can the position of the fulcrum affect the mechanical advantage of a lever?
levers are one type of simple machine. A simple machine is a device that gives the user a mechanical advantage. in science, work is accomplished when an object is moved from one place to another. every object, from the lightest feather to the heaviest rock, requires a force to make it move. This is called the resistance force. The effort force, as the name suggests, is the amount of force (or effort) used to get the work done. if the effort force is greater than an object’s resistance force, then the object will move, and work will be accomplished.
All simple machines reduce the amount of effort force needed to get a job done. They do this by increasing the distance over which the force is applied. The degree that a machine reduces the amount of effort needed is called the mechanical advantage. Simple machines create a mechanical advantage by moving the object a greater distance but with less effort. in the end, the amount of work that is accomplished is the same whether you use a machine or not.
A lever has two parts: a bar and a fulcrum. The bar moves, or pivots, on the fulcrum. The fulcrum divides the bar into two parts called arms (see Figure 1). The arm that holds the object being moved (called the load) is the resistance arm. The arm to which you apply the force is called the effort arm. in this activity, you will discover that moving the position of the fulcrum can give you a large mechanical advantage.
● 12-in. (30 cm) ruler
● 4 quarters
Safety Note Please review and follow the safety guidelines.
1. create a lever by placing a pencil under the 6 in. (15 cm) mark of the ruler. The two ends of the ruler should balance on the pencil. in this setup, the pencil acts as a fulcrum. Place two quarters on the 12-in. (30-cm) end of the ruler and one quarter on the “0” end. The lever should now tip to one side. Record your observations and explain why this happens.
2. Without moving the quarters, change the position of the fulcrum so that the two sides of the lever balance again. Record the number of inches (cm) on the ruler at the point where you get the lever to balance again.
3. Keep the pencil in the new position and predict what will happen if you place a third quarter on top of the two that are already on the 12-in. (30- cm) end of the ruler. Add the third quarter and record your observations.
4. if you wanted to get the two ends of the lever to balance again, predict what you would have to do to the fulcrum. Try to balance the lever again and record your observations.
1. Based on your observations, which end of the ruler was the force arm and which was the resistance arm? Why?
2. in which direction did you have to move the fulcrum in order to get the heavier weight to balance with the lighter weight?
3. in order to balance the three quarters, which end of the lever was longer: the resistance arm or the effort arm?
4. Based on your observations, how does moving a fulcrum affect the mechanical advantage of a lever?
What’s Going On?
With a lever, the amount of mechanical advantage is controlled by the position of the fulcrum. When the fulcrum is in the center of the bar, the force arm and resistance arm are equal lengths and the mechanical advantage is one. This means that the amount of force required to lift the load is equal to the resisting force of the load. one way to increase the mechanical advantage of a lever is to move the fulcrum closer to the load. This makes the effort arm longer, compared with the resistance arm. When you lifted two quarters with one quarter, you doubled the mechanical advantage of the lever. You did not gain free work, though. You had to move the lever a greater distance. in this case, the effort arm was twice as long as the resistance arm was. When you lifted three quarters with one quarter, you tripled the mechanical advantage, and the length of the resistance arm was tripled.
1. The arm with the multiple quarters is the resistance arm, because it has the load. The arm with the single quarter is the effort arm.
2. in order to lift a heavy weight with a lighter weight, the fulcrum must be moved closer to the load. This increases the length of the effort arm.
3. When lifting three quarters with one quarter, the effort arm was much longer than the resistance arm.
4. With a lever, moving the fulcrum closer to a load increases the length of the effort arm and increases the mechanical advantage.
Spear Throwers and Siege Engines
Spears were much better for hunting than simple hand axes. But spears still had a fairly limited range. Also, even the best spears had a hard time penetrating some animal hides. By about 15,000 b.c., hunters had discovered that they could use additional leverage to improve on a simple spear by launching it with a spear thrower. Called an atlatl by the ancient Aztec people, a spear thrower was usually made out of wood, antler, or bone. It looked like a long stick with a curved notch at one end.
A spear thrower extends the length of a hunter’s arm, giving him a greater mechanical advantage. The end of the spear is placed in the notch, and the hunter holds the other end of the spear thrower. When the hunter winds up to throw the spear, the longer effort arm (the hunter’s arm, plus the length of the spear thrower) means that the spear is released with a greater amount of force. This gives it more power to penetrate tough animal hides. This same type of device is still used by some Inuit today to throw harpoons, and it was the primary hunting tool of many people until being replaced by the bow and arrow.
During the Middle Ages, levers had a major impact on warfare. Before the cannon was invented, the only way that an army could bring down a stone fortress or castle was to bombard it with large stones from a siege engine. One of the most important of these siege engines was the trebuchet. Thought to originate in China in about a.d. 100, the trebuchet is designed to throw large rocks or other projectiles using a giant lever. A large lever is mounted unevenly on a fulcrum. The short end is the effort arm; it has a large weight (usually a box of rocks) hanging from it. The long end of the lever is the resistance arm. It is attached to a large sling that holds a rock or some other type of projectile. Unlike a catapult that gets its power from some type of spring, a trebuchet uses the movement of the weight to fire the projectile.
An aboriginal tribe member in Kimberley, Australia, uses an atlatl. By placing a spear in an atlatl, or spear thrower, a person’s arm length increases and this action provides more force in a throw.
A trebuchet uses the movement of weight to fire heavy objects long distances: The weight on the short end of the beam is released, pulling that side down and sending force to the other end of the beam, thereby firing the object.
Before the trebuchet could be fired, a team of men would pull down on the resistance arm, lifting the effort arm and its weight high above the ground. The weight was then locked into place. When the signal to fire was given, the weight was released. As gravity pulled it toward the ground, the force was translated through the fulcrum to the other end of the beam firing the rock. During the fourteenth century, some trebuchets were so large that they could fire a 220-pound (100-kg) rock a distance of 492 feet (150 m). That’s nearly the length of two football fields.
Levers in the Modern World
Everywhere you turn, you can see tools that are levers in action. Just about any tool with a handle uses a lever to give mechanical advantage. Picks, shovels, rakes, and brooms are examples of devices that gain added leverage by having handles. In Experiment 5: Testing Hammer Handle Length, you can discover how important a handle is for the function of a modern-day tool.