GCSE Physics - Liquid Pressure & Upthrust

Summary

This video explains how to calculate the pressure in a liquid and its variation with depth. It covers the relationship between pressure, liquid density, and gravitational field strength, illustrated through a practical example. The video also explores the concept of upthrust, explaining why some objects float while others sink. By comparing the forces of upthrust and weight, the video demonstrates that an object will float if the upthrust is greater than its weight, and sink otherwise. It concludes by explaining how the density of an object compared to the liquid determines whether it will float or sink.

Takeaways

• 😀 Pressure in a liquid increases with depth due to the weight of the liquid above the object.
• 😀 The pressure experienced by an object in a liquid depends on three factors: depth, density of the liquid, and gravitational field strength.
• 😀 The formula for calculating pressure in a liquid is: Pressure = Depth × Density × Gravitational field strength.
• 😀 A person diving from 20 meters to 90 meters experiences a change in pressure of 686,000 pascals.
• 😀 The change in pressure between two depths can be calculated directly using the difference in depth.
• 😀 The density of the liquid affects how much pressure is felt by an object submerged in it.
• 😀 Objects in liquids experience pressure from surrounding molecules, which is generally equal in all directions, but deeper areas experience more pressure from the water above.
• 😀 Up thrust is the upward force exerted by a liquid on an object submerged in it, which is a result of unequal pressure forces from the surrounding liquid.
• 😀 The up thrust force is larger at the bottom of an object due to greater depth, causing a net upward force on the object.
• 😀 An object will float if the up thrust is greater than its weight; if the weight is larger, the object will sink.
• 😀 To determine whether an object floats or sinks, compare its density to the density of the liquid it's in: denser objects sink, less dense objects float.

Q & A

What is the primary factor affecting pressure in a liquid?

The primary factor affecting pressure in a liquid is the depth of the liquid above the object. As the depth increases, the pressure due to the weight of the liquid also increases.

How does the density of a liquid influence pressure?

The denser the liquid, the larger its mass per unit of volume, which results in greater pressure. Denser liquids exert more force on objects submerged in them.

What role does gravitational field strength play in calculating liquid pressure?

Gravitational field strength determines the weight of the liquid. A stronger gravitational field increases the pressure exerted by the liquid on an object.

What is the formula for calculating pressure in a liquid?

The formula for calculating pressure in a liquid is: Pressure (P) = Depth (h) × Density (ρ) × Gravitational field strength (g), where pressure is measured in pascals.

In the given example, how do you calculate the change in pressure when a person moves from 20 meters to 90 meters in depth?

To calculate the change in pressure, first calculate the pressure at both depths using the formula. For 20 meters, the pressure is 196,000 pascals, and for 90 meters, the pressure is 882,000 pascals. The difference in pressure is 882,000 - 196,000 = 686,000 pascals.

Can the pressure difference also be calculated by considering the change in depth alone?

Yes, the pressure difference can also be calculated by considering the change in depth. In this case, the change in depth is 70 meters, and the pressure difference would be 70 meters × 1,000 kg/m³ × 9.8 m/s² = 686,000 pascals.

What is up thrust, and how does it affect whether an object sinks or floats?

Up thrust is the upward force exerted by the liquid on an object submerged in it. It occurs because the bottom of the object experiences a larger force from the surrounding liquid molecules than the top. If the up thrust is greater than the object's weight, the object floats. If the weight is greater, the object sinks.

Why does the object experience more upward force at the bottom compared to the top?

The object experiences more upward force at the bottom because it is submerged deeper in the liquid. The pressure increases with depth, so the force from the liquid molecules on the bottom of the object is stronger than the force on the top.

How does the density of an object determine whether it floats or sinks?

If an object is denser than the liquid it's in, it will sink, because its weight will exceed the up thrust. If the object is less dense than the liquid, the up thrust will be greater, and the object will float.

Can we determine whether an object will float or sink without calculating up thrust and weight directly?

Yes, we can determine whether an object will float or sink by comparing its density to the density of the liquid. If the object's density is higher than the liquid's density, it sinks. If it is lower, it floats.