Density

Mass and Weight

• Mass is the amount of matter an object has.• The weight of an object on earth depends on

the force of gravity between the object and earth.

• Mass and weight are not the same thing!• A rock with a weight of 30 kg will not have a

weight of 30 kg in space, but it will have the same mass.

Volume

• Volume is the amount of space an object occupies.

• When dealing with scientific principles, volume should always be measured in metric units.

• Liters, mL, dL, etc.

Densitya property (or characteristic) of matter

• Which of the following boxes is the most dense? Assume each box is the same volume.

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• Density is the amount of mass in a given volume.

• mass - unit of measure is grams (g) volume - unit of measure is cm3.

• The unit of measure for density is g/cm3.

Calculating Density

• Formula for Density:

Density = mass

volume• All must have correct units (g,cm3,g/cm3)• Important: All units must be in grams or cubic

centimeters. • 1 cm3= 1mL• You must remember this fact!!!!!!!!!

The Magic Triangle• The Magic Triangle is a proven tool to help figure out

density.• Rules for the triangle are as follows:1. Decide what the problem is asking you to find

2. Cover up what you are trying to find

3. Do as the triangle commands

M

D V

Divided by Divided by

Multiply

Know this:

Water has a density of 1.0 g/cm3

•Floaters have a density less than 1 g/cm3

•Sinkers have a density more than 1 g/cm3

•Subsurface floaters have density equal to 1 g/cm3

Density’s Purpose

• If an object is less dense than the fluid in which it is placed, it will "float" on the fluid.

• If it is more dense than the fluid, it will "sink.”

• An object that is more compact tends to be more dense than an object that is more spread out.

Density Practice--Problem 1A three-armed monkey carrying a voo-doo doll of a local

science teacher wanted to see if the doll would float or sink in his cousin’s bathtub. The doll had a mass of 2.5 g and a volume of 3.9 cm3. What is the density of the doll and would it float in the bathtub?

Step 1: Write the formula

Step 2: Plug in the numbers we are given

Step 3: Do the math

Use the 3-step method!

•Answer: D= M/V•2.5/3.9= .641 g/cm3

Problem 2• A blind peanut walks down the street wondering if the

tomato soup he ate for lunch was real soup or just watered down ketchup. He also wanted to know what the density of the crackers were that he put in his soup. The cracker had a mass of .765 g and a volume of 1.34 mL. What was the density of the cracker?

•D=M/V•.765/1.34= .5250 g/mL



Step 3: Do the math


Problem 3A pickle was swimming in a jar of mayonnaise

when he realized that he forgot to shut his car door. He jumped out of the mayonnaise and ran outside. Just as he was about to dive back into the pool of mayo he wondered what his volume was. He knew he had a mass of 22 g and a density of 13.35 g/cm3. Help the pickle out and tell him his volume.



Step 3: Do the math


V= M/D

22/13.35=1.648 cm3

Problem 4• Carlos, the cotton candy vendor, decided to try and

figure out the mass of his glass eye. He knew that the density of his eye was 1.35 g/cm3. He also knew if he placed the eye in a cylinder of 50 mL of water the water rose up to 61.9 mL. What was the mass of Carlos’ glass eye?

•Mass= D x V•1.35 x 11.9=16.065 g



Step 3: Do the math


Problem 5• A 8.23 kg plucked turkey runs down a country road

screaming the phrase “Save the Turkey! Save The World!” The turkey keeps looking over his shoulder and screaming. He comes to a small pond and jumps head first into the murky water. The common plucked turkey is known to have a volume of 7.45 L. What is the density of the turkey? Will he sink or float in the water?



Step 3: Do the math


•D=M/V•8230/7450=1.104 g/mL•He will sink because his density is greater than 1.0

Problem 6

• Which of the following objects is the most dense? Assume all volumes are 2.3 cm3

3300 mg 2.95 g .0042 kg

A B C

• D=M/V

• A: 3.3/2.3=1.43 g/mL

• B: 2.95/2.3= 1.283 g/mL

• C: 4.2/2.3= 1.826 g/mL

Density

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