Demos: 1F-08 50-pound Block
The instructor places a 50 pound block on his/her chest (or have a student do this) and has the student strike the block with a sledge hammer. The person holding the block is not harmed. Although the force of the sledge hammer blow is quite large, the impact time is very, very small. The large mass of the block prevents it from gaining any significant movement before the impact is over.
Directions*: Lie on the lecture desk and have someone (or yourself) place the block on your chest. A student will then be asked to strike the metal plate with the hammer. The student is encouraged to swing the hammer with considerable enthusiasm. (Note: Although you will not be harmed, you will feel the impulse of the blow in your body. It is only a slight discomfort.
Suggestions for Presentation: Talk about how you have a firm trust in the Laws of Newton. You are willing to subject yourself to apparent danger in the belief that Newtons First Law will keep you from harm.
Applications: Relate this to other inertia demos, especially the Inertial Ball, 1F-6.
* An alternative method is to place the block on top of several inverted drinking cups. If the block is dropped onto the cups, they will crush. If the block is hit with the sledge hammer, they will not be crushed.