# Irvine Physics Tutor Tip: What is a Newton? Weight vs. Mass

A common problem for physics students is conceptualizing new types of measures and what they represent.  It’s easy enough to understand meters an,d seconds; we’re used to these concepts of distance (even if it isn’t usually in meters) and time.  Other units, however, aren’t as commonplace in our everyday lives and are harder to understand.  As a tutor and physics teacher, I’ve found that students having a strong understanding of units and being able to trace and connect different units helps them dramatically in both their physics understanding and their physics grades (book your private Irvine physics tutor today).

Here, we will be discussing one of the main culprits in physics confusion that is introduced very early in an introductory physics class: the Newton.

You are likely familiar with the concept of weight in everyday life. Things have different weights, and we measure them in pounds, ounces, and sometimes tons.  You’ve likely also heard metric system units of weight: notably grams, milligrams, and kilograms.

But in physics, this gets more confusing.  Pounds are rarely or never used. Kilograms are still used, but they are a unit for “mass” rather than weight.  Instead, the “newton” is introduced as the new unit of weight.  So, in real life, we use kilograms to describe weight, but really, they’re for mass?  What is a newton and how is it different?

Our answer is in a fundamental physics equation that you will need to quickly commit to memory to succeed in physics: F=ma. This is called Newton’s second law.

Knowing the equations is the key to understanding new units and what they measure.  This equation is defining “force.”  Force is the measure that is describing what we call weight in physics.  Mass is an intrinsic quality of matter.  If we multiply that mass by an acceleration, we get the force.

The reason we often equate these two in everyday life is that the acceleration of gravity is always the same for us on Earth.  Specifically, the acceleration is around 9.8 meters per seconds, squared (m/s2).  So, if we know the mass of an object, we can always multiply by 9.8 to get force it exerts downward or its “weight.”  Because of this, mass and weight have become synonymous when talking about the force that objects exert downward since we are almost always talking about objects on Earth and at a typical altitude.

A way to understand this new unit of force is to break it up into its fundamental components.  We can do this by replacing our equation for force above with the units for each variable: force is measured with Newtons, mass with kilograms, and acceleration in meters per second2.

Using this unit definition can help you check answers during a test.  If your final answer is a force, then the units you end with should be:

Similarly, if you get an answer with those units but didn’t know what is, now you know that it is representing force.

Knowing your units is an important tool in physics to check answers and aid in understanding new concepts and new measures.  Every time you learn a new unit, you should be asking yourself what it is measuring and how it is defined with the more fundamental units.

Our experienced Irvine physics tutors are here to help you succeed in your physics class. Call us today to book your tutor!

Michael C. is currently a private math, science, and standardized test tutor with TutorNerds in Irvine and Anaheim.

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