Apple Ripper

Physics:	Torque, Force
Grade Range:	Middle School, High School
Format:	Hands-on, Stage

This Demonstration is a fun way to introduce torque to students. It is easy for students to replicate this at home, so encourage them to try and show it to their parents! The explanation is easy to adjust between Middle School and High School students, but hard to explain to students in Elementary School.

Materials

• Apples
  • Tall, softer apples (Such as Red Delicious)
  • Short, firmer apples (Such as Granny Smith)

Safety Precautions

Please read the Physical Demonstration section of the Demonstration Safety page before performing this demonstration.

Demonstration

1. Introduce yourself as one of the world's strongest people. Tell the audience that you can prove it by ripping an apple in half before their very eyes!

2. Take the tall apple. Place the base of your thumbs around the stem, and press down and outward. Have your other fingers holding the base of the apple. No matter how strong you really are, you will rip the apple in half, but it may take up to a minute.

3. Show the halves to the class, and ask if anyone would like to try. Give the volunteer the short apple, and watch as they struggle with it. If they don’t know the technique, or if they aren’t strong enough, they will fail. Applaud them for their efforts, and have them return to their seat. If you are confident enough, try with the short apple, but be warned: it will be much harder!

Why This Works

Torque is a rotational force. It is the force you need to apply to something in order for it to rotate on a fixed axis, or point of rotation. The farther you are from the point of rotation, the less force is required, and the easier it becomes to turn. Torque is easy to show using a door; the doorknob is far from the hinges of the door, and it is easy to open the door by pushing or pulling near the doorknob. It is much harder when you push or pull nearer to the hinge, because of the affect that distance has on the force you need to apply.

When we are ripping an apple in half, we are using the base of the apple as the point of rotation. By placing your thumbs at the stem and your fingers at the base, you are using your hand much like a lever, with the distance from your thumb to the base of the apple determining the force you need. The larger the gap between your thumb and the other fingers, the greater the leverage you will have, and therefore the easier it gets to rip the apple in half. A tall apple gives enough height to make a noticeable difference in leverage, which is why the short apple will be much harder to do this with.

Additional Information

• By using a more ripe apple, it will be easier to rip it in half. Also, the softer apple types (like the red delicious) will be easier to rip than the firmer apples (Like granny smith), regardless of the height.