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Posted by on Feb 24, 2018 in Tell Me Why |

What Is the Law of Inertia?

What Is the Law of Inertia?

Hundreds of years ago, scientists made simple observations and believed that the natural state of an object was to be at rest. It seemed obvious that an outside force had to be applied to make an object move. For example, the merry-go-round won’t simply turn on its own. To enjoy a fun ride, you need to grab the handle and start pushing it in a circle before you jump on and spin around. Eventually, the ride will come to a stop.

Long ago, people believed this simply represented the merry-go-round returning to its natural state of rest. To keep an object moving, they believed that the outside force applied to the object must be continuous. If you’re a fan of the merry-go-round, this certainly seems to make sense.

However, one scientist Sir Isaac Newton built upon the work of others and made a great intuitive leap to understand how objects and motion actually work. Newton understood that the merry-go-round doesn’t come to a stop because it’s returning to a natural state of rest. Instead, it comes to a stop because an outside force is acting upon the merry-go-round to stop its motion.

Newton built upon the work of other scientists, including Rene Descartes and Galileo Galilei. Galileo showed that all objects accelerate at the same rate regardless of size or mass. In doing so, he was the first to develop the concept of inertia. Inertia is the scientific concept that describes the property of objects with mass to resist changes in their states of motion. You may have heard this concept stated another way: an object at rest tends to stay at rest, and an object in motion tends to stay in motion. The more inertia an object has, the more mass it has. As a result, the more massive an object is, the more it will resist changes in its state of motion.

Newton stated this principle in his First Law of Motion, which holds that a body at rest will remain at rest and a body in motion will remain motion, unless and until it’s acted upon by an outside force. We can observe this principle in action when we see that objects don’t start moving, stop moving, or change direction by themselves. They need an outside force acting upon them to do those things.


Going back to the merry-go-round, you can see that a force is required to make it start moving. It would keep moving forever if it weren’t for another force — friction — acting to stop its motion. We must continue to apply force to keep it moving, but not because force is needed to start motion. Instead, the force we must continue to apply to the merry-go-round is necessary to overcome the force of friction that works to stop motion.

Friction is a key force that often works to stop the motion of objects. The more you can reduce friction, the farther an object will move before being stopped. For example, if you pushed a brick with equal force across concrete and across an ice rink, on which surface would it travel farther? If you said the ice rink, you’re correct, because the ice causes much less friction than the concrete.

Content for this question contributed by Mary Roediger, resident of Willits, Mendocino County, California, United States, USA