Unit 9 – Forces in Action Lesson 1 - Density Starter Activity • Which is heavier? Circle the correct answer. 100 kg of rocks 100 kg of feathers • Which is heavier? Circle the correct answer. A tree trunk A metal coin Density! The density of a substance is a measure of the amount of matter that is present in a certain volume of it. š·ššš šš”š¦ = ššš š šššš¢šš šššš” ā¶ g/cm3 or šš/š3 3 š/šš To convert to multiply by 1000. 3 šš/š you have to Example: 0.920 g/cm3 = 0.920 x 1000 = 920 kg/m3 Try these: 1. 32.5 g/cm3 = 2. 0.895 g/cm3 = 3. 0.0376 g/cm3 = 3 kš/š To convert divide by 1000. to 3 š/šš you have to Example: 935 kg/m3 = 935 ÷ 1000 = 0.935 g/cm3 Try these: 1. 325 kg/m3 = 2. 9850 kg/m3 = 3. 7.12 kg/m3 = Density helps you decide which material has more mass in a certain volume Can you tell: 1. Is steel or copper less dense? 2. Which of the materials in the table will float in water? FUN FACT: Ice is less dense than water! This means ice is going to float in water. Can you think why is ice less dense than water? Measuring the density of a regular block • Remember: šššš šš”š¦ = ššš š š£ššš¢šš Step 1: Find out the mass of the shape by placing it on a weight machine/top-pan balance Step 2: Find out the volume of the block by calculation (cube/cuboid volume) Step 3: Density = mass ÷ volume Question 1: • Remember: šššš šš”š¦ = ššš š š£ššš¢šš The length of each side of this cube is 3 m. Its mass is 54 kg. What is its density? Question 2: • Remember: šššš šš”š¦ = ššš š š£ššš¢šš The dimensions of the cuboid are shown in the diagram. If the mass of the cuboid is 540 g, what is its density? 12 cm 6 cm 3 cm Question 3: • A block of aluminium has a mass of 54 kg. If the density of the aluminium is 2700 kg/m3 , what is its volume? Question 4: • A block of steel has density 7900 kg/m3 . A block of aluminium has density 2700 kg/m3 . If you take a cube of 1 m3 of steel and a cube of 1 m3 of aluminium, which will weigh more on a weighing machine? How do you measure the density of an irregular shape? What is the density of this tin can? Step 1: Find out the mass of the shape by placing it on a weight machine/top-pan balance Step 2: Find out the volume of the shape by placing it in a measuring cylinder (next slide) Step 3: Density = mass ÷ volume Initially, the volume of water in the measuring cylinder is 150 cm3. Place the can inside the measuring cylinder. ļ ļ Now the reading is showing 250 cm3. So, volume of the can is 250 – 150 = 100 cm3. Question 5: • Based on the mass and the volume of the tin can in the previous slides, calculate its density. The value of density can help us decide if a material is pure or impure. • Ashmit buys a gold necklace for his mother, but he wants to make sure its made of pure gold. He measures the mass of the necklace as 500 g. He places the gold necklace in a measuring cylinder and measures the initial and final volume of water in the cylinder. Initial volume = 120 cm3. Final volume = 140 cm3. • The density of pure gold is 19.32 g/cm3. • Calculate the density of the gold necklace Ashmit bought and determine if it is made of pure gold or not. Answer to Ashmit’s gold necklace problem:
