This cargo ship displaces an amount of water such that the weight of the displaced water is exactly equal to the weight of the ship and its cargo. The displacement of water is what produces the buoyancy to float this ship. When this photo was taken, the ship was empty so it did not sink very deep in the water to displace the necessary weight. When this ship is fully loaded with cargo, the water line will be where the black paint meets the red paint.
Archimedes’ Principle and Buoyancy
If an object is submerged in a liquid, the object will displace a volume of the liquid equal to the volume of the submerged object. Legend has it that this observation was made by Archimedes when he sat in a bath tub that was filled to the top of the tub. The volume of water that overflowed was equal to the volume of the bather. The forces exerted by the fluid on the sides of the submerged object are balanced but the forces exerted by the fluid on the top and bottom of the object are not equal. The force exerted on the bottom of the object is greater than the force exerted by the liquid on the top. Therefore, the liquid exerts a net upward force on the submerged or floating object. The force is called buoyancy and its magnitude is equal to the weight of the displaced water. Archimedes Principle states that the buoyant force is equal to the weight of the displaced liquid.
Example Problem: The density of steel is 9000. kg/m ^{ 3 } and the density of water is 1000. kg/m ^{ 3 } . If a cube of steel that is 0.100 m on each side is placed in a tank of water and weighed while under water, what is the apparent weight of the cube?
Solution: The volume of the cube is 0.00100 m ^{ 3 } .
The mass of the cube is 9.00 kg.
The weight of the cube when not submerged in water = (9.00 kg)(9.80 m/s ^{ 2 } ) = 88.2 N
The mass of water displaced by the cube = 1.00 kg
The weight of the water displaced by the cube = 9.80 N
The buoyant force on the steel cube = 9.80 N
Apparent weight of cube under water = 88.2 N - 9.80 N = 78.4 N
Example Problem: A hollow metal cube 1.00 m on each side has a mass of 600. kg. How deep with this cube sink when placed in a vat of water?
Solution: Since the weight of the cube is 5880 N, it will need to displace 5880 N of water in order to float.
Volume of submerged portion of cube
Mass of water displaced
Weight of water displaced
The cube will sink to the level that 0.60 meters are under water and 0.40 meters are above water.
Summary
- If an object is submerged in a liquid, the object will displace a volume of the liquid equal to the volume of the submerged object.
- The forces exerted by the fluid on the sides of the submerged object are balanced but the forces exerted by the fluid on the top and bottom of the object are not equal.
- The liquid exerts a net upward force on the submerged or floating object.
- This force is called buoyancy and its magnitude is equal to the weight of the displaced water.
- Archimedes Principle states that the buoyant force is equal to the weight of the displaced liquid.
Practice
The following video is on buoyancy. Use this resource to answer the two questions that follow.
http://www.youtube.com/watch?v=-U1YVZMifMM
- Name two things Bill Nye puts in a tank of water to demonstrate the displacement of water.
- What was Archimedes doing when he discovered an object displaces its own volume when submerged in water?
Additional Practice Questions:
- If a ship has a mass of 200,000,000 kg, how much water would it need to displace in order to float?
- A 40.0 kg cylinder that is 1.2 m tall and has a 0.20 m diameter falls off the pier and gets completely submerged when it hits the water. What is the buoyant force on the cylinder when it is under water? Will it sink or float? The formula for finding the volume of a cylinder is .
- The formula for the volume of a sphere is . A 1.7 × 10 ^{ -6 } kg air bubble has a diameter of 1.00 cm and is submerged in water. What is the buoyant force on the bubble?
Review
- A cylinder with a radius of 11 cm and a height of 3.4 cm has a mass of 10.0 kg. (a) What is the weight of this cylinder? (b) What is the weight of this cylinder when it is submerged in water?
- A wooden raft is 2.00 m wide, 3.00 m long, and 0.200 m deep. The raft and its occupants have a mass of 700. kg. How deep will the raft sink below the water when floating?
- For the raft in problem #2, how many 50. kg people can be added to the raft before it sinks completely under water?
- The density of gold is 19,320 kg/m ^{ 3 } and the density of mercury is 13,500 kg/m ^{ 3 } . If a cube of gold that is 0.100 m on each side is placed in a tank of mercury and weighed while under the surface, what is the apparent weight of the cube?