Physics: Motion, Acceleration, and Forces

Vocabulary and core definitions covering kinematic equations, Newton's laws of motion, force classifications, inertia, momentum, and speed calculations.

Scooter Acceleration at Constant Speed

A condition where a scooter rider accelerates while keeping a constant speedometer reading by changing direction.

Car Average Acceleration Calculation

For a car starting from rest ($u = 0\,\text{m/s}$) and reaching $v = 24\,\text{m/s}$ in $t = 6\,\text{s}$, average acceleration $a = \frac{v - u}{t} = \frac{24 - 0}{6} = 4\,\text{m/s}^2$.

Car Distance Travelled Calculation

The distance travelled in $6\,\text{s}$ by a car accelerating at $4\,\text{m/s}^2$ from rest, calculated using $s = ut + \frac{1}{2}at^2 = 0 \times 6 + \frac{1}{2} \times 4 \times 6^2 = 72\,\text{m}$.

Derivation of First Kinetic Equation

Using the definition of average acceleration $a = \frac{\text{final velocity} - \text{initial velocity}}{\text{time}} = \frac{v - u}{t}$, rearranging gives $at = v - u$, which yields $v = u + at$.

Effects of Force

Four major changes force can produce on an object: change position, change direction, change shape, and change speed.

Newton's First Law of Motion

Every body continues to be in the state of rest or in uniform motion unless an external imbalanced force acts on it.

Newton's Second Law of Motion

The rate of change of momentum is directly proportional to the net force applied and acts in the direction of force applied.

Newton's Third Law of Motion

For every action there is an equal and opposite reaction; action and reaction act on two different objects.

Inertia

The property of an object to resist the change of state, which is measured in terms of mass (higher mass means greater inertia).

Momentum

The product of mass and velocity ($P = m v$), denoted by $P$, with the SI unit of $\text{kg}\,\text{m/s}$.

Frictional Force

A force that acts in the direction opposite to the motion of an object.

Sarang's Swimming Pool Problem

For Sarang swimming $50\,\text{m}$ back and forth in $50\,\text{s}$ with $0\,\text{m}$ displacement: average speed is $\frac{50\,\text{m}}{50\,\text{s}} = 1\,\text{m/s}$ and average velocity is $\frac{0\,\text{m}}{50\,\text{s}} = 0\,\text{m/s}$.

Uniform Speed

Motion where a body covers equal distance in equal intervals of time (e.g., rotation of Earth, a clock).

Non-Uniform Speed

Motion where speed changes over time, exemplified by a vehicle moving in a crowded street.

Force

An external agent which can change the position of an object.

Balanced Force

Two forces that are equal in magnitude but opposite in direction.

Unbalanced Force

Two forces of different magnitudes acting in opposite directions.