SIMPLIFIED: "5.4: Newton's Second Law"

Article Link: https://phys.libretexts.org/Bookshelves/University_Physics/University_Physics_(OpenStax)/Book%3A_University_Physics_I_-_Mechanics_Sound_Oscillations_and_Waves_(OpenStax)/05%3A_Newton's_Laws_of_Motion/5.04%3A_Newton's_Second_Law 

Note: The image above is from the original article.

This article talks about Newton’s second law. There is a section at the bottom that talks about momentum, but this is another topic that we have simplified if you search for momentum simplified articles. 

This simplification will essentially tell you exactly what you need to know about Newton’s second law and its meaning.

In essence, Newton’s second law is an equation: Force = mass * acceleration. Let’s analyze this equation. We know that a force is something that causes an object’s speed to change. For instance, if you have a bowling ball that is just sitting there, in order to make it change its speed from 0 to anything else, you need to push it, therefore exerting a force on it. Looking at the equation now, if the force on an object is the same, then an object with more mass will have less acceleration because of its weight. If that does not make much sense, look at it this way: if you put the same amount of force between two objects, one that is very light and one that is very heavy, the object that is very heavy will have less acceleration and thus a less change in speed. Connecting back to the equation, if you want to keep force the same, increasing the mass means you have to decrease the acceleration and vice versa.

The article also briefly mentions external versus internal forces. This is initially a confusing concept. Essentially, you need to define a system. A system can literally be anything, including the whole Earth. Any force that happens INSIDE of the system is an internal force and any force that occurs OUTSIDE of the system is an external force. Here is an example: let’s say that there are two boxes stacked on top of each other. If you say the system is only the two boxes stacked on top of each other, when you shoot an arrow at the two boxes causing the top box to fall onto the ground, you have exerted an EXTERNAL force because the force is outside of the system. However, if you define the system to be the arrow AND the two boxes, then the force is internal to the system. For both scenarios, the force of gravity from the Earth is an external force because you did not include Earth in the system. If you include Earth in the system, then the force of gravity becomes an internal force too.