Motion, Forces, and Newton's Insight
Key ideas from Newton's three laws, gravity, and their everyday applications from braking cars to orbiting moons.
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Questions Covered in This Set
10 cards to master
What was Newton's genuinely radical insight?
That the apple and the Moon obey the SAME rules — one unified set of laws covers falling objects, cars, rockets, and planetary orbits, replacing separate explanations for earthly and heavenly motion.
State Newton's first law and the question it tells you to ask.
An object at rest stays at rest and an object in motion stays in constant velocity unless acted on by a net external force. The practical question: what is the NET force (what's left after forces cancel)?
Why does a car cruising at constant 100 km/h still need its engine running?
The engine thrust exactly cancels drag plus rolling resistance, giving net force zero. The engine isn't 'maintaining' speed mystically — it's cancelling friction. Lift off the gas and the cancellation breaks, so you decelerate.
Why is a car going around a roundabout at steady 30 km/h accelerating?
Acceleration means ANY change in velocity, including direction. The velocity vector swings around, so there's centripetal acceleration supplied by sideways tyre friction pointing toward the centre of the curve.
Why do you skid off a curve on ice without touching the brakes?
The required centripetal force exceeds what the icy road can supply, so the car keeps going straight — perfectly obeying the first law.
Work the braking example: 1,500 kg car from 100 km/h (27.8 m/s) to rest in 4 s.
a = -27.8/4 ≈ -6.9 m/s²; F = 1500 × 6.9 ≈ 10,400 N. Weight = 1500 × 9.8 = 14,700 N, so friction coefficient ≈ 0.7 — the limit of dry asphalt.
Why does stopping distance roughly double in the rain?
Wet asphalt's friction coefficient drops from ~0.7 to ~0.4, nearly halving maximum deceleration. Since distance scales as v²/2a, halving a roughly doubles the stopping distance — so leave more following distance.
What is the key clause people miss in Newton's third law?
Action–reaction pairs act on TWO DIFFERENT bodies. When you push a wall, the wall pushes you; the forces don't cancel because they're applied to different objects.
How does a rocket work in the vacuum of space?
It does not push against air. It throws mass (hot exhaust) backward, and the exhaust pushes the rocket forward with equal and opposite force.
Newton's law of universal gravitation — formula and meaning.
F = G·m₁m₂/r², with G ≈ 6.67×10⁻¹¹. Every mass attracts every other mass; force grows with the masses and falls off with the square of the distance. The Moon is simply falling — continuously — around Earth.
