Unit II: Newton's Laws of Motion
The Three Simple Rules That Govern the Universe.
This is the second chapter of Unit II. It builds on our understanding of forces.
← Go back to Chapter 1: Introduction to Forces.
Law 1: The Law of Inertia
In the last chapter, we saw that a book on a table has balanced forces and doesn't move. But what if an object is *already* moving? What does it take to change its motion? This brings us to Newton's First Law, the Law of Inertia:
An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Inertia is an object's resistance to a change in its state of motion. The more mass an object has, the more inertia it has. In the simulation below, imagine a box floating in deep space, far from any gravity. Click the buttons to apply a brief "push" (a force) and see what happens.
The "Aha!" Moment
Notice that after you push the box, it continues to glide at a constant velocity forever. It doesn't slow down because there's no friction in space. The only way to stop it is to apply another force in the opposite direction. This is inertia in action!
Law 2: The Law of Acceleration (F = ma)
The First Law tells us that a force is needed to change motion (to accelerate). The Second Law tells us *exactly* how much acceleration we get for a given force. It's one of the most important equations in all of physics:
F = ma
This means: The Force required is equal to the object's mass times its acceleration. We can rearrange this to a = F/m, which tells us that acceleration is directly proportional to force (more force = more acceleration) and inversely proportional to mass (more mass = less acceleration).
Use the simulator below to discover this relationship. Adjust the force and mass, then hit "Run" to see the resulting acceleration.
The "Aha!" Moment
Try doubling the force while keeping the mass constant. What happens to the acceleration? It doubles! Now, try doubling the mass while keeping the force constant. The acceleration is cut in half! You've just proven a = F/m for yourself.
Law 3: The Law of Action-Reaction
Newton's final law reveals a deep truth about the universe: forces never occur alone. They always come in pairs.
For every action, there is an equal and opposite reaction.
This means if you push on a wall, the wall pushes back on you with the exact same force. When a rocket expels gas downwards (the "action"), the gas pushes the rocket upwards (the "reaction"). The key is that the action and reaction forces always act on *different* objects.
In the simulation below, observe the classic example of a cannon firing a cannonball. This perfectly illustrates the principle of action and reaction.
The "Aha!" Moment
When the cannon fires, it exerts a powerful forward force on the cannonball (the "action"). At the exact same instant, the cannonball exerts an equal and opposite force back on the cannon (the "reaction"), causing it to recoil backward. This demonstrates that forces always occur in pairs.
You've Mastered Newton's Laws. What's Next?
You now understand the fundamental rules that connect forces and motion. The next step is to apply these laws to more complex, real-world situations involving forces like friction and drag.