Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define Mechanical Advantage (MA).
• Identify types of simple mechanisms like levers, pulleys, and gears.
• Calculate the mechanical advantage of levers.
• Explain how simple mechanisms combine to form complex systems.

Lesson summary

This week, we delve deeper into the fascinating world of simple mechanisms and how they give us a mechanical advantage. Simple mechanisms are the building blocks of more complex machines, and understanding how they work is crucial for developing problem-solving skills and innovative thinking. From the gears in a bicycle to the lever of a see-saw, these mechanisms are all around us. Understanding mechanical advantage empowers us to design and build solutions to everyday challenges, making tasks easier and more efficient. Thinking critically about how simple machines function can also help us become more conscious consumers and appreciate the ingenuity behind the technologies we use daily.

Lesson notes

Simple Mechanisms: These are basic devices that use force to do work. They make tasks easier by changing the direction or magnitude of a force. Key examples include levers, pulleys, gears, inclined planes, wedges, and screws.

Mechanical Advantage (MA): This is the ratio of the output force (the force the machine exerts) to the input force (the force you exert). In simpler terms, it tells us how much the machine multiplies our force. MA = Output Force / Input Force A mechanical advantage greater than 1 means the machine multiplies your force, making the task easier. A mechanical advantage less than 1 means the machine requires you to exert more force, but it might increase the distance or speed.

Levers: A lever is a rigid bar that pivots around a fixed point called a fulcrum. There are three classes of levers, defined by the relative positions of the fulcrum, the effort (input force), and the load (output force).

Class 1 Lever: Fulcrum is between the effort and the load (e.g., see-saw, crowbar).

Class 2 Lever: Load is between the fulcrum and the effort (e.g., wheelbarrow, nutcracker).

Class 3 Lever: Effort is between the fulcrum and the load (e.g., tweezers, fishing rod). Calculating Mechanical Advantage of Levers: The MA of a lever is determined by the distances from the fulcrum to the effort and load. MA = Distance from Fulcrum to Effort / Distance from Fulcrum to Load Pulleys: A pulley is a wheel with a grooved rim around which a rope, cable, or belt passes. Pulleys are used to change the direction of a force or to gain a mechanical advantage.

Fixed Pulley: A pulley attached to a fixed point (e.g., a wall or ceiling). It only changes the direction of the force (MA = 1).

Movable Pulley: A pulley attached to the load. It reduces the amount of force needed to lift the load (MA > 1).

Block and Tackle: A system of multiple fixed and movable pulleys. The mechanical advantage is equal to the number of rope segments supporting the load.

Gears: Gears are toothed wheels that mesh together to transmit rotary motion. They can change the speed, torque, and direction of rotation.

Gear Ratio: The ratio of the number of teeth on the driven gear (output gear) to the number of teeth on the driving gear (input gear). Gear Ratio = Number of Teeth on Driven Gear / Number of Teeth on Driving Gear If the gear ratio is greater than 1, the output speed is slower, but the output torque (rotational force) is higher. If the gear ratio is less than 1, the output speed is faster, but the output torque is lower.

Worked example

Example 1: Lever (Class 1)

A crowbar is used to lift a rock. The fulcrum is placed 0.2 meters from the rock (load), and the effort is applied 1 meter from the fulcrum. What is the mechanical advantage?

Distance from Fulcrum to Load = 0.2 m

Distance from Fulcrum to Effort = 1 m

MA = Distance from Fulcrum to Effort / Distance from Fulcrum to Load = 1 m / 0.2 m = 5

The crowbar has a mechanical advantage of

5. This means the force you apply to the crowbar is multiplied by 5 when lifting the rock.