Problem 37 - Physics for Scientists and Engineers with Modern Physics

Question

(II) Is it possible to whirl a bucket of water fast enough in a vertical circle so that the water won't fall out? If so, what is the minimum speed? Define all quantities needed.

Step-by-Step Solution

Answer

Yes, minimum speed is \( \sqrt{rg} \).

Step 1: Understanding the Problem

We need to find out if a bucket of water can be spun in such a way that the water won't fall out when spun vertically. This involves understanding the forces acting on the water in the bucket during circular motion.

Step 2: Identifying the Forces

When the bucket is at the top of the circle, the water inside should experience a centripetal force that equals or exceeds the gravitational force pulling it downward. This ensures the water stays in the bucket.

Step 3: Introducing Relevant Quantities

Let \( v \) be the speed of the bucket, \( r \) be the radius of the circular path, and \( g \) be the acceleration due to gravity (approximately \( 9.8 \ m/s^2 \)).

Key Concepts

Circular Motion

Circular motion occurs when an object moves along a circular path. The object continuously changes direction, which means there is acceleration involved, even if the object's speed is constant. This type of motion is common in many everyday situations, such as a car turning around a curve or the Earth orbiting around the sun.
In circular motion, two essential components are necessary: the plane of the motion and the axis of rotation. In our case, as the bucket of water moves vertically, we must consider both gravity and centripetal force to ensure that the water remains inside the bucket.

• The **velocity vector** is always tangent to the circle, meaning it points in the direction the object is moving at any moment.
• The **centripetal force** is directed towards the center of the circle, keeping the object in its circular path. Without this inward force, the object would move in a straight line.

Remember, if an object's speed changes, it can result in changes to the radius or frequency of its circular path, but in this scenario, we're considering a steady speed to maintain circular motion.

Gravitational Force

Gravitational force is the force of attraction between two objects with mass. On Earth, gravity is a constant force acting downwards toward the planet's center. It has an acceleration of approximately 9.8 m/s², often denoted as **g**.
For the water in the bucket, gravitational force plays a crucial role in determining whether it stays or spills out as the bucket moves. At the top of the circular path, gravity acts downward on the water, trying to pull it out of the bucket. For the water to remain in the bucket, the centripetal force must counteract this pull.

• When the bucket is at the circle's top, the gravitational force and centripetal force work together towards the center of the circle.
• If the combined forces are enough to keep the water inside, the water won't spill out even as gravity tries to pull it down.
  This makes understanding the relationship between gravity and motion essential when examining if the water will stay inside the bucket.

In summary, gravitational force is a major player in this situation, influencing whether the centripetal force is strong enough to keep the water from falling out.

Minimum Speed Calculation

To keep the water from spilling out at the top of the circle, the speed of the bucket must be high enough to maintain the necessary centripetal force.
The centripetal force needed is provided entirely by gravitational force when at the circle's peak. Thus, the key to finding the minimum speed, **v_min**, is to set the gravitational force equal to the centripetal force.

• The formula for centripetal force is given by \[ F_c = \frac{mv^2}{r} \]
• The gravitational force formula is \[ F_g = mg \]

At the bucket's highest point in the circle, we need \[ F_c = F_g \], leading to \[ \frac{mv^2}{r} = mg \].
This simplifies to \[ v^2 = rg \] or, solving for speed, \[ v = \sqrt{rg} \].
Here, **r** represents the radius of the circle and **g** is the acceleration due to gravity. By plugging in these values, you can calculate the minimum speed required to keep the bucket's water in place.
Through this understanding, you see that maintaining enough speed ensures the centripetal force can overcome gravity’s pull, keeping the water safely inside the bucket even at challenging vertical positions.