What is Newton’s 2nd Law of Motion?

Newton’s 2nd Law of Motion is probably the most confusing of the three, at least in how it’s defined. Why? Because it is defined in a few different ways.

No matter how you put it, the 2nd law describes the “how” of it all. How are force, mass, and acceleration related?

To get a better picture, we have to think of one. That’s right, let’s use our imaginations.

You’re standing in a field on a nice summer day. There’s a gentle breeze, the smell of fresh cut grass, and there’s just enough clouds overhead to keep the sun out of your eyes. You have a baseball in one hand and decide to throw it as hard as you can. You walk to pick it up, and this time you give it a gentle toss. What was different? 

Well for one, we know the mass of the ball didn’t change, so we can rule that out. What else?

When you throw the ball as hard as you can, you’re using more force (ie, as hard as you can).

When you give it a gentle toss, you’re using less force (ie, gently).

Let’s take a step back a moment.

Remember, force is calculated by multiplying mass and acceleration, right? So if the mass of the ball doesn’t change, how do we end up with different amounts of force? 

Acceleration.

The harder thrown ball is also moving faster, or with greater velocity. That velocity also changes over a shorter period of time, which means it has a higher acceleration (change in velocity over time).

Make sense?

If we understand that, then this next part ought to be pretty easy, too!

What about throwing, I don’t know, a baseball versus a bowling ball. It doesn’t take a lot to imagine that, even if you throw both as hard as you can, the baseball is going to go a lot farther than the bowling ball. 

What’s changing for the equation this time? The mass, of course, but also the acceleration. The bowling ball is going to have a lower velocity, therefore a lower acceleration, too.

Which brings us back to the 2nd Law. It is commonly stated as “an object’s acceleration will be in the same direction as the net force being applied and is equal to the net force divided by its mass.

Or, the acceleration of an object is equal to the net force divided by the mass.

Or, a=F/m

OR you can simply rearrange the law and equations to say 

No matter which way you put it or use it, it’s the law.