Balanced and unbalanced forces, Newton's first and second laws and F equals ma, the difference between mass and weight, terminal velocity, and the thinking, braking and stopping distances of a vehicle.

What this dot point is asking

CCEA wants you to explain balanced and unbalanced forces, use Newton's first and second laws and $F = ma$, distinguish mass and weight, explain terminal velocity, and describe the thinking, braking and stopping distances of a vehicle.

Balanced and unbalanced forces

Newton's second law

The bigger the resultant force, the bigger the acceleration; the bigger the mass, the smaller the acceleration for the same force.

Mass, weight and terminal velocity

A falling object reaches terminal velocity when air resistance equals weight, so the resultant force is zero and (by Newton's first law) it falls at a constant maximum speed.

Stopping distances

Examples in context

Example 1. Why a skydiver reaches a steady speed. When a skydiver jumps, only weight acts at first, so they accelerate. As they speed up, air resistance grows until it equals their weight; the resultant force is then zero and they fall at a constant terminal velocity. Opening the parachute increases air resistance sharply, so they slow to a new, lower terminal velocity safe for landing. This is Newton's first law applied to falling.

Example 2. Why braking distance rises so steeply with speed. Doubling a car's speed does not double the braking distance, it roughly quadruples it, because the kinetic energy that the brakes must remove depends on the speed squared. This is why speed limits matter so much, and why CCEA asks you to explain that braking distance increases more than thinking distance as speed rises.

Try this

Q1. State Newton's second law as an equation. [1 mark]

• Cue. $F = ma$ (resultant force equals mass times acceleration).

Q2. Calculate the weight of a 3 kg object where g is 9.8 N/kg. [1 mark]

• Cue. $W = mg = 3 \times 9.8 = 29.4$ N.