Table of Mechanics Demonstration List of Mechanics Equipment & Supplies Lecture Demonstrations

# Unknown Bricks, 1F10.25

Topic and Concept:

Location:

Abstract:

A lead brick and a wooden block(balsa wood) of the same dimensions are both wrapped in aluminum foil to look identical. Each block is then hit with a hammer. Then toss the wooden block to a student.

 Equipment Location ID Number Wrapped Wood and Lead Blocks ME, Bay B5, Shelf #1 Rubber-Coated Mallet ME, Bay , Shelf #

Important Setup Notes:

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Setup and Procedure:

1. Place the two blocks on the table, and do not tell the audience which two materials are concealed before their eyes.
2. Tell the audience you will do your best to apply equivalent forces via the hammer to each block then do it.
3. Hit the wooden block first (hard) and observe it sliding across the table and onto the floor.
4. Then hit lead block (with the same force) and observe that it barely moves at all.
5. Ask the audience what they think the blocks are made of and why one flew across the table well the other didn't move much at all.

Optional:

1. Don't tell the audience anything.
2. Hit the lead brick with a b
3. Act like the wooden block is very heavy and toss it to an audience member to catch.
4. Toss the light, wood block to a student. If they understand physics well enough, they shouldn't freak out.

Cautions, Warnings, or Safety Concerns:

• The lead block is heavy. Don't drop it - it could cause serious damage to the floor, table, or your foot.

Discussion:

The lead block is much more dense than the wood block. Therefore, since the two blocks have the same dimensions, the lead block will be much heavier than the wood block. In other words, the lead block has much more inertia than the wood block. When hitting each block with the hammer, due to the large mass of the lead block, it will barely move and due to the comparatively small mass of the wood block, it will go sliding across the table. These ideas are quantified in Newton's second law: F = m*a, the larger the mass of the object, the less acceleration it will obtain by a given force.

Videos:

References:

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