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AP Physics 1: Exam Prep12 chapters | 136 lessons

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Lesson Transcript

Instructor:
*Sarah Friedl*

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

We often talk about mass and weight as if they are the same. While they are proportional to each other, they are not in fact the same. In this video lesson you will learn to distinguish between the two, as well as convert one to the other.

Though you may not like what it says, when you stand on a scale, the reading you see is your weight. **Weight** is the force on an object due to gravity, so this is how much the Earth is pulling you toward it. The more you weigh, the greater the force, so the scale reads a higher number. Want to lose weight? Try thinking of it as lessening your force on the earth due to gravity!

Another option is to go to a higher elevation on Earth. While you probably wouldn't notice the difference, the force due to gravity decreases slightly as you go higher in elevation. Basically, the earth is pulling you down with slightly less force than at sea level, so your weight is reduced!

This is also why objects in space are 'weightless' - weight is relative to the amount of gravity. In space, where gravity is minimal, weight becomes almost insignificant. You might weigh 150 pounds on Earth where there is a lot of gravitational force pulling you down, but travel to the moon and your weight will drop significantly - to about 25 pounds! This is because gravity on the moon is about 1/6 of the gravity on Earth, so the force pulling you down is less.

When you travel to the moon you weigh less, but do you actually become smaller? Is there less of your body than on Earth? Not at all! Your body has the same amount of 'stuff' on the moon as it does on Earth. This measurement is your **mass**, the amount of matter in an object.

On Earth, we often use mass and weight interchangeably because they're directly proportional to each other, but they aren't the same thing at all. An elephant has more mass than a mouse because the elephant has more matter than the mouse. It also weighs more than the mouse because the force due to gravity is greater.

What we mean when we say they are proportional is that even though the weight of an object might change with its location, that change is the same for every object. So the elephant weighs more than the mouse on both Earth and the moon by the same amount. Twice the mass means twice the weight, no matter where you are! So if you take that elephant into space and you try and push it, you'll find it's just as difficult as on Earth. This is because it still has the same amount of mass - the same amount of 'stuff' that you're trying to move no matter what it weighs.

Luckily, if we know how much something weighs, we can easily calculate its mass. Likewise, we can also calculate the weight of something if we know how much mass it has. People usually refer to weight in pounds in everyday life, but in physics the standard unit of this force is the Newton. The symbol we use for Newton is N. Mass also has a standard unit, the kilogram, or kg.

The conversion factor between these two is 1 kg to 9.8 N. An object that has a mass of 1 kg weighs about 9.8 N. So if you already know the mass of an object and you want to know its weight in Newtons, you simply multiply the number of kilograms by 9.8 N. Conversely, if you know an object's weight in Newtons, you can divide by 9.8 N, and you'll get that object's mass.

Using Newtons is standard in physics, but what exactly is a Newton? Well, it's about 0.225 pounds. To look at it the other way, one pound (symbol lb) is the same as 4.45 N. So if you weigh 150 lbs on Earth, you weigh 4.45 times as much in Newtons - 667.5 N, in fact! Next time you think you need to lose weight, just think about how much more your scale would read if it was in Newtons instead of pounds!

You can also convert pounds and kilograms directly, because 1 kg weighs 2.2 lbs on Earth. So in order to get an object's weight in pounds, simply multiply the mass by 2.2 lbs. If you want to know an object's mass, divide by 2.2 lbs, and you'll get the object's mass in kilograms.

On Earth, we often use weight and mass interchangeably, but they're definitely not the same thing. **Mass** is the amount of matter an object has and **weight** is the force on an object due to gravity. Your mass won't change from Earth to the moon - you'll still be made of the same amount of stuff. However, your weight will change because there is less gravity on the moon, so the force pulling you down is also less.

Though they're not the same, mass and weight are proportional. This means that no matter where you are, a more massive object will weigh more than a less massive object. An elephant will weigh more than a mouse on both the Earth and the moon because it's more massive than the small mouse, no matter where they are!

You can easily calculate an object's weight from its mass, or the mass from its weight. In physics the standard unit of weight is Newton, and the standard unit of mass is kilogram. On Earth, a 1 kg object weighs 9.8 N, so to find the weight of an object in N simply multiply the mass by 9.8 N. Or, to find the mass in kg, divide the weight by 9.8 N.

Working in pounds? Have no fear! On Earth, a 1 kg object weighs 2.2 lbs, which also means that 1 lb is the same as 4.45 N. So if you have an object's weight in pounds, but want to know the mass, simply divide by 2.2 lbs. To find the weight from the mass, multiply the number of kilograms by 2.2 lbs, and you'll end up with the object's weight in pounds. Just be careful converting pounds to Newtons, because you may not like your sudden weight 'gain'!

When this lesson is complete, you should be able to:

- Describe the difference between weight and mass
- Explain how you can calculate for mass and weight and accomplish conversions between them using Newtons and kilograms

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AP Physics 1: Exam Prep12 chapters | 136 lessons

- Newton's First Law of Motion: Examples of the Effect of Force on Motion 8:25
- Distinguishing Between Inertia and Mass 6:45
- Mass and Weight: Differences and Calculations 5:44
- Force: Definition and Types 7:02
- Forces: Balanced and Unbalanced 5:50
- Free-Body Diagrams 4:34
- Solving Mathematical Representations of Free-Body Diagrams
- How to Use Free-Body Diagrams to Solve Motion Problems
- Net Force: Definition and Calculations 6:16
- Force & Motion: Physics Lab
- Newton's Second Law of Motion: The Relationship Between Force and Acceleration 8:04
- Determining the Acceleration of an Object 8:35
- Action and Reaction Forces: Law & Examples 8:15
- Determining the Individual Forces Acting Upon an Object 5:41
- Implications of Mechanics on Objects 6:53
- Air Resistance and Free Fall 8:27
- Newton's Third Law of Motion: Examples of the Relationship Between Two Forces 6:05
- Newton's Laws and Weight, Mass & Gravity 8:14
- Identifying Action and Reaction Force Pairs 8:12
- The Normal Force: Definition and Examples 3:56
- Friction: Definition and Types 4:15
- Inclined Planes in Physics: Definition, Facts, and Examples 6:56
- Go to AP Physics 1: Newton's Laws

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