Physics Week Seven

With all our trebuchets successfully built last week, it was time to learn a little about their history and the science that makes them work before testing them.

We started by reviewing the simple machine called a lever, which is simply a bar or beam that rotates around a fixed point called the fulcrum.  The amount of force applied by weight on one end of a lever can be adjusted by changing the distance the weight is from the fulcrum.  We briefly discussed first, second, and third class levers with examples of each.

A trebuchet is an ancient battle machine that was used to throw heavy payloads a considerable distance, to smash fortress walls or to strike enemies inside their stronghold.  The trebuchet has a sling that swings from the bottom to the top of the machine, giving it more momentum and projectile force.  It uses gravity to produce the force, and the throwing arm is a simple lever with a heavy weight at one end.  When the trigger is released, gravity pulls the weight down, swinging the other end of the lever up.  The swinging motion pulls the sling around and over the trebuchet until it releases its ammunition.  The trebuchet was the most accurate of the catapult types, and extremely efficient in transferring potential energy to the projectile.  Also, it was much more consistent in throwing, because the same amount of energy could be delivered every time when using the same weight.

Finally, we bent paperclips into the right shape to hold our counterweights (zinc washers) and then went to our "throwing field" to try out the trebuchets.  We had a fair amount of success, although we had to experiment a bit with the respective masses of the projectiles and the counterweights.

The diagrams and some of the info came from the very cool science website: Real World Physics Problems. Other info came from Pitsco Education's book Siege Machines.

This is the seventh post in a series -

• Physics Week One (gravity and friction)
• Physics Week Two (structural engineering)
• Physics Week Three (suspension bridges)
• Physics Week Four (constructing catapults)
• Physics Week Five (catapults)
• Physics Week Six (constructing trebuchets)

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