Our family has always had a little fascination with demolition and wrecking balls. We love to watch demolition videos where they lay charges in just the right areas of one building making it fall straight down without touching the surrounding buildings. That is brilliant engineering with a firm grasp on physics.
Video: Best Building Demos
We, also, love to watch buildings being demolished by wrecking balls. It is less about finesse and more about brute strength, but it’s still awesome to watch. Even though wrecking balls function with less precision than other demolition methods, the effectiveness of wrecking balls is still determined by physics.
Video: Wrecking Ball Demolition
The Wrecking Ball Is A Pendulum
If we look at a wrecking ball, it is basically a pendulum. A pendulum is a weight hung from a pivot point and allowed to swing freely until friction between the weight and the air slows and stops the swinging of the pendulum and it eventually comes to a stop.
Pendulums work because of the energy involved in their systems. When the pendulum weight is raised, it contains gravitational potential energy. This is the energy stored by the weight because of its position above the ground. When the weight is released, that potential energy is converted into kinetic energy, or energy of motion.
As gravity pulls the weight down toward earth, both the kinetic energy and velocity, or speed, of the weight increase until the weight reaches the bottom point of the pendulum arc. This is the point of highest kinetic energy and velocity. After the weight passes the bottom of the arc, these both decrease until the weight stops at the top of the arc at opposite point of the release.
At this point, the kinetic energy is back to zero and the weight now contains gravitational potential energy again. The weight only stops for an instant until gravity pulls the weight back down and the process starts again.
Wrecking Balls And Momentum
As stated before, wrecking balls are giant pendulums. The wrecking ball is the weight held by a heavy cable to a pivot point held up by a crane. The crane operator swings the wreaking ball up to a point where the ball stops and then falls and swings through an arc just like a pendulum.
The entire point of a wrecking ball is to knock things over. To do this, the wrecking ball must apply force to the structure it is trying to destroy. To create the amount of force necessary, the crane operator needs to create a lot of momentum. As momentum increases, force increases.
Momentum is a function of mass and velocity and can be calculated as:
momentum= mass x velocity
Since the mass of the weight of the wrecking ball is constant, the velocity of the weight as it swings must be increased if momentum is going to be increased. In our pendulum discussion, we learned that the released weight increases in velocity as it nears the bottom of the pendulum arc. If the weight gains velocity as it falls, then to increase the velocity, the distance the weight falls must increase. This means the weight needs to be released from a higher point.
If the peak velocity is more when the weight is released from a higher point, then the momentum of that weight increases, as well. Therefore, the force of the weight hitting a structure increases. This means the destructive power of the wrecking ball should then be more when the weight is released from a higher point.
The Wrecking Ball Physics Experiment
If the theory above is true, the crane operator controlling the wrecking ball should be able to control the force of the wrecking ball by changing the height from where the ball is “released” or from the top of the arc of the swing. This can be tested quite simply using children’s blocks.
Spielgaben educational toy sets are perfect for this. The Spielgaben Complete Package has everything you need. First, create a pendulum to simulate a wrecking ball. Then, stack blocks as a structure that the “wrecking ball” can hit. Set sticks in the Spielgaben peg board as marks to determine two heights from where the ball is to be released.
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Release the ball from the first height and note how many blocks were knocked over. Restack the blocks and then repeat releasing the ball at the second height.
When the ball is released from the second, higher height, more blocks should be knocked over than when the ball was released from the first, lower height. This proves our theory. Test it for yourself.
Further experiments could be designed with a Spielgaben set to test the strength of structures. Build structures of varying styles and then use your wrecking ball released from the same height to test the strength of the structures.
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