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### Course: Class 11 Physics (India)>Unit 14

Lesson 2: Buoyant force and Archimedes' principle

# Buoyant force example problems

A couple of problems involving Archimedes' principle and buoyant forces. Created by Sal Khan.

## Want to join the conversation?

• Could one walk on water by wearing shoes on their feet that are far less dense than water? Or would the entire body's volume and density contribute in determining whether the person with very low-density shoes on their feet remains afloat?
• An object will float if its average density is less than the average density of the fluid it is immersed in. But you might ask, how can a cruise ship, which is constructed of metals weighing thousands of kilograms, float on water if all those dense metals have a higher average density than water?

Well, air has a lower average density than water and cruise ship's have air submerged beneath the water (ie, between the ship's lower decks) which lowers the ship's average density to be less than that of water.

So if you built shoes that lowered your average density to be less than water than yes, you would be able to walk on water.
• What will be the buoyant force if the gravity is zero?
• If there is no gravity, there will be no buoyancy. Buoyancy results from pressure differentials caused by gravity.
• He states a cubic meter = 27 "square" feet.
A cubic meter = 35.3147 "cubic" feet.
A square meter = 10.7639 "square" feet.
A cubic yard = 27 "cubic" feet.
Am I missing something?
• When he said "square" feet at , he actually meant to say "cubic" feet
• Can someone explain, why water doesn't rise even the ice melts?
I am facing a doubt ,if ice floating on water melts more water will be added to existing water so the water level should rise.Why isn't so ?
• see when ice floats on water 11th part out of its remaining 12 parts remain in the water and only one part floats above the water level, hence when the ice melts its fills the gap created by it during its ice form, thus the water level does not rise .But when in the polar caps the ice melts as the ice is collected above the land mass hence it does not create any gap in the surface of the water hence when it melt it forms extra water , thus increasing the water level of the waterbody it falls on.
• If W = m * g and the mass of the object and the force of gravity don't change when the object is submerged, then how can the weight of the object change when it's put into water?
• The gravitational weight doesn't change, but the apparent weight (F_gravity - F_buoyancy) will change since the buoyant force will counteract the gravitational weight.
• the cube in the begining..will it go downward?? coz force down is 10 N while up is 8N..
• Yes, as the (net) weight force acting on the cube is 2N (you can see from 10N - 8N, but Sal also mentions that it's 2N in the video)
• If you're standing on the bottom of a swimming pool how is a buoyant force exerted on you from below? Does the normal force become the buoyant force or is no buoyant force exerted on you from directly below?

Obviously there will still be buoyant force on your arms, etc. but does that combined with the normal force constitute the total buoyant force?
• If you form a solid seal with the floor, like a suction cup, there will be less upwards pressure. Otherwise, as long as water is capable of getting under your feet, it will push up with the same pressure as usual. The normal force is separate from the bouyant force.
• If two objects have same mass but different volume, do they have same buoyant force?
• No, the buoyant force is the weight of the displaced fluid.

Consider 1 kg block of solid of iron and 1 kg block of solid styrofoam, the iron will sink but the styrofoam will float.