Understanding Newton’s First Law of Motion

Keerthana Harwalkar
23 March 2018

Above: Image © FatCat21, iStockPhoto.com

Did you know that every time you close a door, you’re demonstrating a law of physics?

Newton’s laws of motion were first formulated by Isaac Newton, a physicist who lived in the 1600-1700s. These laws revolve around objects, the forces that act on them, and the response of the object because of this force.

For example, whenever you close a door, you have to push the door, and the door shuts. Here:

  1. The object is the door
  2. The force is you pushing the door
  3. The response to the force is the door shutting

There are three laws of motion involved in the simple action of shutting the door. Let’s look at the fist one.

Newton’s First Law of Motion

This law states that every object remains in its original position, either at rest or in constant motion in a straight line, unless a force acts upon it.

In the example above, the door is the object. You pushing the door is the force. The door closing is the response.

If you do not push the door, there is no force. In other words, that door would remain open if you did not decide to close it.

Did you know? Newton’s First Law of Motion is also often called the Law of Inertia.

Here are some other ways of understanding this law:

Example 1: On a winter’s morning

Imagine a cold, winter day. It’s 7:00 a.m. It is -20 degrees outside, and you are extremely cozy and comfortable beneath your thick blanket. You really don’t want to get up, but you know that your mother will come by to wake you up for school.

In this case, you are the object, and the force that makes you get off your comfortable bed is your mother. The response is you getting off the bed to get ready.

If your mother forgets to force you out of your bed, you would most definitely remain in your original position in bed for the entire day (even forever, if it was possible!).

The object (you) will remain in its original position of rest (in bed) unless a force (your mother forcing you to wake up) acts upon it.

Example 2: Over the holidays

Imagine that your younger brother received a large train set for Christmas. You spend the whole night setting it up for him, even though you are tired after all the Christmas day excitement! Finally, at 6 a.m, the train tracks are complete. It’s a huge train set, and you are super-excited to see your brother’s expression when the train choo-choos all around the room!

You place the toy train on the rails and watch it go around the room for one run. But by this point, you’re completely exhausted! Before the train completes its second run around the room, you lay down and fall asleep - right on the rail! The train runs along the rails and stops as soon as it hits your sleeping body.

Here, the train is the object is the train, the force is your sleeping body blocking its path, and the response is the train suddenly stopping.

If you had not fallen asleep on the rails, there would have been no force on the object, and the train would have continued choo-chooing around the room.

The object (the train) will remain in its original position in constant motion (choo-chooing around the room) unless a force (your sleeping body) acts upon it.

Example 3: In outer space

Imagine you are an astronaut on board a spaceship far from planets and other bodies. The spaceship needs some repair work, so you step outside with your toolbag. You place the toolbag carefully in space, right next to you, and it sits beside you.

Why does this happen? Well, outer space does not have air or other forces (except maybe gravitational pull) that would act on the object. That means your object would stay in its current state of rest or motion.

In this example, the bag is at rest. The bag would only start moving under one of these circumstances:

  1. If an asteroid or other object hit it
  2. If you moved it yourself
  3. If the gravitational pull of a planet or another body in space moved it

In this example, if you were revolving around the Earth, your toolbag would have floated towards the planet. That’s because Earth has gravitational pull. Gravity would be a force acting on the object. Then, your toolbag would have orbited the Earth for quite some time before burning up in the Earth’s atmosphere. This actually happened to Ed White, the very first American spacewalker, when he accidentally let go of his glove! The glove was in orbit for a month before it burnt up in the Earth’s atmosphere. Here’s a cool video of his floating glove:

Let’s get back to you in space. The toolbag is the object, and it will remain where you placed it unless a force acts on it. It’s a bit like you in bed.

On the other hand, if you had given it a little push (the force), it would begin moving in the direction it was pushed (response to the force). It would never stop unless something in space caused it to change course - say, an asteroid bumping into it.

Through these examples, you can see how Newton’s first law applies to stationary objects (objects at rest) as well as objects in motion. Every object remains in its original position unless a force acts upon it.

Think of everything you did today. How many examples of Newton's First Law of Motion can you remember?

Learn More

Newton’s First Law (2018)
The Physics Classroom

What is Newton’s First Law? (2018)
Khan Academy

More on Newton’s First Law (2011)
Khan Academy

Newton’s Laws (2016)
Crash Course Physics


Lost in Space: 8 weird pieces of space junk (2009)

Newton’s Laws of Motion (2015)

Organizational Physics (2011)
The Physics of Fast Execution

Do we truly understand Newton’s First Laws? (2017)

Keerthana Harwalkar

I am originally from Bengaluru, India and am currently pursuing a master’s degree in Human Genetics at McGill University, Montreal. My research revolves around oviduct homeostasis and cancer. I spend most of my free time (when I’m not in class or at the lab) engrossed in novels or crying hysterically as my favorite anime/book character gets murdered.

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