# Baseball and string activity to find the surface area of a sphere

1 hour

### Prerequisites/Instructions, prior preparations, if any

Circles should have been covered.

### Materials/ Resources needed

Non digital: Baseballs, string, marker pens, scale, scissors and chart paper

Digital: Computer and projector

This activity has been taken from the website ehow.com

### Process (How to do the activity)

• Process:
1. Divide the class into groups and give each group a baseball and string.
2. Ask the students to lay flat the string around the widest part of the baseball and mark.
3. The meaured length would be the circumference.
4. The measurement will be divided by pi to find the diameter (Remember: circumference/diameter = pi).
5. The diameter divided by 2 would be the radius. Note down the radius.
6. Now take a long piece of string and carefully wind it around the entire surface of the ball taking care not to overlap.Cut and discard the extra string.
7. Next unwind the string,measure it using a measuring tape. This measure would be teh surface area of the sphere.

To understand the formula $\displaystyle 4πr^2$ for deriving surface area of a sphere :

1. Consider the length of string obtained from step 7 , divide it into exactly 4 equal parts.
2. Make a circle with each part. Measure the radii.( Hint: Can draw circle of radius from step 5 and lay the string around it)
3. Observe that 4 circles would be equal and the radii in all 4 cases would be the same as the radius of sphere (derived initially by measuring circumference).
4. The length of string covering the entire surface area of the sphere has been formed into 4 circles of equal radius.
5. The area of 1 circle is $\displaystyle πr^2$
6. The area of 4 such circles would be 4 times the area of such circle.
7. Hence teh area of a sphere would be $\displaystyle 4πr^2$
• Developmental Questions
• Evaluation
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