Lesson Notes By Weeks and Term v5 - Grade 10
Simple mechanisms and mechanical advantage – Week 7 focus
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Simple mechanisms and mechanical advantage – Week 7 focus
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Subject: Mechanical Technology
Class: Grade 10
Term: 2nd Term
Week: 7
Theme: General lesson support
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Welcome, Grade 10 Mechanical Technology learners! This week, we delve into the fascinating world of simple mechanisms and mechanical advantage. These aren't just abstract concepts; they are the fundamental building blocks of many tools and machines we encounter daily. From the pliers your father uses to fix the bakkie, to the gears in a bicycle, and even the levers used to lift heavy loads on construction sites – simple mechanisms are everywhere. Understanding how they work allows us to design, repair, and optimize these systems for greater efficiency and safety.
What are Simple Mechanisms? Simple mechanisms are basic mechanical devices that amplify an applied force (effort) to overcome a larger force (load) or change the direction of the force. They are the foundation for more complex machines.
We will focus on: Levers: A rigid bar that pivots on a fixed point called a fulcrum.
Pulleys: A wheel with a grooved rim around which a rope or belt passes.
Wheel and Axle: Two circular objects of different diameters rigidly attached and rotating together.
Inclined Plane: A sloping surface used to raise or lower objects.
Wedge: A triangular-shaped tool used to separate or split objects.
Screw: An inclined plane wrapped around a cylinder.
Mechanical Advantage (MA): Mechanical Advantage is the ratio of the load force (F load ) to the effort force (F effort ). It tells us how much the machine multiplies the force we apply. MA = F load / F effort MA > 1: The machine multiplies the force. MA effort ) to the distance moved by the load (d load ). It is a purely geometrical property. VR = d effort / d load Efficiency (η): Efficiency is a measure of how well a machine converts input work (effort) into output work (load). Real machines always have some energy losses due to friction. η = (Work Output / Work Input) x 100% = (MA / VR) x 100% Where Work = Force x Distance.
Detailed Explanations of Each Mechanism: Levers: Levers are classified into three classes based on the relative positions of the fulcrum, load, and effort: Class 1:* Fulcrum between load and effort (e.g., see-saw, crowbar). MA can be > 1,
1. Class 3:* Effort between fulcrum and load (e.g., tweezers, fishing rod). MA is always effort /d load = length of effort arm / length of load arm Pulleys: Pulleys can be fixed or movable.
Fixed Pulley:* Changes the direction of the force (MA = 1, VR = 1).
Movable Pulley:* Multiplies the force (MA > 1, VR > 1).
Pulley System:* A combination of fixed and movable pulleys. The MA and VR are approximately equal to the number of rope segments supporting the load (ignoring friction).
Wheel and Axle: The wheel and axle rotate together. MA = Radius of Wheel / Radius of Axle VR = Radius of Wheel / Radius of Axle Inclined Plane: Reduces the force required to move an object vertically. MA = Length of Slope / Height of Incline VR = Length of Slope / Height of Incline Wedge: Similar to an inclined plane, but used to separate objects. MA = Length of Wedge / Thickness of Wedge (at the back) VR = Length of Wedge / Thickness of Wedge (at the back)
Screw: An inclined plane wrapped around a cylinder. The distance between the threads is called the pitch. MA = 2πr / Pitch (where r is the radius of the screw) VR = 2πr / Pitch
Lever Example (Class 2): A worker uses a wheelbarrow (Class 2 lever) to lift a load of bricks weighing 500 N. The distance from the wheel (fulcrum) to the center of the load is 0.5 m, and the distance from the wheel to where the worker applies the effort is 1.5 m. Calculate the effort force required.
Solution:*
F load = 500 N
Load arm = 0.5 m
Effort arm = 1.5 m
MA = Effort arm / Load arm = 1.5 m / 0.5 m = 3
MA = F load / F effort => 3 = 500 N / F effort
F effort = 500 N / 3 = 166.67 N
Commentary:* The mechanical advantage is 3, meaning the wheelbarrow multiplies the worker's force by a factor of
3. The worker only needs to apply 166.67 N of force to lift the 500 N load.
Pulley System
Example: A construction worker uses a pulley system with 4 rope segments supporting a load of 800 N. He applies an effort of 250 N.
Calculate the Mechanical Advantage (MA)
Calculate the Velocity Ratio (VR).
Calculate the efficiency.
Solution:*
F load = 800 N
F effort = 250 N