Science: Density and Buoyancy Concepts
Exploring why objects sink, float, and balance in fluids
Science: Density and Buoyancy Concepts
Exploring why objects sink, float, and balance in fluids
Science - Grade 6-8
- 1
A block has a mass of 120 grams and a volume of 60 cubic centimeters. What is its density?
Use the formula density = mass ÷ volume.
The density is 2 grams per cubic centimeter because density equals mass divided by volume, so 120 g divided by 60 cm³ equals 2 g/cm³. - 2
Water has a density of about 1 g/cm³. If a solid object has a density of 0.7 g/cm³, will it sink or float in water? Explain.
The object will float in water because its density is less than the density of water. - 3
A metal cube has a mass of 270 grams and a volume of 30 cubic centimeters. A plastic cube has a mass of 40 grams and a volume of 50 cubic centimeters. Which cube is denser?
Calculate the density of each cube, then compare the two values.
The metal cube is denser. Its density is 9 g/cm³, while the plastic cube's density is 0.8 g/cm³. - 4
Explain the difference between mass and volume in your own words.
Mass is the amount of matter in an object, while volume is the amount of space the object takes up. - 5
A student places three objects in water. Object A floats high on the surface, Object B floats with most of it underwater, and Object C sinks to the bottom. Rank the objects from lowest density to highest density.
Objects that float higher in a liquid usually have lower density than objects that float lower.
The order from lowest density to highest density is Object A, Object B, and Object C. Object A floats highest, so it is least dense, and Object C sinks, so it is most dense. - 6
What is buoyant force, and what direction does it act on an object in a fluid?
Buoyant force is the upward force a fluid applies to an object. It acts upward, opposite the pull of gravity. - 7
A rock weighs 15 newtons in air. When it is completely submerged in water, it appears to weigh 9 newtons. What is the buoyant force on the rock?
Subtract the apparent weight in water from the weight in air.
The buoyant force on the rock is 6 newtons because 15 N minus 9 N equals 6 N. - 8
A large ship made of steel can float, even though a small steel nail sinks. Explain how this is possible.
A steel ship can float because its hollow shape makes its overall density, including the air inside, less than the density of water. A solid steel nail has a density greater than water, so it sinks. - 9
An object is placed in a liquid and stays suspended in the middle without rising or sinking. What can you conclude about the object's density compared with the liquid's density?
Think about what happens when the upward buoyant force balances the downward weight.
The object's density is about equal to the liquid's density because it stays suspended instead of floating upward or sinking downward. - 10
A sealed bottle floats in fresh water but sinks in salt water, according to a student's claim. Is this claim likely correct? Explain why or why not.
The claim is not likely correct because salt water is denser than fresh water. An object that floats in fresh water should usually float even better in salt water. - 11
A liquid has a density of 1.2 g/cm³. An object with a density of 1.0 g/cm³ is placed in the liquid. Will the object sink or float? Explain.
Compare the object's density to the liquid's density.
The object will float because its density, 1.0 g/cm³, is less than the liquid's density, 1.2 g/cm³. - 12
A graduated cylinder contains 50 mL of water. A small stone is placed in the cylinder, and the water level rises to 68 mL. The stone has a mass of 45 grams. What is the stone's density?
First find the volume of the stone using water displacement.
The stone's volume is 18 mL because 68 mL minus 50 mL equals 18 mL. Its density is 2.5 g/mL because 45 g divided by 18 mL equals 2.5 g/mL.