Falling Chimney

Age

Elementary School, Middle School, High School

Format

Stage Show, Hands-on

Materials

   "Chimney" (Board with cup and hole for the ball)
   Ball
   Stick to hold up the chimney until you are ready to let it fall 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience.

No real safety gear is needed for this demo. Just make sure that ALL hands are away from the chimney when it falls. Also be sure that nobody accidentally knocks the chimney down before you are ready to let it happen.

Preparation

Once you have the materials, there is very little preparation. Set the stick under the chimney so that it won't fall, and add the ball to the top of the chimney.

Demonstration

Once you have prepared, the demonstration is as easy as pulling out the stick and watching the chimney fall. If everything goes well, the ball will end up in the cup.

What to Say

Has anyone ever seen a chimney fall? (Very unlikely, but if someone has, ask if the chimney broke in 2 as it was falling)

Well an interesting thing about a chimney falling is that the top of the chimney will actually end up falling faster than if it were just in free fall.

So if I put a ball at the very top of the chimney and let it fall in free fall, it would fall slower than the very top of the chimney.

When I take the stick out, the chimney will hit the ground first, with the cup ending up underneath the ball and then the ball will fall into the cup. It all happens very fast, so watch closely.

(Count down to the release of the stick so that everything can fall into place)

So this is why sometimes if you see a chimney fall, it will break into 2 pieces. The top part is trying to fall faster than it would in free fall, and it puts a lot of stress on the chimney. This causes the chimney to break, making the top part fall slower than it was previously.

Why It Is

Some simple kinematics can explain this one. Say that instead of a chimney, we have two balls that are allowed to drop in free fall. They are both held in place along an imaginary straight line from the origin at 45 degrees (this is where the chimney would be). One ball is held 0.5 meters from the origin along the line and the second ball is held 1 meter from the origin along the line. If we were to let both balls fall at the same time, they would have the same linear velocity. If we examine the angular velocities of the 2 balls though, where ω = v/r, we see that it is inversely dependent upon the radius. So the ball that is closer to the origin has an angular velocity that is twice as much as that of the ball at 1m.

Now, if we turn this imaginary line into a real line (the chimney) and let it fall from 45 degrees, since it is a rigid line all points on it must have the same angular velocity. So in effect, the points lower down on the chimney are "making" the top part of the chimney fall faster than it would have if in free fall.

Real Life Examples

A chimney in Glasgow falling. As you can see, after the break, the lower part of the chimney is at a steeper angle than the higher part. This is because the angular velocity of the lower part is faster than the higher part.