moved

Graphs help us understand motion visually and make solving problems easier.

Newton’s Laws of Motion

Sir Isaac Newton gave us three famous laws to explain how and why things move

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Newton’s First Law (Law of Inertia)

An object will stay at rest or move in a straight line unless something (a force) makes it change.

Example: A book on a table doesn’t move unless you push it.

Newton’s Second Law

Force = Mass × Acceleration

This law tells us how much force is needed to move something. The bigger the mass, the more force needed.

Newton’s Third Law

Every action has an equal and opposite reaction.

Example: When you jump, you push the ground, and the ground pushes you up.

Types of Forces

In physics, forces are what cause motion or stop motion. Some common forces are:

• Gravity: Pulls everything toward Earth
• Friction: Slows things down when they touch
• Normal force: The support force from a surface
• Tension: Force in a rope or string
• Spring force: Push/pull from a stretched or compressed spring

Free Body Diagrams (FBD)

To solve force problems, we draw a Free Body Diagram. This means drawing an object and showing all the forces acting on it. It helps us understand what’s happening to that object.

8. Work, Energy, and Power

➤ Work

Work is done when a force moves an object.

W=F×d×cosθ

If nothing moves, no work is done—even if you push hard.

➤ Energy

Energy is the ability to do work.

Types:

Kinetic Energy (KE) – energy of motion:

KE=1/2mv²

Potential Energy (PE) – stored energy due to height:

PE= mgh

➤ Power

Power is the rate of doing work.

P= work/time

Time Work​ tells us how fast energy is used. Measured in watts (W).

Law of Conservation of Energy

Energy can’t be created or destroyed. It can only change from one form to another.

Example: In a roller coaster, potential energy at the top becomes kinetic energy as it goes down.

Motion in a Plane

So far, we talked about motion in a straight line. But objects also move in two dimensions—like a football flying through the air.

This is called Projectile Motion. A projectile moves in a curved path. Its motion is broken into:

• Horizontal motion (no acceleration)
• Vertical motion (accelerated by gravity)

Together, they make a parabolic (U-shaped) path.

Center of Mass

Every object has a point called the center of mass, where its mass is balanced.

For example:

• A ball's center of mass is at its center.
• For irregular shapes, it's at a special point where it balances perfectly.

In motion, we can imagine all mass is at this point.

Collisions and Momentum

When objects collide, they exchange momentum.

Momentum=mass×velocity

In a closed system (no external force), total momentum stays the same before and after collision.

There are two types:

• Elastic collision: Energy and momentum both are conserved
• Inelastic collision: Only momentum is conserved