Magic Spoon
| Physics: | Torque |
| Grade Range: | Elementary School, Middle School, High School |
| Format: | Hands-on, Stage |
This is a theatrical demonstration that is easy to explain, but requires clever acting and some sleight of hand to be performed well. If you have a rowdy student in the audience, they might make the best volunteer for this demonstration!
Materials
- A Spoon
Safety Precautions
Please read the General Safety Precautions section of the Demonstration Safety page before performing this demonstration.
Demonstration
Note: These steps will be written with a bullet point below each step. The steps will be covering how you theatrically present it, while the bullet point will be how you perform the actions in each step.
- Show the spoon to the audience, and exclaim how it is a Magic Spoon that can grant you strength and balance! Elaborate on the mystical powers of the Magic Spoon, and really ham it up. Ask for a volunteer to help you show how magical the spoon is, and use the power of the Magic Spoon to find the "ideal" volunteer.
- When you ask for a volunteer, ask for an adult if being done with Elementary or Middle school. For high school, choose a volunteer who looks strong.
- Have your volunteer stand facing the audience. Ask them to hold their right arm out straight sideways, and to resist you when you push down on it. Show how you can easily push their arm down while holding the Magic Spoon! Then, Give the volunteer the Magic Spoon and have them hold their right arm out again. Now it is a lot harder to push their arm down, if you can at all!
- When the volunteer holds out their right arm, stand behind their arm and face the audience. Hold the spoon in your right hand, being sure to show that you are holding the spoon, and push their forearm down using your right hand. When you give them the spoon, be sure to point out that they are holding it, and then try to push their upper arm down using your left hand.
- Have your volunteer turn sideways, and hold both of their arms out in front of them. Ask them to keep their arms rigid and to try to not fall over when you push down on their arms. Show that while you have the Magic Spoon in your hand, and that you can easily push both their arms down and make them fall over!
- When the volunteer turns sideways, make sure to note which way their forearms face when you stand behind them. Hold the magic spoon in the same hand as that side, and point out the spoon before pushing down on their forearms with the corresponding hand.
- Give your volunteer the Magic Spoon and repeat step 3. This time, they don't fall over!
- When you repeat step 3, be sure to have the volunteer turn around and face the other direction. That way, you can then use the same hand as before, but this time you will be pushing down on their upper arm, which will fail.
- Thank the volunteer, and ask them if they think the Magic Spoon is truly magical. If not, can they guess what was going on? Can anyone from the audience guess how you did it?
- After getting ideas, ask for a new volunteer to help show how this demonstration works. Your old volunteer will likely want to take a seat!
Why This Works
Short Explanation
The Magic Spoon isn't actually magical. Instead, the presenter is showing Torque or a rotational force, and using the spoon as a distraction. To think of torque, think of how a door works. If you push on a door next to the doorknob, the door will open easily. But, if you push on a door near the hinges, it is really hard to open! This is because the amount of force you need is affected by how far you are from the hinges; the further you get from the hinges, the easier the door is to open.
In this demo, the presenter is pushing on the volunteer's arm to make it move. The volunteer's shoulder is where their arm rotates, so think of it being similar to a door hinge. When the presenter pushes on the volunteer's forearm, it is far from their shoulder and easy to push down! Likewise, when they push on their upper arm, it is really close to the shoulder and much harder to push down!
Full Explanation
Many of us have heard of "magic" items like this before. Often these items are bracelets or charms that supposedly help with balance and strength, but in actuality don't affect you in any way. Rather, the salesman is using sleight of hand and clever tricks to hide the truth. In reality, they are using torque to make the illusion of "magical" properties with the object. Torque is a rotational force, or when you have a force applied to an object while it rotates on an axis:
| τ = r F sin θ |
| τ: Torque |
| r: Radius from axis |
| F: Force applied, where F = ma |
When you apply a force to an object on an axis, the distance you are from the axis is inversely related to the amount of force you have to apply. An easy way to think of this is by thinking about a door; If you push on the door while close to the hinges, you have to apply more force. Likewise, if you push on the door where the doorknob is, which is far from the hinge, you have to apply less force.
In this demonstration, we are taking advantage of torque to make it seem like the "Magic Spoon" works. When the presenter pushes down on the volunteers arm(s), they are making sure to stay aware of where they are applying the force. The volunteer's shoulder is the point of rotation, and so the radius from the axis can be determined by where on the volunteer's arm the presenter pushes down. If the presenter is holding the spoon, they will push down on the forearm, which is far from the shoulder and requires less force to be applied to it. If the volunteer is holding the spoon, the presenter will push down on the upper arm, which is close to the shoulder and requires more force to be applied to it.
If the presenter feels like being extra sneaky about this, they can take this demonstration one step further, and switch positions with the volunteer. In this case, by carefully positioning themselves in regard to the volunteer, the presenter can get the same effects to happen without the volunteer catching on!
Additional Information
- This demonstration pairs well with the Where's the Doorknob and Walk the Spool demonstrations.
- This demonstration is a part of the Forces and Motion Show.