Physics 33.5 Buoyancy Force: What is Buoyancy Force? (1 of 9) Fraction Submerged

Summary

This video lecture focuses on buoyancy force and its role in understanding why objects float in liquids. The speaker explains how to calculate the fraction of an object submerged beneath the surface, using the densities of the object and the liquid. By balancing the forces of buoyancy and gravity, the lecture presents a formula for determining the submerged portion, highlighting that the ratio of densities influences how much of the object remains above or below the surface. This principle is essential for objects like ice or wood floating in water, where part of the object stays above the surface.

Takeaways

• 😀 The buoyant force is the force exerted by a liquid on an object submerged in it, and it is equal to the weight of the displaced liquid.
• 😀 An object will float if its density is less than that of the liquid it's in, with a portion of the object submerged below the surface and a portion above it.
• 😀 The buoyant force is expressed as: BF = Density of liquid × Volume submerged × g, where g is the acceleration due to gravity.
• 😀 When an object is floating, the buoyant force is equal to the weight of the object, which can be written as: Weight of object = Density of object × Volume of object × g.
• 😀 If the object is floating, the equation for the buoyant force and the weight of the object can be set equal to each other, allowing for calculation of the submerged volume.
• 😀 The density of the liquid and the object play a crucial role in determining how much of the object is submerged in the liquid.
• 😀 The volume of the submerged liquid is related to the total volume of the object, and this relationship helps in calculating the submerged portion.
• 😀 The fraction of the object submerged (X) is given by the formula: X = (Density of object / Density of liquid) × H, where H is the height or thickness of the object.
• 😀 The ratio of the submerged portion to the total height of the object is equal to the ratio of the object's density to the liquid's density.
• 😀 If the density of the object is less than the liquid's density, the fraction submerged will be smaller than 1, and only part of the object will be underwater.

Q & A

What is buoyancy force?

Buoyancy force is the upward force exerted by a liquid on an object submerged in it. It is equal to the weight of the displaced liquid.

When does an object float on a liquid?

An object floats on a liquid when its density is less than the density of the liquid. In this case, a portion of the object will be submerged while the rest will be above the surface.

How do we calculate the portion of an object submerged in a liquid?

The portion of an object submerged in a liquid can be calculated using the ratio of the density of the object to the density of the liquid, multiplied by the height or thickness of the object.

What is the relationship between the buoyancy force and the weight of the object when the object is floating?

When an object is floating, the buoyancy force is equal to the weight of the object. This is because the upward force exerted by the liquid matches the downward gravitational force on the object.

How is the weight of the displaced liquid related to the buoyancy force?

The buoyancy force is defined as the weight of the displaced liquid. This means the amount of liquid displaced by an object, multiplied by its density and gravity, determines the buoyant force on the object.

Why is it important to use correct subscripts when setting up equations for buoyancy?

Using correct subscripts helps differentiate between the properties of the object and the properties of the liquid, ensuring clarity and preventing confusion when solving equations related to buoyancy.

What happens if the density of the object is greater than the density of the liquid?

If the density of the object is greater than the density of the liquid, the object will sink to the bottom because the buoyancy force will be insufficient to support the weight of the object.

How do we express the portion of an object submerged relative to its total height?

The portion of the object submerged can be expressed as the ratio of the density of the object to the density of the liquid, and this ratio is multiplied by the total height or thickness of the object.

Why is the fraction X (portion submerged) always less than 1 when an object is floating?

The fraction X is always less than 1 because the density of the object is less than the density of the liquid. Therefore, the object only displaces a fraction of its volume, which results in part of it floating above the surface.

What happens to the buoyancy force and weight of the object when an object is sinking?

When an object is sinking, the buoyancy force is less than the weight of the object. This imbalance causes the object to continue moving downward until it reaches the bottom.