SQA National 5 Physics Area 1 Dynamics: a complete overview of motion, forces and energy

Area 1 Dynamics is the study of motion, forces and energy, and it is the most calculation-heavy area of National 5 Physics. This guide maps the five key areas, the relationships you select from the relationships sheet, and the skills markers reward. Each key area has its own answer page with worked examples; this page ties them together.

Vectors and scalars

A scalar has size only (distance, speed, mass, energy, time); a vector has size and direction (displacement, velocity, acceleration, force, weight). The two practical consequences tested at National 5 are the difference between distance and displacement and between speed and velocity, and combining two vectors at right angles. Distance is the total path length and is a scalar; displacement is the straight-line distance and direction from start to finish. To combine perpendicular vectors, use a scale diagram or $R = \sqrt{a^2 + b^2}$ with $\theta = \tan^{-1}(b/a)$ for the direction.

Velocity and acceleration

Acceleration is the change in velocity each second, $a = \dfrac{v - u}{t}$, in $\text{m s}^{-2}$. A negative value means slowing down. It is measured in the lab with light gates timing a card of known length. On a velocity-time graph the gradient is the acceleration and the area under the line is the distance travelled, so a problem is solved by reading slopes and splitting the area into triangles and rectangles.

Newton's laws

Newton's first law: balanced forces give no change in motion (rest or constant velocity). Newton's second law: an unbalanced (resultant) force causes acceleration, $F = ma$. Always combine the forces to find the resultant before using $F = ma$. Mass (kilograms, constant everywhere) differs from weight (newtons, $W = mg$), and friction is a force opposing motion. Terminal velocity is a balanced-forces situation reached when air resistance grows to equal weight.

Work, energy and power

Work done is the energy transferred by a force, $E_w = Fd$. Gravitational potential energy is $E_p = mgh$ and kinetic energy is $E_k = \frac{1}{2}mv^2$. The unifying idea is the conservation of energy: energy is only changed in form, so a falling object converts $E_p$ to $E_k$ and a braking car converts $E_k$ to heat through the work done by the brakes. Equating the two energies solves many problems without forces or time.

Projectile motion

A projectile is split into two independent motions: the horizontal motion is constant velocity ($d = \bar{v}t$) and the vertical motion is constant acceleration at $g$ ($v = u + at$ with $u = 0$ for a horizontal launch). The two are linked only by the shared time. This explains why a thrown ball and a dropped ball land together, and it leads into the idea of a satellite as a projectile in continuous free fall.

How Dynamics is examined

Dynamics questions are mostly numerical. The marks reward selecting the right relationship from the relationships sheet, substituting correctly, and quoting the unit. Show the relationship, the substitution and the answer on separate lines, and always give a direction for a vector answer. Practise reading velocity-time graphs and energy-conversion problems, because these appear every year.

For the official course specification

The SQA publishes the full National 5 Physics course specification, data sheet, relationships sheet and past papers at sqa.org.uk. Always revise from the current specification and SQA past papers.