Newton's First Law of Motion - dummies

By Steven Holzner

Whether you’re asked to give it in physics class or not, you need to be familiar with Isaac Newton’s First Law of Motion: “An object continues in a state of rest or in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force.”

What’s the translation? The idea is that if you don’t apply a force to something in motion, it will stay in that same motion along a straight line. Forever!

And what is force? Simply defined, force is a push or a pull on an object. When you pull on a wagon, you’re applying a force to that wagon. When you push a shopping cart in the grocery store, you’re applying a force to that shopping cart.

The property of an object to stay in constant motion is called inertia, and mass is a measure of that inertia. In physics, it’s very important to realize that there’s a difference between mass and weight; objects have mass built in, but they don’t acquire weight until you put them in a gravitational field.

Physics measures mass using the unit kilograms in the MKS system and grams in the CGS system. What’s the unit of mass in the FPS (foot-pound-second) system? You might think it is the pound, but pounds are actually a unit of force. The unit of mass in the FPS system is the oddly named slug. On Earth’s surface, a slug weighs approximately 32 pounds. (You will learn more about units of weight for the MKS and CGS systems soon enough.)

Sample question

  1. Suppose that you have an SUV and want to measure its mass in the MKS system. What unit of measurement do you use?

    The correct answer is kilograms because the MKS unit of mass is the kilogram.

Practice questions

  1. You decide to measure the mass of an aircraft carrier in the CGS system. What unit does your measurement end up in?

  2. You decide to measure your own mass in the FPS system. What unit is your measurement in?

Following are answers to the practice questions:

  1. grams

    The units of mass in the CGS system are grams.

  2. slugs

    The units of mass in the FPS system are slugs.