Science for Writers: Newton's Second Law 'Apple on books' from stockvault.net. Science for Writers Logo and text created by me. |

I have put important words in

**bold**. These words are important in physics and I will refer to them throughout the post. It isn't overly important for you to know the exact meaning, so long as you get the gist of what I'm talking about you will be fine following this post.

Writing Links are in

*italics*and these discuss how the science could be used in writing.

## What does it state?

*Principa*:

Law II: The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressedLets break it down before going further:

**Alteration**, means change.- To
**motion**is movement. **Proportional**means a change in the first thing causes a change in the second.- A
**motive force**is what makes something move - The
**right line**means the direction in which the movement is happening **Impressed**means acting on the object

Now we've split it up, it is easier to understand. In actuality it is just a wordy way of saying, 'the amount an object's motion changes is equal to the force making it change'.

As with all good laws in physics there is an

**equation**to go with it.
However, the actual equation is beyond the scope of this article, but we can

**derive**another equation from it. This is one I am sure you have all heard of:
F = ma

Where the F is force, the m is mass, and the a is acceleration.

I should point out the F=ma is actually a special case where the mass is constant and the object is

**not**moving close the speed of light. In ideal conditions it can be applied to balls rolling and things falling.F=ma means that if you accelerate an object (that is to change its velocity/speed) you can calculate the force making it accelerate that much. Let's do an example:

An object of 3kg is pushed. It accelerates from stationary to 10 metres per second in 2 seconds. What force is acting to make this change?

First let's calculate the acceleration. Acceleration is the change in velocity over time. So 10 divided by 2 which is 5 metres per second per second (m/s/s).

Well we know that F=ma so we simply need to do F=3x5, which everyone knows is 15. But 15 what? Elephants? Oranges? No. The unit for force is the Newton (N), named after Isaac Newton.

You may be wondering why I chose kilograms and metres per second. These are

**SI units**. An SI unit is one that a central organisation has decided will be the unit for that measurement. The SI unit for mass is the kilogram, for speed is metres per second, for force is the Newton, for distance is the metre, and for time is the second. By using these units the equations will always work. If you start using non-SI units then the equations don't work without further fiddling.

*Writing Link: I mentioned that F=ma is a special case where mass doesn't change. Imagine a rocket. The mass decreases as fuel is used up and less and less force is needed to keep it at a constant acceleration. Now imagine this as a very convoluted analogy for a story. At the start it is 'heavy' and to get it going you need a lot of force. As the story gets going it gets 'lighter'. This means you can either put less force into making the story continue as it is, or you can keep the force and allow the story to gain momentum and increase the pacing.*

## Another example

Apple Falling Image from kozzi.com Creator: Dan Kosmayer |

Well, we need to know the acceleration. It starts off at 2 m/s and 1 second later is 0.5 m/s. So the maths is (0.5-2)/1. / means 'divided by'. This equals -1.5 m/s/s

Now we'll put it into F=ma. F= 0.2kg x -1.5 m/s/s

=-1.3N

That means that water exerts a force of 1.3N on the apple and causes it to decelerate.

But what makes the water exert this force on the apple? Well that is to do with Newton's Third Law which we'll look at next week.

*Writing Link: You may have been able to tell that Newton's Second Law doesn't lend itself to writing that nicely. It is essentially maths so aside from doing maths in your book there's not much to say. However, you could have a maths-y or science-y character that refers to the laws in conversation. Perhaps the other characters don't like this, perhaps they enjoy the lessons. You decide.*

So, there you have it, Newton's Second Law. Please comment on this post below; I'd love to here from you. Share this post if you enjoyed it. There are social media buttons at the bottom of the post for your convenience.

Thanks