Question Video: Understanding Why the Water Pressure on an Object Is Greatest at Its Base | Nagwa Question Video: Understanding Why the Water Pressure on an Object Is Greatest at Its Base | Nagwa

# Question Video: Understanding Why the Water Pressure on an Object Is Greatest at Its Base Physics • Second Year of Secondary School

A solid object falls through water. The water exerts a pressure on the top, base, and sides of the object. The water pressure on the object is greatest on its base. Which of the following most correctly explains why? [A] The object is falling and so is pushing water below it downward. This water exerts a reaction force on the base of the object. If the object was suspended in the water then the pressure on its base would be equal to the pressure on its other sides. [B] The pressure due to the weight of the water acts equally in all directions and increases with depth.

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### Video Transcript

A solid object falls through water. The water exerts a pressure on the top, base, and sides of the object. The water pressure on the object is greatest on its base. Which of the following most correctly explains why? (A) The object is falling and so is pushing water below it downward. This water exerts a reaction force on the base of the object. If the object was suspended in the water, then the pressure on its base would be equal to the pressure on its other sides. (B) The pressure due to the weight of the water acts equally in all directions and increases with depth.

In this question, we are asked to consider a solid object that is falling through water. We are told that the pressure exerted on the base of the object is greater than the pressure on the top and side. Given this, we are asked to figure out why this is and choose the option that most correctly explains why.

Let’s first recall Pascal’s principle and figure out how we can apply it to this situation. But Pascal’s principle involves pressure, so let’s first consider the equation for pressure.

The pressure 𝑃 over an area 𝐴 is equal to the force 𝐹 divided by that area. Pascal’s principle states that at any point in a fluid, the pressure exerted by the fluid at that point is equal in all directions. Also remember that two points in a fluid that are at the same depth and have no forces acting on them except the weight of the fluid are at an equal pressure. The pressure they are at increases with depth; more fluid above the points means more weight exerting pressure on them.

Now, let’s think about the solid object from our problem with a top, base, and sides. If we consider the faces of the object to be a series of points, we can use Pascal’s principle to help solve this problem.

The points that make up the top of the object, the side at the shallowest depth, are all at the same height so they all feel the same force. The points that make up the sides of the object are at varying heights. And the pressure increases as the points get closer to the bottom of the object. So, if we were to find the pressure at each height and add them up, we would see that the sides would experience more pressure than the top because most of the sides are at a greater depth than the top.

Now the points that make up the bottom of the object are all of the same depth, which means they will all experience the same pressure. The bottom is also at the greatest depth, which means this side would feel the greatest force.

Looking at option (B), it states that the pressure due to the weight of the water acts equally in all directions and increases with depth. This matches what we found using Pascal’s principle and what we know about the pressure from fluids. So, option (B), the pressure due to the weight of the water acts equally in all directions and increases with depth, is the correct answer.

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