(4) Change is hard! (Inertia & Momentum)

Which is easier, to stop a bad habit or start a good one?  What about modifying an existing habit? 

Do you feel like any change is hard?  You’re right!

Inertia is real, both in science and in personal growth. 

But I have good news – change is easier if you get some “outside help”.


SCIENTIFIC DEFINITION:   “a property of matter by which it remains at rest or in uniform motion in the same straight line, unless acted upon by some external force”   (from Merriam-Webster.com )

GEOFF’S DEFINITION:  Change is hard.


SCIENTIFIC DEFINITION: “(noun) a: a property of a moving body that the body has by virtue of its mass and motion and that is equal to the product of the body’s mass and velocity.
b: a property of a moving body that determines the length of time required to bring it to rest when under the action of a constant force or moment” (from Merriam-Webster.com)

GEOFF’S DEFINITION: the power that comes from getting our matter moving.  (Get your mass in gear!)  



The next time you are in your car, watch what happens to the things “in” your car that are not attached “to” your car. When you quickly accelerate from being stopped, what happens to the coffee cup you carefully balanced on the dashboard? It wants to stay exactly where it was (in relation to the earth). So, even though it looks like the cup falls backward and spills all over your lap, in reality your lap got pushed underneath the cup by the moving car.  The cup, and the coffee inside, wants to stay still – it’s inertia was zero and no outside force was acting on it.

The reverse is true when you hit the brakes. All those groceries you put in the back seat want to keep moving at the exact same speed, and in the exact same direction, as they were just before you hit the brakes. So, they go tumbling onto the floor behind your seat. The inertia of the bags was moving at a constant speed in a straight line with no outside force acting on it.  That’s inertia at work.

Momentum is a measurement of “mass in motion”. Any object that is moving has momentum. In physics, the momentum of an object is equal to the mass times the velocity.

momentum = mass * velocity

Momentum is usually abbreviated using the letter “p” making the equation look like:

p = m * v

From this equation you can see that both the velocity of the object and the mass have an equal impact on the amount of momentum. You have more momentum when you are running than when you are walking. By the same token, if a truck and motorcycle are traveling down the street at the same velocity, the truck will have more momentum because it has more mass.  

Because velocity is a vector, momentum is also a vector.
This means that in addition to magnitude momentum also has a direction.  (If you are not familiar with “vectors”, they are discussed in my blog on the difference between speed and velocity.) 

Impulse is a term that defines the “change in momentum” and quantifies the overall effect of a force acting over time.

NASA APPLICATION: How will it help humans get to Mars

When spaceships are sitting on the launch pad, its easy for us to look at it and say, “It’s momentum is zero since  it isn’t moving at all”.  And we know that inertia wants to keep it motionless until some force acts on it.   But “everything is relative”!  Relative to you the rocket isn’t moving.  Relative to the launch pad the rocket isn’t moving.   But relative to Mars the rocket IS moving – because it’s sitting on the surface of the Earth which is flying through space at 67,000 miles per hour! (Check this site for more amazing speeds).   

Earth’s gravity is an “outside force” that acts on everything close to it.  It’s also a significant obstacle to getting to Mars. Gravity is a force pulling the rocket toward  the center of the Earth,  while the surface  of the Earth  (as well as the launch pad) are stopping the rocket from moving that direction – they are pushing back. The two forces are equal and opposite,  so the rocket has no motion relative to the Earth.  Some additional force will be needed to get it off the launch pad.  There will need to be an “impulse” to cause a change in momentum.  In fact, it will take a lot of thrust from the rocket engines to get the rocket into the air.  In other words, a major outside force will need to act upon the rocket to allow liftoff, and cause a change in it’s inertia relative to the Earth.

Once the rocket escapes from Earth’s  gravitational force and leaves Earth atmosphere (where there are frictional forces from wind resistance) and enters the vacuum of outer space it will be traveling over 25,000 mph.  

When it shuts off the engines, will it start to slow down?   

No!  There will be no longer be any outside forces acting on it (no gravity, no wind resistance),  so inertia  will continue to keep it moving in the SAME direction at the SAME velocity.   

However, if it has to make a flight course correction (change of direction) on its way to Mars it will require a change in momentum.  Or when it’s ready to slow down before reaching the Martian surface, that will require a major change in momentum (or impulse).  So NASA will need to calculate how much “outside force” from the rocket engine thrust will be necessary to produce the needed change. 

LIFE APPLICATION: How does it get YOU to your M.A.R.S.

LIFE APPLICATION: I’m going to guess that anyone reading this has enough experience traveling in either a car, a bus or a train, to fully appreciate the demonstration/example given above. The car, bus or train are “outside” of our bodies and when they push against us, we start or stop moving (change in motion) or make a sharp turn (change in direction). We are all experts in understanding the effects of inertia on our bodies in these situations.

We are also all experienced in the fact that personal, internal, or life change, is hard.
It’s really, really hard.
In rocket science, we call this “inertia”, in personal growth we call this a HABIT!

Combining his 1st and 2nd Laws of motion, Sir Isaac Newton said something like this:

“Every BODY endeavors to preserve its present state, whether it be of rest or moving uniformly forward in a straight line, until acted upon by a net force.”

In his definition, designed for physicists, he put a space between the words “every” and “body” because he was referring to separate physical entities.  But the same statement is equally true if we take out the space and push these two words together: 

EVERYBODY endeavors to preserve their present state, 

whether it be of rest or moving uniformly forward in a straight line, 

until acted upon by a net force.

I love that he used the word “endeavors”.   It shows that our lack of motion is not simply a passive act.  Everything within us is doing all that it can to maintain the status quo.  Not just in our physical space but in our personal space. 

When a habit is truly formed, you do it without even thinking.  It  becomes “automatic” – that’s inertia.  It wants to continue on with no additional outside force.  But, what happens when you do have a situation that “forces” you not to do the habit?  (Oh yeah, we’ll be talking a lot about “force” in future blogs)

If you always brush your teeth before bed, but you forget to pack a toothbrush on a trip and now it’s time for bed, what does that feel like?  Some of you are cringing even at the thought of this.  Your body has not “remained in motion” (brushing habit) because it was “acted upon” by an “outside force” (lack of a toothbrush).

What if you always stop for coffee at the same place on the way to work, but today they are unexpectedly closed.  This can throw off your whole day – even if the coffee next door is just as good (and probably cheaper).

Habits are a great way to see the effects of inertia in our personal lives.  

Notice that both examples involve ACTION.  Please don’t accept the common misconception that inertia only refers to inaction, or laziness, or lack of motion.  It also means that once we are going in a certain direction, at a certain rate (speed), then we want to stay on the path at that speed.  These actions become “automatic” – either good or bad.

While that limited view of inertia in our lives is incomplete, it is still valid.  Inertia can also be tied to inaction.  How hard is it for someone who hasn’t exercised in years to get started?   How hard is it for someone who hasn’t been in school for years to go back and finish a degree?   These are examples of how inactivity in some areas of our lives (either physical or intellectual) can make it hard to get started.

“Personal Growth” is also a form of movement.   We talked about movement in the previous blog post.

Whenever we grow, whether personally, internally or intellectually, we are not “preserving our present state”.  We are changing!  We are expanding our knowledge or our views.  Now that we better understand inertia, we know that growth won’t just happen.  It’s the not the “default condition” for us to grow.  We must be intentional about growing.  So, if we want to change our “present state” we need some “net” force to be working on, or in, us.  We’ll talk more about what these forces are in a future blog.

Remember in the Physics section above I told you that the symbol for momentum is the letter “p”?  Well, no one is really quite sure why “p” was chosen.  (There are a few theories, but nothing for sure).

I like to think that I know the reason!

I think it was because subconsciously whoever picked it knew that PASSION is the personal growth equivalent of momentum.   Check this out…

When something matters so much to a person that they can’t stop talking about it, we often say that they have a “passion” for it.
In this example, you can see this amazing parallel between personal growth and physics:

Let’s say that weight loss is important to you, so it “matters”.

  • The amount of it, how much it matters to you, is its “mass
  • The good eating habits you’ve recently formed to lose weight is your “velocity
  • If we multiply “how much it matters” times the “number/strength of your habits” we can measure your “passion”.

In other words, multiplying your mass times your velocity gives you your momentum (m*v = p)

How awesome is that?!?

Over the last few years, my wife and I have had to overcome inertia in several parts of our lives.  Consider these examples:

  • Inertial state:  In Motion, Needed to change direction
    • We were in the habit of staying up until 11pm, midnight, 1am, then getting up just in time for work.
    • We needed to get up earlier, but still needed 7-8 yours of sleep.
    • So, we started going to bed earlier.
  • Inertial state:  In Motion, Needed to stop or change direction
    • We were in the habit of watching TV during dinner and then wasting another hour or two afterward with more shows.
    • We canceled our cable service and started listening to short podcasts and/or watching TED talks online.
  • Inertial state:  At Rest (no motion)
    • I wasn’t getting any exercise, walking fewer than 5,000 steps a day
    • I hired a trainer to help me get through a workout once a week, and then pushed myself to do at least on additional day on my own.
  • Inertial state:  In Motion, needed to stop
    • We were in the habit of eating at restaurants for most of our weekly meals (high in calories and sodium, and expensive)
    • We started to plan meals and eat at home more.

Do you ever do something “on impulse”?   Guess what – that’s a physics concept too!

“Impulse” is the measure of “change in momentum”.   Imagine if you wanted to lose weight but one of the following was true:

  • Your weight loss is really your doctor’s idea and it doesn’t matter that much to you, or
  • You haven’t developed any good habits around healthy snacking yet

So, you go to the grocery to buy all of your vegetables and lean meats when you see your favorite candy right at the checkout counter.

Let’s work the formula:

  • The weight loss doesn’t matter much to you so the mass (m) is low, or
  • You don’t have any good habits yet so your velocity (v) is low
  • Either a low “m” or a low “v” will result in a low “p” (m * v = p)

Therefore, the desire for the candy leads to a change in your momentum, and that’s the very definition of “impulse”! 

So, you buy the candy and eat it on the way home.   But, then you feel guilty and I’m not sure what the physics equivalent of guilt is.

A Special Case of Inertia

Another really cool application of inertia to both the physical world and our personal world is the “Area Moment of Inertia”.  This is an idea that helps us understand the “stiffness” of something – it’s resistance to bending.  

If I want to build a simple bridge to walk across a ravine, and I had two 2×4 boards to make it from, which of these arrangements below would work best?   (assume we are looking at the end of the board, the board is pointing away from us)

In these two options NOTHING about the inherent nature of the board has changed.  

Don’t miss this!!!  Only its posture (or position) is different. 

I hope we can all agree that that (A) will give us the best bridge because it will be the most rigid.   When we reach the middle of the boards in (B), they would be sagging under our weight and could easily break, dropping us into the gap.  In this case rigidity or “inflexibility” is a good thing. 

How does this play out in our personal lives?  In most cases, we value flexibility over rigidity.  

But do you see that we can actually decrease our rigidity (our “Area Moment of Inertia”) if we will just change our posture?  If we put ourselves in someone else’s shoes, or see things from their perspective, then we can become more flexible.  Which, in turn, can help us to grow.

How many times have we met someone (never ourselves, of course) who is “twisted” or “bent” and can’t seem to get “straightened out”?  This is also the result of inertia.  It may have taken a lot of force earlier in their lives to make them this way.  But once they are in this state, inertia wants to keep them here. 

(Check out this video on both the mathematical calculations and a physical demonstration (skip to 6:03 if you don’t like math))

So, if we know in our minds that we should grow, and we make the decision that we “want” to grow, what stops us?  

Several forces are at work here, and we’ll talk about many of them in other blog posts, but one of the most prevalent forces is FRICTION.

Since we know that inertia already wants us to stay on the same path, it won’t take much friction to overcome forces like “desire” and “need”.


I would like to close with this challenge.  

Take some time this week (right now might be a good time), to write down (yes, literally write these down on paper or in your phone or tablet) a list of habits (or lack of a habit) where you see inertia at work in your life.

Once you have the list, identify two things about each one:

  1. Is this a positive or negative habit in my life
  2. Is this an example of:
    1. Stationary inertia – I resistant to starting this
    2. Movement inertia – I am resistant to stopping this (already in the habit, good or bad)

As we move forward in future blogs, I’ll ask you to add additional thoughts to this list.

In my previous blog posts we’ve talked about “change”…
  • Change in location is movement
  • Change in location over the change in time is speed
  • Change in speed in a specific direction is velocity
  • Change in velocity over the change in time is acceleration

In this blog, I talked about Impulse being the change in momentum.

In science and math there is a symbol that is used to represent a “change” in some variable.  That symbol is the capital Greek letter DELTA, shaped like a triangle.  

If you are examining company profits, someone might ask “what was the delta”, meaning what was the amount of change in the profits. 

My challenge for you in this post is to find one area of your life that you can apply a “delta”.  What is a change that you could focus on in the next day or two to help you improve your life in a meaningful (even if small) way.

The following  items combine lessons from my previous  blog posts  and this one.  If you are not familiar with any of these ideas,  please take time to read them.
  1. Decide what direction you need to move
  2. Take the first step in that direction (movement)
  3. If you can measure the amount you moved and the time it took you, then you know your speed
  4. Add to your speed the direction, then you know your velocity
  5. Since you started from a standstill (velocity = 0) then you can calculate your acceleration
  6. Decide how much it matters to you to keep moving in this direction, then you know your mass
  7. Multiply your mass times your latest velocity and you know your momentum

If you aren’t already doing so, I would like to invite you to “Like” and “Follow” our Facebook page.

Check out these two (very different) songs about “Momentum”

Here is a great parady of “Cake By the Ocean” that is all about Sir Isaac Newton’s 3 Laws of Motion.

If you want to read more about the idea of Inertia in human behavior, check out these great articles from Dr. Jim Taylor, Ph.D., on the Psychology Today website:

5 thoughts on “(4) Change is hard! (Inertia & Momentum)”

  1. Something that’s interesting (and I imagine you’ll discuss in future posts) is the effect of our “environment” (that is, everything outside of ourselves) on our growth and inertia.

    Fortunately for us as humans, the universe of our “human experiment” is not a static, sterile place where every variable is known, but rather a chaotic environment where every second things are changing and acting (and reacting) on and to us.

    As hard as personal growth can be at times, even the introduction of new environmental factors (such as highly motivated and highly talented peers) can produce and inspire changes in us to the point that its easier for us to overcome our inertia!

    1. Thanks for your reply Isaac. Yes, a great point. When I get to a later post I’ll talk about how much “STP” is an important factor in physics experiments. STP = Standard Temperature and Pressure. But in reality, to your point, as much as we like to remove these as variables, they are always changing. Just as the environment you talked about is changing. Thanks for sharing. I hope you enjoy the post and will continue to share your thoughts. Geoff

  2. Just gotta say that I needed that boost. I’ve been more on the path of getting projects done instead of getting Netflix done lately😊 But partly because your blog was fresh in my mind this morning I was up plenty early and rather than waste 45 minutes with Facebook, I cleaned my car. Baby-steps!! You’re right about replacing. I really do have plenty of time to get what I need done if I get rid of something I do that I don’t need to do or that has not positive impact on my life.

    1. Thanks for sharing April. It’s awesome to hear stories of people making the changes they want to see in their lives. You are so right, “baby steps”! Chinese philosopher Lao Tzu famously said, “A journey of a thousand miles begins with one step”. The size of the step is irrelevant, but the direction does matter. In my next blog (#2, “Hamster Wheel”), I talk about the difference between speed and velocity – the difference is direction. In my third blog (#3, “What does it matter”) I go to the next level and talk about momentum (mass x velocity). Now that you have some momentum you need to keep it going. In a later post I’ll talk about “friction” and its attempts to slow us down and stop us. So, don’t stop now! Maybe next time you can clean my car. 🙂 Ha ha ha!!!

  3. Geoff, I really love these posts. They are very engaging and your voice and tone come through so well. I must confess I watched a child’s video on “inertia” this morning to help me understand this post. Science isn’t my thing, but you did a good job of making it simple. I look forward to reading more.

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