Newton’s First Law of Motion: Mass and Inertia

Professor Dave here, let’s talk about Newton’s first law. In Newton’s Principia he describes three laws of motion, and the first of these three was revolutionary. It can be summarized as follows. An object at rest will remain at rest and an object in motion will remain in motion unless acted upon by some external force. The first part seems to make sense, since motionless objects typically don’t just start moving, but the second part contradicts much of our everyday experience. When you throw a ball it doesn’t keep moving forever, it eventually stops. For a wheelbarrow to roll or for your car to move or for a plane to stay in the air, a constant force must be applied. So where did Newton come up with this stuff? To understand Newton’s first law of motion we have to understand all of the forces that act on a body in motion. Our planet is a complicated system and when we move around in our atmosphere we experience things like wind resistance and other forms of friction, which is a force that resists motion. This is what slows us down, requiring the application of constant force to keep moving. But think of something like a skating rink. The ice is very slippery. It is a surface with very little friction. If you gently push a hockey puck it will glide rather far, much farther than if you pushed it on a table or on the ground. But there is still some friction so it eventually does stop. Now imagine, as Galileo and Newton did, that there is a surface that is completely free of friction. If you push the hockey puck on this surface, whatever velocity it has when it leaves the stick would be retained indefinitely, and it would glide forever, unless some other force were to act upon it. This is the kind of motion that can be seen in the vacuum of space. Stars and planets and asteroids and satellites, these objects have roughly constant velocity and they never stop moving. As it happens, most of the universe is in space and very little of it is on earth, so we should consider motion in space where there is no friction and no wind resistance to be normal motion. It is motion within Earth’s atmosphere that is the special case, where objects are subjected to a variety of variables. This is why a car needs an engine that is always running, because there is resistance from the atmosphere and friction with the road. But a spaceship can just accelerate to a certain velocity and then turn the engine off and it will cruise at that final velocity without end. The capacity of an object to resist changes in motion is called its inertia. Force must be applied to get an object moving and again to slow it down to a stop. Only motion with constant velocity including zero velocity, requires no application of force. An object will have some amount of inertia that is proportional to its mass, a scalar value measured in kilograms. More massive objects have greater inertia or greater resistance to motion. Think of a tiny sailboat versus a huge luxury cruise ship. A strong gust of wind is enough to get the sailboat going, and if it hits a rock it will stop on impact. But the cruise ship needs huge engines to accelerate from rest. A lot of force is needed to get it going and a lot of force is needed to get it to stop. Similarly, it is easy to set a ping-pong ball in motion. Just give it a flick. But a bowling ball will need quite a bit more force, and more force to stop as well, as any bowling pin will tell you. In this way mass is essentially a quantitative measure of an object’s inertia. As we said, this applies to objects that are already in motion as well. If you get into a car accident, the force of impact will stop the car, but your inertia dictates that your body will tend to continue moving, and this is why we wear seatbelts, because without them we might continue moving right through the windshield which wouldn’t be very fun. So Newton’s first law can be summarized as the law of inertia, and it tells us that in absence of a net force an object will preserve its state of motion, wether at rest or moving with constant velocity. We must understand what net force means because there will often be multiple forces operating on an object, and the sum of the vectors representing these forces is called the net force. If the net force is not zero, there will be some acceleration. If the net force is zero there will be no acceleration. Now we can move on to the second law of motion, but first let’s check comprehension. Thanks for watching, guys. Subscribe to my channel for more tutorials, support me on patreon so I can keep making content, and as always, feel free to email me: