Newton’s Three Laws Through the Eyes of Animals

Note:  I did this for a school project, and I have adapted it into a blog post. Special thanks to Professor Jacob Willig-Onwuachi, who teaches physics at Grinnell College for his clarifying suggestions and feedback.

Isaac Newton’s Three Laws of Motion. Everyone lives, breathes, and moves according to these laws. In this, we always get the perspective of humans and their relationship to Newton’s Laws. But what about animals?

Ruff Ruff? This doggy says “What’s Newton’s Laws?” Picture from “The Animal Den”

Of course, even if animals don’t necessarily understand Newton’s Laws, they still abide by them. Here are some easy-to-understand examples of each of Newton’s Laws in the animal kingdom.

The first law is called the Law of Inertia. It is Newton’s First Law.

First, let’s talk about what Inertia actually is. The Law of Inertia is the idea that an object at rest stays at rest, and an object in motion stays in motion, so Inertia is just the tendency of objects to stay in their single state of motion. There is another part to this rule, however. That is that an object in motion or at rest stays that way, unless acted on by an outside force. It sounds complicated at first, but what this means is that, for the most part, an object cannot move or stop moving simply by itself.

I’ll give an example of the Law of Inertia, then we’ll break it down.

For instance, an apple is hanging from the branches of a tree. Suddenly, a bear comes along, brushes into the tree, and knocks it over. The apple falls to the ground, and hits the earth below the tree, before rolling to a pause.

The Law of Inertia is presented twice in this situation.

The Law of Inertia applies to this situation, with the bear and the apple. The apple at rest would stay at rest in the tree, if it wasn’t acted upon by an external force: the bear. But how does it happen twice? Well, after falling, the apple would stay in motion, if it wasn’t again acted on by the force of the ground, causing it to going back to being still once more. This is a basic explanation of the Law of Inertia, because there is more to it.  In some ways, the example is a little off, because gravity is also a force being exerted on the apple as it falls, meaning that it doesn’t exactly fit the descriptor of an “object in motion”, because Newton is assuming, in this law, that the object in motion is in constant motion, which the apple is not, because it is technically speeding up while falling. Complicated, right? Thanks, Jacob!

Newton’s Second Law is the Law of Force, Mass and Acceleration. It describes the relationship between the three. The equation of this law is F (force) = M (Mass) x A (Acceleration). Because Mass and Acceleration are the numbers being multiplied together, if you scale back on one of them, you’ll lose force. (Again, butting in here to say that this explanation also has more to it. You can’t technically “lose” or “gain” force–actually, it is more of a thing that is distributed between mass and acceleration, like a push and pull scenario. If you have two objects, one with a lighter mass, and another with a heavier mass, which one will have more acceleration when pushed? That’s right, the lighter mass. The force isn’t being lost, it is just being distributed between the two.) Acceleration is the change in velocity, and velocity is the change in position. For instance, if a car is going at a constant rate, it’s position in space is changing, so it has a non-zero velocity, even though the acceleration is zero. If the car was speeding up or slowing down, that would be the acceleration.

Here is an example:

A group of dogsledders rush by on a glacier. One of the dogsledders has three dogs attached to their sled. This dogsledder will switch the brunt of the force if 1. The dogs slow down, or 2. They unattached the harness of one of the dogs in the group.

If the dogs slow down, that’s losing acceleration, and if they get rid of one of the dogs pulling the sled, that’s losing mass. This law basically means that, the faster an object is going in space, and the heavier it is, the more force exerted. Back to the dog example, a group of four dogs going 20 MPH is going to have more force than a group of three dogs, which would go a slower speed.

Newton’s Third and Final Law, the Law of Action and Reaction, it states that, “for every action, there is an equal and opposite reaction”. Well, first we must define the equal and opposite reaction.  For each force, the reaction is equal in size, and opposite in direction. This means that, when a force hits something, a force is exerted that matches that same amount, which is often shown in the object pushing backwards.

One instance of this law could be two goats crashing into each other with their horns. At the point of impact, they crash into each other and push each other back. But why is this an example of Newton’s third law?

When the first goat’s force pushes forward, the second goat’s power pushes backward. Then, both goats move opposite in direction, after a hit of equal size. The size of the force from one goat is equal to the size of the force that it pushes back. This law basically means that, for every force, there is an equal and opposite one to it.   A caveat here, according to Professor Willig, this example fits the general idea of Newton’s third law, but it’s actually more complicated. We’ll save that for a later day – perhaps high school!.

In conclusion, Newton’s Three Laws of Motion may be hard to understand at first, but they are much more interesting when used in the context of animal activities.