Problem 39 - College Physics

Question

If the radius of a circle of area \(A\) and circumference \(C\) is doubled, find the new area and circumference of the circle in terms of \(A\) and C. (Consult Chapter 0 if necessary.)

Step-by-Step Solution

Answer

The new area is \( 4A \) and the new circumference is \( 2C \).

Step 1: Recall Key Circle Formulas

The area of a circle is given by the formula \( A = \pi r^2 \), and the circumference is \( C = 2\pi r \) where \( r \) is the radius of the circle.

Step 2: Express Radius in Terms of Area and Circumference

From the area formula \( A = \pi r^2 \), solve for \( r \) to get \( r = \sqrt{\frac{A}{\pi}} \). Similarly, from the circumference formula \( C = 2\pi r \), solve for \( r \) to get \( r = \frac{C}{2\pi} \). These expressions will help to substitute back once the radius is doubled.

Step 3: Double the Radius

If the radius \( r \) is doubled, the new radius \( r' \) is \( 2r \).

Step 4: Calculate the New Area

Substitute \( r' = 2r \) into the area formula: The new area is \( A' = \pi (2r)^2 = 4\pi r^2 \). Since \( A = \pi r^2 \), the new area is \( A' = 4A \).

Step 5: Calculate the New Circumference

Substitute \( r' = 2r \) into the circumference formula: The new circumference is \( C' = 2\pi (2r) = 4\pi r \). Since \( C = 2\pi r \), the new circumference is \( C' = 2C \).

Step 6: Summarize the Results

The new area of the circle is \( 4A \) and the new circumference is \( 2C \).

Key Concepts

Area of a Circle

The area of a circle is a measure of the space enclosed within its boundary. Imagine the circle filled with tiny squares, the sum of those squares gives you the area. For any circle, the area can be calculated using the formula: \[A = \pi r^2\]where \( A \) represents the area, \( \pi \) (pi) is approximately 3.14159, and \( r \) is the radius of the circle. When the radius is doubled, in this context, the area of the circle changes significantly. Since the area is proportional to the square of the radius, doubling the radius increases the area by a factor of four. This means if the original area was \( A \), the new area will be \( 4A \). This scaling property is crucial in geometry and physics, especially when calculating areas in different shapes or changing dimensions.
Understanding this relationship helps us recognize how areas grow in relation to changes in size.

Circumference of a Circle

The circumference of a circle is the total length around the circle, akin to a perimeter of a polygon. For a circle, the circumference provides insights into the boundary's length, wrapped around the circle once. The formula for calculating the circumference is: \[C = 2\pi r\]where \( C \) stands for the circumference and \( r \) is the radius.
When the radius of a circle is doubled, the circumference is also directly affected. It is directly proportional to the radius, meaning if the radius doubles, so does the circumference. So, if the initial circumference is \( C \), when the radius becomes twice its size, the circumference becomes \( 2C \). This linear relation between the radius and circumference is foundational in geometry, showing how linear dimensions influence boundary lengths.
It also is important in practical applications, such as determining distances traveled by wheels or circular tracks.

Radius of a Circle

The radius of a circle is the distance from the center to any point on its edge. It represents half of the diameter, which is the longest distance across the circle. The radius is a fundamental part of the key circle formulas, as it helps define both the area and the circumference:

• From the formula \( A = \pi r^2 \), we understand the significant impact of the radius on the area.


• In the circumference formula, \( C = 2\pi r \), the radius directly scales the circle's boundary length.

The beauty of geometry lies in how these measurements interrelate. In our example, when the radius is initially solved through the formulas \( r = \sqrt{\frac{A}{\pi}} \) or \( r = \frac{C}{2\pi} \), it reveals its overarching influence.
When doubled, the new radius allows us to explore the expanded properties of larger circles, evidencing how a simple linear change has exponential impacts on a circle's area, and linear yet proportional effects on its circumference.

Geometry in Physics

Geometry plays a significant role in physics, especially when describing the physical world. Circles and circular motions are common in physical systems and engineering, manifested in gears, orbits, and rotations. When examining systems through geometry, understanding the relationship between radius, area, and circumference helps us to understand different physical phenomena. For instance,

• The area can represent sections of wheels impacting friction or contact area.
• Circumference might relate to the path in circular motion or revolution cycles.

In problems like the one we tackled, doubling the radius can symbolize scaling up systems to handle more load, speed, or volume. Understanding these geometric principles aids in conceptualizing and calculating physical possibilities efficiently. This connection between mathematics and physics highlights how foundational concepts such as circle formulas are, not just in pure mathematics but across numerous physical applications.