Motion and Forces
8th Grade Science · 7 lessons
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Lessons
- 1Inertia Is Measured by MassNewton's first law is usually read as a rule about what objects do, but it is better read as a definition of inertia. Inertia is measured by mass in kilograms, so a 2,000 kg car resists a change in its motion exactly twice as strongly as a 1,000 kg car under the same net force. Nothing in the law says an object prefers to stay still: it says that with zero net force the acceleration is zero, whatever the velocity already was.Experiment
- 2Action and Reaction Act on Different ObjectsThe two forces in Newton's third law are equal in size and opposite in direction, but they never act on the same object, which is exactly why they never cancel. A rifle and its bullet feel forces of identical size; the bullet accelerates enormously more because a = F/m and its mass is thousands of times smaller. Two forces cancelling on one body is the first law, not the third — confusing the two is the commonest error in the topic.Experiment
- 3Momentum Is Conserved Even When Energy Is NotTotal momentum p = mv is unchanged in any collision, because the two bodies exert equal and opposite forces on each other for exactly the same length of time. A 2 kg trolley at 3 m/s striking a stationary 1 kg trolley and sticking to it carries 6 kg m/s into a combined 3 kg, so the pair leaves at 2 m/s. Kinetic energy is not conserved in that collision — some is dissipated as heat and sound — which is why momentum, not energy, is the reliable bookkeeping tool.Experiment
- 4Drag Grows with the Square of SpeedAir resistance is not a fixed force. For everyday objects it rises roughly with the square of speed, so doubling a cyclist's speed roughly quadruples the drag — and because power is force times speed, it demands about eight times the power. That one relationship explains why the last few km/h of top speed are so expensive, why vehicles are streamlined to keep the airflow attached rather than merely polished smooth, and why a parachute slows you by adding area rather than by adding weight.Experiment
- 5Pressure: the Same Force Over a Different AreaPressure is p = F/A, measured in pascals, where one pascal is one newton spread over one square metre. A 600 N student standing on shoes of total area 0.03 m² exerts 20,000 Pa; the same student on snowshoes of 0.3 m² exerts 2,000 Pa and stays on top of the crust. The force never changed — only the area it was spread over — and the pressure fell by exactly the factor the area rose. That inverse relationship is why a drawing pin has a wide head and a sharp point on the same shaft.Experiment
- 6Terminal Velocity Is a Balance, Not a LimitA falling object accelerates until the drag pushing up equals the weight pulling down; from that moment the net force is zero and the velocity stops changing. Nothing forbids faster speeds — the balance simply moves. Because drag depends on speed and area while weight depends on mass, a heavier skydiver in the same posture settles at a higher terminal velocity, and opening a parachute does not reduce weight at all: it multiplies the area, so the same weight is balanced at a far lower speed.Experiment
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