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.
Newton's first law says an object at rest stays at rest, and an object in motion stays in motion at constant speed in a straight line, unless a net force acts on it. This property is called inertia. A spacecraft in space keeps moving without engines because there is no friction or air resistance to slow it down. A seatbelt stops you from flying forward when a car brakes suddenly because your body wants to keep moving.
Newton's second law: F = ma. The acceleration of an object equals the net force divided by its mass. Bigger force means bigger acceleration. Bigger mass means smaller acceleration for the same force. This explains why a heavy truck needs more force to accelerate than a light car. The unit of force is the newton: 1 N = 1 kg m/s².
Newton's second law gives the quantitative relationship between force, mass, and acceleration. External forces change an object's motion. Internal forces cancel out. The net external force equals mass times acceleration. This law lets you calculate any of the three quantities if you know the other two. For example, if you know the force on a rocket and its mass, you can find its acceleration.
Acceleration is the rate of change of velocity. You can calculate it from force and mass using a = F/m. If a 1000 kg car experiences a 2000 N force, its acceleration is 2 m/s². You can also find acceleration from change in velocity over time. Newton's first law is the special case when net force is zero: acceleration is zero, so velocity stays constant.
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?
Learn first
Optional connections
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.