83 Wrecking Ball vs. Strong Wall-Mid

Wrecking Ball vs. Strong Wall FOCUS Question How does a wrecking ball knock down a building?

Demolition Day Have you ever tried to knock over a sand castle? Wrecking a weak building made of sand is easy. What if your job were to demolish a strong building made of brick and concrete? You could use a wrecking ball! Before knocking down an old building, workers first make sure that no one is inside. Then they bring a crane with a chain and a very heavy metal ball attached. This is a wrecking ball machine. Once the workers are sure that everyone is safe, they can swing the wrecking ball.

Gravity Drop One way to use a wrecking ball is to drop it straight down. Think about holding a bowling ball. Suppose you could lift it over an old building and drop it straight down. It would smash through the roof! The higher the ball is lifted, the more damage it causes when it falls. This is due to gravity. Gravity always pulls things down. When an object falls gravity makes it fall faster and faster. An object that falls a long way moves with more energy than if it falls a short way. Dropping the wrecking ball straight down is often used to punch through roofs or floors.

Tall Crane, Swinging Chain Another way to use a wrecking ball is to swing it. A cable pulls the ball toward the crane or the crane swings the ball away from the building. Then the ball swings forward and smashes into a wall. The force of this forward motion is how wrecking balls knock down walls. When a wrecking ball machine is used in this way, it acts as a pendulum.

Pendulums A pendulum is an object that swings back and forth from a fixed point. It has a weight at the end of a line. When the pendulum is lifted and released, it swings back and forth in a curved path, or arc. At the top of its swing, gravity pulls the weight down. It speeds up as it goes down. After it passes the bottom part of its swing, the pendulum starts to move up along the arc. Gravity slows the pendulum down until it stops at the other end of the arc. Then gravity pulls it down again. It swings back in the other direction. A wrecking ball works in a similar way. But a wall gets in the way before the ball can make one full swing through its arc.

Wrecking Ball Energy A wrecking ball machine uses a lot of energy to demolish a building. Energy is the power to cause movement or change. When the ball swings away from the building, the machine has stored-up energy. Whoosh The wrecking ball swings. Thunk! The ball smashes into the wall. The building starts to crumble. A cloud of dust swirls into the air. Workers use other machines to haul away the rubble.