Kinematics and dynamics

Introduction to kinematics and dynamics

To begin understanding physics, it is necessary to know two important parts of it: kinematics and dynamics. These terms may seem complicated, but they are simply a way of talking about motion and forces. Kinematics describes motion without considering the causes, while dynamics considers the forces that cause motion. Let's dive into each concept!

Dynamics: the study of motion

Kinematics is the branch of physics that talks about the motion of objects. It tells us how things move, what their speed is, and how their position changes over time.

Position, distance and displacement

Position: This tells you where something is at a particular time. Think of it as locating a point on a number line.

Distance: It is the entire path traveled by an object. If you walk in a circle and return to your starting point, the distance you have traveled is the length of the entire circle.

Displacement: Unlike distance, displacement is about direction. It's the change from the starting position to the final position. If you start at one place and end up at another, your displacement is a straight line in a particular direction between those two points.

In the image above, we see a red circle that represents the starting point and a blue circle that represents the ending point. The dotted line represents the distance, and the straight line is the direction of displacement.

Speed and velocity

Speed: It refers to the speed of something. It does not take into account the direction. If a car moves at 50 kilometers per hour, then this is its speed.

Velocity: Velocity is a little different. It includes both speed and direction. If the car travels north at 50 kilometers per hour, its velocity is 50 kilometers per hour due north.

To find the average speed we can use the following formula:

Average Speed = Total Distance / Total Time

And for velocity, you use:

Velocity = Displacement / Time

Imagine you are traveling from point A to point D. If point B is the place where you change direction, your speed from A to B may be the same as your speed from B to D, but your velocity from A to D is the total change in position over time.

Acceleration

Acceleration: This is the rate of change of velocity over time. If a car speeds up or slows down, it is accelerating. Acceleration can be positive (speeding up) or negative (slowing down).

Acceleration = (Final Velocity - Initial Velocity) / Time

If your car goes from 0 to 60 km/h in 10 seconds, you can calculate its acceleration using the above formula.

The red dot is where you start; the blue dot shows where you are after 5 seconds, and the green dot shows your position after 10 seconds. This change in time is the acceleration.

Dynamics: forces and their effects

Now, let's talk about dynamics, which is about forces and why objects move or don't move. Forces can push or pull objects, affecting their motion.

What is force?

Force is the push or pull applied to an object. It is measured in newtons (N). Force can make an object move, stop the motion, or change the direction.

Some common types of forces are:

• Gravity: The force that pulls objects toward one another. Earth's gravity pulls everything toward it.
• Friction: When two surfaces rub against each other, they exert a force that resists motion.
• Applied force: The force applied to an object by someone or another object.

The diagram shows how gravity acts on an object and pulls it toward the Earth. Forces combined together can produce different effects.

Newton's laws of motion

Isaac Newton gave us three laws to better understand how forces affect motion.

Newton's first law (law of inertia)

An object at rest stays at rest, and an object in motion continues moving at the same speed and in the same direction unless an external force is applied to it. This means that if you slide a hockey puck across the ice, it will continue moving until friction or another force stops it.

The first part of the figure shows the blue circle (the puck) moving along a path until an external force (the black dot) stops it.

Newton's second law (law of acceleration)

The force acting on an object is equal to the mass of the object multiplied by its acceleration. It can be expressed by the formula:

F = m * a

Here, F is the force, m is the mass, and a is the acceleration. If you push two objects with the same force, the lighter object will move faster.

It shows two boxes, an orange light box and a purple heavy box, both being pushed by the same force. The lighter box will speed up more than the heavier box.

Newton's third law (action and reaction)

For every action there is an equal and opposite reaction. This means that if you push on a wall, the wall also pushes back with the same force.

This simple illustration shows the brown block (the wall surface) and the action-reaction force, as represented by the blue lines with two forces moving in opposite directions. You exert an 'action' force on it, and it exerts a 'reaction' force on you.

Balanced and unbalanced forces

When multiple forces act on an object they can either be balanced or unbalanced.

Balanced forces: These forces are equal in size but opposite in direction, so they cancel each other out. For example, when a book rests on a table, the force of gravity pulling it down and the force of the table pushing it up are balanced.

Unbalanced forces: These occur when the forces are not equal, resulting in a change in motion. If you kick a ball, the force of your foot is greater than the force stopping it, so the ball moves.

On the left side of the scene, red arrows represent unbalanced forces, where opposite forces act but are unequal; while on the right side, blue arrows represent balanced forces, because they cancel each other out.

These concepts of kinematics and dynamics form the fundamental understanding of how things move and interact in our world. Whenever we watch a car drive, kick a ball, or just sit quietly, forces and motions are at work, making physics an integral part of our daily lives.

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