The Physics Philes, lesson 5: May the Forces be with You

In which equilibrium is reached, forces are explained, and natural laws are introduced.

Hello! I had a great time at SkepchickCON last week, but I’m very sorry it kept me from writing another Physics Philes. This week I’ll start on Newton’s Laws of Motion. Specifically, I’ll be discussing Newton’s First Law of Motion. Before we dig too deep into the laws of motion, we need to know what we’re dealing with.

Newton’s laws deal with forces. A force is an interaction between two bodies or between a body and its environment. In simpler terms, it’s a push or a pull on an object. There are a couple different kinds of forces: long-range forces and contact forces. Long range forces act on a body even when those bodies are separated by empty space. Contact forces, on the other hand, involves direct contact between two bodies.

There are a few different types of contact forces. There is the normal force, which is a force exerted on an object by any surface the object is in contact with. The normal force is always perpendicular to the surface. Like this:

See how in picture 2 the force isn’t straight up and down? It’s perpendicular to the surface.

Another type of contact force is the friction force. This force is exerted on an object by a surface that acts parallel to the surface. Like this:

Remember! A normal force is still acting on the body perpendicular to the surface!

The friction force is what keeps the box from sliding around!

The final type of contact force is the tension force. The tension force is the pulling force exerted by a stretched rope or chord on an object the rope or chord is attached to. The tension force looks like this:

As you can tell from my most beautiful paintings, a force is a vector quantity. As you know from a previous post, a vector must have a direction and a magnitude. The unit of magnitude we use for force is the Newton (N).

There is one more concept we need to understand before we can plow into the first of Newton’s three laws, and that is the superposition of forces. When two forces F1 and F2 act at the same time on a point on a body, the effect on the body’s motion is the same as if a single force R ere acting equal to the vector sum of the original forces. We call the vector sum of all the forces acting on a body the net force. Or, in picture terms:

This picture is horrible. But it’s a rough approximation of what I’m trying to say.

Remember when we talked about vector addition? That’s what we’re doing here. In fact, this method works when you have any number of forces acting on a body. Just add those force vector together. You’ll find all those forces have the same effect as a single force equal to the net force of those other forces. This also means that any force can be replaced by its component vectors, acting at the same point.

Now, the moment you’ve been waiting for: Newton’s First Law! It’s one of the most famous scientific notions I can think of. Don’t pretend you don’t know it. Say it with me!

A body acted on by no net force moves with constant velocity (which may be zero) and zero acceleration.

This is true because of the effect of inertia. Inertia is what keeps an object moving when it’s already moving and it keeps an object at rest when it’s already at rest. Think about a bottle of ketchup. You shake the glass bottle forwards and backwards to get the ketchup out. As you bring the bottle backwards, the ketchup wants to keep moving forward. Voila! Now you have ketchup on your burger and you’ve seen inertia in action.

It’s important to recognize that what is important in Newton’s First Law is the net force on a body. Zero net force is tantamount to no force at all. When the net force on a body is zero – that is, when a body is either at rest or moving at a constant velocity – that means the body is in equilibrium.

Because Newton’s First Law is so fundamental, there isn’t really any math to prove it. But it’s been proven time and again through experiments. The same is true for the Second and Third laws, but I’ll get to those in the next couple of weeks.

Featured image credit: Wikimedia Commons

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Mindy is an attorney and Managing Editor of Teen Skepchick. She hates the law and loves stars. You can follow her on Twitter and on Google+.


  1. June 19, 2014 at 11:36 pm —

    It seems to be starting with Lesson 5; have lessons 1-4 been taken down?

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