Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Identify and describe levers, pulleys, and inclined planes.
• Explain the concept of mechanical advantage.
• Calculate the mechanical advantage of simple mechanisms.
• Distinguish between effort force and load force.
• Design simple solutions using these mechanisms.

Lesson summary

Simple mechanisms are the building blocks of many machines we use every day. Understanding how they work, and especially the concept of mechanical advantage, is crucial for designing and building efficient and effective solutions to practical problems. Imagine trying to lift a heavy bucket of water from a deep well in a rural South African community. Without a simple mechanism like a pulley, this task would be extremely difficult, if not impossible. This week, we will delve into the world of simple mechanisms and discover how they can make our lives easier. We will also learn how to calculate their mechanical advantage, which is the ratio of the output force to the input force.

Lesson notes

What is a Simple Mechanism? A simple mechanism is a basic device that changes the direction or magnitude of a force. It is a fundamental building block of more complex machines. Simple mechanisms allow us to do work with less force, although it might require applying the force over a longer distance.

Types of Simple Mechanisms: Lever: A rigid bar that pivots on a fixed point called a fulcrum. Levers are used to multiply force or distance. There are three classes of levers, depending on the relative positions of the fulcrum, load, and effort.

Examples in South Africa: a see-saw in a park (Class 1), a wheelbarrow used in construction (Class 2), and a fishing rod (Class 3).

Pulley: A wheel with a grooved rim around which a rope, chain, or belt passes. Pulleys are used to change the direction of a force or to multiply force. There are fixed pulleys (which only change the direction of the force) and movable pulleys (which multiply the force).

Example in South Africa: a pulley used to lift water from a well.

Inclined Plane: A flat surface set at an angle to the horizontal. Inclined planes make it easier to move objects vertically by increasing the distance over which the force is applied.

Example in South Africa: a ramp used to load goods onto a truck.

Effort Force and Load Force: Effort Force (Fe): The force applied to the simple mechanism. This is the force you exert. It's also sometimes called the input force.

Load Force (Fl): The force exerted by the simple mechanism on the object being moved or acted upon. This is the force that does the work. It's also sometimes called the output force.

Mechanical Advantage (MA): Mechanical advantage (MA) is the ratio of the load force (Fl) to the effort force (Fe). It tells us how much a simple mechanism multiplies the force we apply.

Formula: MA = Load Force (Fl) / Effort Force (Fe) A mechanical advantage greater than 1 means that the mechanism multiplies the force, making it easier to move a heavy object. A mechanical advantage less than 1 means that the mechanism requires more force than lifting the object directly, but it might allow us to move the object faster or through a greater distance.

Examples & Calculations: Example 1: Lever A person uses a crowbar (lever) to lift a rock. The rock (load) weighs 500 N (Newtons). The person applies a force of 100 N to the crowbar. What is the mechanical advantage of the crowbar?

Solution: Load Force (Fl) = 500 N Effort Force (Fe) = 100 N MA = Fl / Fe = 500 N / 100 N = 5 The mechanical advantage of the crowbar is

5. This means that the crowbar multiplies the person's force by a factor of

5. Example 2: Pulley System A pulley system is used to lift a bucket of water from a well. The bucket weighs 200 N. The person pulling the rope needs to apply a force of 50

N. What is the mechanical advantage of the pulley system?

Solution: Load Force (Fl) = 200 N Effort Force (Fe) = 50 N MA = Fl / Fe = 200 N / 50 N = 4 The mechanical advantage of the pulley system is

4. This means that the pulley system multiplies the person's force by a factor of

4. Example 3: Inclined Plane A worker uses a ramp to push a box weighing 300 N onto a truck. The worker has to apply a force of 100

N. What is the mechanical advantage of the ramp?

Solution: Load Force (Fl) = 300 N Effort Force (Fe) = 100 N MA = Fl / Fe = 300 N / 100 N = 3 The mechanical advantage of the ramp is

3. Guided Practice (With Solutions)

Question 1: A Class 2 lever is used to lift a crate of oranges. The crate weighs 300 N and is placed 0.5 meters from the fulcrum. The effort force is applied 1.5 meters from the fulcrum. Calculate the effort force required to lift the crate and then calculate the M

A. Solution: We can use the principle of moments: Effort Force x Effort Distance = Load Force x Load Distance Fe x 1.5 m = 300 N x 0.5 m Fe = (300 N x 0.5 m) / 1.5 m Fe = 100 N MA = Load Force / Effort Force = 300 N / 100 N = 3

Commentary: This problem uses the principle of moments, which is fundamental to understanding how levers work. The effort distance is greater than the load distance, resulting in a mechanical advantage greater than one.

Question 2: A single movable pulley is used to lift a bag of maize. If the bag weighs 150 N, what is the ideal effort force required to lift the bag, assuming no friction? What is the MA?

Solution: With a single movable pulley, the effort force is theoretically half the load force. Fe = 150 N / 2 = 75 N MA = Load Force / Effort Force = 150 N / 75 N = 2

Commentary: A single movable pulley theoretically halves the required effort, giving a mechanical advantage of

2. In real-world scenarios, friction would increase the required effort force.

Question 3: A worker needs to get a 400 N drum of oil onto a platform. They can either lift it directly or use a ramp. If the ramp allows the worker to push the drum with a force of 100 N, what is the mechanical advantage of the ramp?