Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define key terms like 'mechanical system', 'linkage', 'input' and 'output'.
• Identify and describe three basic types of linkages: lever, crank, and push-pull.
• Explain how linkages transform motion and force.
• Analyze and design simple linkage systems.

Lesson summary

This week, we're diving into the fascinating world of mechanical systems and linkages! Mechanical systems are all around us, from the simple bottle opener in your kitchen to complex machinery like the water pumps that bring clean water to our communities. Understanding how these systems work is not just about technology; it's about understanding the world around you and being able to design and build solutions to everyday problems. Linkages, in particular, are fundamental components in these systems, controlling movement and force. They are the 'muscles' of many machines.

Lesson notes

What is a Mechanical System? A mechanical system is a combination of interconnected parts that work together to perform a specific task. These parts usually involve mechanical components that transmit or transform motion and force. Think of a pair of scissors; it's a simple mechanical system. What is a Linkage? A linkage is a series of rigid members (links) connected by joints that allow relative motion. These links are connected to each other, or to a fixed point (ground), to create a specific motion or transmit force. Linkages are used to convert one type of motion into another, change the direction of force, or amplify force. They're fundamental to many mechanical devices.

Key Terms: Link: A rigid bar in a linkage.

Joint: A connection between two links, allowing movement. Common joints include pivots (revolute joints) and sliding joints.

Input: The force or motion applied to the system.

Output: The resulting force or motion produced by the system.

Motion: Movement. This can be linear (straight line), rotary (circular), or reciprocating (back and forth).

Force: A push or pull that can cause an object to accelerate or deform.

Types of Linkages (with examples): Lever: A lever is a simple machine consisting of a rigid bar that pivots about a fixed point (fulcrum). Levers are used to multiply force. There are three classes of levers, based on the position of the fulcrum, load, and effort.

Example:* A crowbar used to lift a heavy rock. The fulcrum is the point where the crowbar rests on the ground. The effort is applied at one end of the crowbar, and the load is the rock being lifted. Bottle openers are also levers.

Motion Transformed:* The input motion of pushing down on the lever transforms into the output motion of lifting the load.

Force Transformation:* A small input force can be multiplied into a larger output force, making it easier to lift heavy objects.

Crank: A crank is a rotating lever connected to a connecting rod. Cranks are often used to convert rotary motion into reciprocating (back-and-forth) linear motion, or vice versa.

Example:* The pedals on a bicycle. The circular motion of pedaling is converted into the reciprocating motion of the pistons in your legs. The crankshaft in a car engine also converts the reciprocating motion of the pistons into rotary motion that turns the wheels.

Motion Transformed:* The rotary motion of the crank is transformed into reciprocating motion.

Force Transformation:* The force applied to the crank is converted into a torque (rotational force).

Push-Pull Linkage: This type of linkage uses a sliding joint to transmit motion linearly. It's often used to convert rotary motion to linear motion, or vice-versa over a short distance.

Example:* The steering mechanism of a toy car. Turning the steering wheel (rotary motion) moves a rod back and forth (linear motion), which turns the wheels. Also, consider the mechanism that opens and closes some window blinds.

Motion Transformed:* The rotary motion can be converted to linear motion, and the force can also be manipulated.

Force Transformation:* Can be used to increase or decrease the force depending on the linkage design.

How Linkages Transform Motion and Force: Linkages transform motion and force through their geometry and the placement of joints and links. The length of the links, the position of the fulcrum (in levers), and the angles between the links all influence how the input motion and force are converted into output motion and force. By changing these parameters, we can design linkages to achieve specific tasks.