What is a trebuchet? How can a trebuchet use gravity? What does gravity have to do with launching an object? To begin, a trebuchet is a medieval catapult used for hurling heavy stones. The trebuchet is thought to have been invented by the Chinese and made it’s way to Europe by AD 500. It was used by armies to throw stones to break down barriers like castle walls. Sometimes, bodies of people and animals, that had died from disease like the Black Plague, were even thrown over the walls to try to infect people. The stones (and bodies) were thrown by the trebuchet using gravity.
Gravity is defined as the force of attraction by which terrestrial bodies tend to fall toward the center of the earth. In other words, gravity is what keeps people and objects on the ground! So, if gravity keeps things on the ground, how did people use a trebuchet to throw things? The answer is very simple–they used a “counter-weight. ” To understand what a counter-weight is, think of two kids on a see-saw. One is in the eighth grade and the other is in kindergarten. The eighth grader is going to be a lot bigger than the kindergartener.
So when the eighth grader sits down, what’s going to happen to the kindergartener? You guessed it! That little kid is going to go sailing through the air! The eighth grader was bigger and heavier than the other kid and the force of his weight caused an action, in this case his end of the see-saw came down quickly, that created an equal and opposite reaction, the little kid’s end came up quickly. But that’s not the end of it! The speed created, combined with the force, caused the little kid to be thrown through the air.
The eighth grader was the counter-weight! For this project, a simple trebuchet was constructed of wood, with a cloth sling, and counter-weights in the forms of lead weights and a brick. The projectile was a golf ball. The trebuchet was secured by a pin, the “bucket” was loaded with counter-weights, and the golf ball was loaded into the sling. When the pin was released the counter-weight was pulled down by gravity, releasing the arm that held the sling, which released the golf ball through the air.
The force was so great that the golf ball traveled through the air, landing seventy-two feet away from the trebuchet. The first part of the trebuchet that was built was the arm which acted as a lever. A lever is a simple machine. Next, the frame was built to support the functioning parts of the trebuchet. After the frame was finished, the basket for the counter weights was built. The last part that was constructed was the sling. All of the components were assembled to create the trebuchet.
Once the trebuchet was built, it was time to test the machine to see that it functioned properly. The trial runs were conducted with a “practice” (hollow plastic) golf ball and twenty ounce counter-weight to test the operation. This first trial resulted in a distance of six and a half feet. With an increase in the counter-weight to 23 ounces, a distance of three feet was achieved. In investigating the shorter distance in the second trial run, it was discovered that the trebuchet was releasing too late, so the hook was adjusted to correct the problem.
Then the counter weight was put back to 20 ounces resulting in a distance of nine feet for the third trial. With the counter-weight increased to 31 ounces the result of the fourth trial was a distance of 16 ? feet. Another adjustment was made to the hook creating a distance of 22 ? feet for the fifth trial. The sixth and final trial run was done using a 34 ounce counter-weight which gave a distance of 30 feet. Then a real golf ball was used as a projectile along with a 34 ounce counter-weight resulting in a distance of 28 due to the increase in mass of the real golf ball.
The second run with a real golf ball was done with a 64 ounce counter-weight giving a distance of 43 feet. The last few runs were done with a real golf ball and a 98 ounce counter-weight giving distances that ranged between 68 to 72 feet. Building and testing the trebuchet with a lever and weight system was a good way to witness science “at work. ” This system uses gravity to activate propulsion. The force of gravity applied to the counter-weight of the trebuchet creates the action that moves the projectile through the air as the trials show.