State and apply Newton's First Law (an object stays at rest or constant velocity unless acted on by a resultant force) and Second Law (force = mass × acceleration), including the relationship between mass, force, and acceleration
You already learned about resultant forces. Newton's First Law builds on that idea: an object at rest stays at rest, and a moving object keeps moving at the same speed and direction, unless a resultant force acts on it.
Once a spacecraft is moving through space, it can switch off its engines. There is hardly any air resistance or friction out there, so no resultant force acts on it. It keeps moving in a straight line at the same speed, sometimes for years, until something else pushes or pulls on it.
In a moving car, your body travels at the same speed as the car. If the car stops suddenly, your body keeps moving forward, exactly as Newton's First Law predicts, until a force stops it. A seatbelt gives you that force safely.
Newton's Second Law tells you how much force is needed to change motion. Force equals mass times acceleration, written as F = m × a. Pushing harder gives more acceleration. But a bigger mass needs more force to reach the same acceleration.
One more word earns its place: acceleration. It means how quickly the speed changes, and it is measured in metres per second squared (m/s²). A cart that goes from standing still to 6 m/s in 3 seconds gains 2 m/s of speed every second, so its acceleration is 2 m/s². That is the a in F = m × a.
An object keeps its motion unless a resultant force changes it, and F = m × a tells you exactly how much force that takes.
You know how to add forces into one resultant. Newton's First Law says what happens when that total is zero. A spacecraft glides through space with its engines switched off, and out there almost nothing rubs or pushes back.
What does the spacecraft do?
Jobs that lean on this skill. Follow one to see everything it is built on.
32 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.