Introduction to Kinematics and Newton's Laws

What is kinematics?

Kinematics is the branch of physics that qualitatively and quantitatively describes motion.

What is linear translational motion?

Motion along a straight path without spinning.

What does uniform linear motion mean?

An object moves at constant speed in one direction.

What is uniformly accelerated motion?

An object moves in one direction with speed changing at a constant rate.

What is the symbol and unit for time?

Symbol: t or Δt; Unit: seconds (s).

How do you calculate change in time?

Δt = tf - ti (final time minus initial time)

What is distance?

The total path length between two points; scalar; symbol: d; unit: metres (m).

What is speed?

Distance per unit time; scalar; unit: m/s.

Speed formula (uniform motion)

S = d / Δt

What is average speed?

Average speed = total distance / total time

What is instantaneous speed?

Speed at a specific instant in time.

What is displacement?

Overall change in position; vector with magnitude and direction; unit: metres (m).

What is the difference between distance and displacement?

Distance is scalar and total path length; displacement is vector and overall change in position.

What are vectors and scalars?

Vectors have magnitude and direction (e.g. displacement); scalars have only magnitude (e.g. distance).

Example of zero displacement

Walking in a full circle back to the start — displacement = 0.

Displacement vector symbol

Usually written as an arrow above a letter, e.g. \vec{d}

What is velocity?

Displacement per unit of time; vector.

Average velocity formula

v = Δdisplacement / Δt

Instantaneous velocity

Velocity at a particular moment; if uniform, average = instantaneous.

SI unit of velocity

Metres per second (m/s)

How is direction represented in velocity?

Positive = one direction (e.g. east), Negative = opposite (e.g. west)

What is acceleration?

Rate of change of velocity; vector; unit: m/s²

Acceleration formula

a = Δv / Δt

What does a negative acceleration mean?

Velocity is decreasing (slowing down); direction matters.

What is the direction of acceleration?

Same as direction of change in velocity; not necessarily same as velocity direction.

In a displacement vs time graph, what does slope represent?

Velocity

What does a flat line in a displacement-time graph mean?

Object is stationary (velocity = 0)

What does a curved displacement-time graph show?

Acceleration is present.

In a velocity-time graph, what does slope represent?

Acceleration

What does area under a velocity-time graph represent?

Displacement

What does a flat line in a velocity-time graph mean?

Constant velocity (a = 0)

SUVAT Variables

s = displacement, u = initial velocity, v = final velocity, a = acceleration, t = time

First SUVAT equation

v = u + at

Second SUVAT equation

s = ut + ½at²

Third SUVAT equation

v² = u² + 2as

Acceleration due to gravity near Earth

9.8 m/s² downwards

What causes objects to fall at the same rate in free-fall?

Gravitational acceleration is constant; mass cancels out.

What is force?

A vector representing a push or pull; can cause acceleration or balance other forces.

SI unit of force

Newton (N) or kg·m/s²

What is weight?

The force of gravity on a mass; W = mg

Difference between mass and weight

Mass is amount of matter (kg); weight is force due to gravity (N)

What is tension?

Force exerted by a rope or string; always pulls in direction of string.

What is the normal force?

Perpendicular force exerted by a surface to prevent motion through it.

What is friction?

Force that resists sliding between two surfaces; opposite to motion.

Difference between static and kinetic friction

Static: object not sliding; Kinetic: object is sliding.

What is drag?

Air or fluid resistance; opposes motion and increases with speed.

What is net force?

The vector sum of all external forces acting on an object.

How do you find net force from multiple forces?

Add all x- and y-components of external forces.

What is a free-body diagram?

Diagram showing all external forces acting on an object's centre of mass.

Newton's First Law

If no net external force acts, an object's velocity remains constant.

What is inertia?

An object's resistance to a change in its velocity.

Newton's Second Law

F = ma; net force equals mass times acceleration.

Direction of acceleration

Always the same as direction of net external force.

Newton's Third Law

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

Do action-reaction forces cancel out?

No, because they act on different objects.

Why do all objects fall at the same rate?

Gravitational force and mass both increase proportionally, so acceleration remains constant.

How does Newton's 2nd law apply to falling objects?

F = ma and W = mg ⇒ a = g regardless of mass.