Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Identify three types of simple mechanisms.
• Define and explain mechanical advantage.
• Calculate the mechanical advantage of levers and pulleys.
• Understand how mechanisms combine to form complex machines.
• Relate simple mechanisms to real-life South African scenarios.

Lesson summary

This week, we delve into the fascinating world of simple mechanisms and mechanical advantage. Simple mechanisms are the fundamental building blocks of more complex machines, allowing us to perform tasks more easily. Understanding how they work and how to calculate their mechanical advantage is crucial for designing and analyzing machines that improve our lives. Imagine a wheelbarrow helping someone transport heavy loads of mealie meal, or a simple pulley system used to lift water from a well in a rural community.

Lesson notes

What is a Simple Mechanism? A simple mechanism is a basic mechanical device that changes the direction or magnitude of a force. They make work easier by reducing the amount of force needed to perform a task, although you might have to apply that force over a longer distance. The main types of simple mechanisms we'll focus on this week are: Lever: A rigid bar that pivots around a fixed point called a fulcrum. Levers are used to lift, pry, or multiply force.

Pulley: A grooved wheel with a rope or cable running along the groove. Pulleys are used to lift objects, change the direction of force, or multiply force.

Inclined Plane: A flat surface set at an angle to a horizontal surface. Inclined planes are used to reduce the force needed to move an object vertically. Mechanical Advantage (MA) Mechanical advantage is a measure of how much a simple machine multiplies the force you apply. It tells you how much easier the machine makes the work.

It's calculated as: MA = Output Force / Input Force Output Force (Fout): The force exerted by the machine. This is the force needed to overcome the resistance or load.

Input Force (Fin): The force you exert on the machine. This is the effort you put in. A mechanical advantage greater than 1 means the machine multiplies your force. A mechanical advantage less than 1 means the machine reduces your force (but often increases the distance you can move something).

Levers: Levers are classified into three classes, depending on the position of the fulcrum, load, and effort.

Class 1 Lever: Fulcrum is between the effort and the load (e.g., see-saw, crowbar). MA = Distance from fulcrum to effort (Effort Arm) / Distance from fulcrum to load (Load Arm)

Class 2 Lever: Load is between the fulcrum and the effort (e.g., wheelbarrow, bottle opener). MA = Distance from fulcrum to effort (Effort Arm) / Distance from fulcrum to load (Load Arm) Class 2 levers always have a mechanical advantage greater than

1. Class 3 Lever: Effort is between the fulcrum and the load (e.g., tweezers, fishing rod). MA = Distance from fulcrum to effort (Effort Arm) / Distance from fulcrum to load (Load Arm) Class 3 levers always have a mechanical advantage less than

1. While they don't multiply force, they increase speed and distance.

Example 1 (Lever): A farmer uses a crowbar (Class 1 lever) to lift a large rock blocking his irrigation ditch. The distance from the fulcrum to the rock (load) is 0.5 meters, and the distance from the fulcrum to where the farmer applies force (effort) is 2 meters. If the rock weighs 500 N (Newtons), what force does the farmer need to apply? MA = Effort Arm / Load Arm = 2m / 0.5m = 4 MA = Output Force / Input Force => 4 = 500N / Input Force Input Force = 500N / 4 = 125N The farmer only needs to apply a force of 125N to lift the 500N rock, thanks to the crowbar's mechanical advantage.

Pulleys: Fixed Pulley: A pulley attached to a fixed point. It changes the direction of the force, but the force required is the same as the weight of the load (MA = 1). Lifting a bucket from a well using a fixed pulley makes pulling down easier than lifting directly upwards.

Movable Pulley: A pulley attached to the load. It reduces the force needed to lift the load, but the distance you pull the rope is greater. The MA is equal to the number of rope segments supporting the load.

Pulley System (Combination): A system with multiple fixed and movable pulleys. The MA is equal to the number of rope segments supporting the load (ignoring friction).

Example 2 (Pulley): Workers use a pulley system with 3 supporting ropes to lift bags of cement onto a construction site. If each bag of cement weighs 200 N, how much force do the workers need to apply to lift one bag? MA = Number of supporting ropes = 3 MA = Output Force / Input Force => 3 = 200N / Input Force Input Force = 200N / 3 = 66.67N (approximately) The workers only need to apply about 66.67N of force to lift the 200N bag of cement.

Inclined Plane: An inclined plane (ramp) reduces the force needed to move an object vertically by increasing the distance over which the force is applied. MA = Length of the inclined plane / Height of the inclined plane Example 3 (Inclined Plane): A worker uses a ramp that is 3 meters long to push a heavy box onto the back of a truck. The height of the truck bed is 1 meter. What is the mechanical advantage of the ramp? MA = Length of ramp / Height of ramp = 3m / 1m = 3 The ramp allows the worker to use 1/3 of the force it would take to lift the box straight up. Guided Practice (With Solutions)

Question 1: A Class 2 lever (like a bottle opener) has an effort arm of 15 cm and a load arm of 3 cm. What is the mechanical advantage of this lever?

Solution: Lever type: Class 2 Effort arm = 15 cm Load arm = 3 cm MA = Effort Arm / Load Arm = 15 cm / 3 cm = 5

Commentary: Remember the formula for the mechanical advantage of a lever: MA = Effort Arm / Load Arm. Plugging in the values gives us a mechanical advantage of 5.