Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define and solve problems for force, work, energy, and power.
• Identify and calculate mechanical advantage for simple machines.
• Explain properties of materials and their uses.
• Apply safety procedures in a workshop.
• Describe different joining methods.

Lesson summary

This week focuses on consolidating your understanding of key concepts covered earlier in the term in Mechanical Technology. Effective revision is crucial not just for passing the exams but also for building a solid foundation for future learning in the subject. Mastering these concepts will enable you to understand the mechanical principles behind many everyday technologies we use in South Africa, from basic tools used in construction and agriculture to more complex systems like vehicle engines and manufacturing processes. Understanding these principles is essential for South Africans aiming for careers in engineering, manufacturing, and technical fields.

Lesson notes

2.1 Force, Work, Energy, and Power Force (F): A force is a push or a pull that can cause an object to accelerate (change its velocity). Its SI unit is the Newton (N). 1 N is the force required to accelerate a 1 kg mass at 1 m/s². Forces are vector quantities, meaning they have both magnitude and direction.

Work (W): Work is done when a force causes displacement of an object. It's the transfer of energy. The SI unit is the Joule (J). 1 J is the work done when a force of 1 N moves an object 1 m in the direction of the force.

The formula for work is: W = F x d x cos(θ)

Where: W = Work (in Joules) F = Force (in Newtons) d = Displacement (in meters) θ = Angle between the force and the direction of displacement (in degrees)

Energy (E): Energy is the capacity to do work. It exists in various forms, including kinetic (motion), potential (stored), thermal (heat), and electrical energy. The SI unit is also the Joule (J).

Kinetic energy is given by: KE = 1/2 m * v² Where: KE = Kinetic energy (in Joules) m = Mass (in kilograms) v = Velocity (in meters per second) Potential energy due to gravity is given by: PE = m g * h Where: PE = Potential energy (in Joules) m = Mass (in kilograms) g = Acceleration due to gravity (approximately 9.8 m/s²) h = Height above a reference point (in meters)

Power (P): Power is the rate at which work is done or energy is transferred. The SI unit is the Watt (W). 1 W is the power when 1 J of work is done in 1 second.

The formula for power is: P = W / t or P = F v Where: P = Power (in Watts) W = Work (in Joules) t = Time (in seconds) F = Force (in Newtons) v = Velocity (in meters per second)

Example 1: A worker in a construction site uses a force of 50 N to push a wheelbarrow 10 meters across level ground. Calculate the work done by the worker.

Solution: F = 50 N d = 10 m θ = 0° (Assuming the force is applied horizontally in the direction of motion) W = F x d x cos(θ) = 50 N x 10 m x cos(0°) = 500 J Example 2: A 2 kg brick is lifted to the top of a 5-meter-high wall by a construction worker. Calculate the potential energy gained by the brick.

Solution: m = 2 kg g = 9.8 m/s² h = 5 m PE = m g * h = 2 kg x 9.8 m/s² x 5 m = 98 J 2.2 Simple Machines Simple machines are basic mechanical devices that amplify force or change the direction of force to make work easier.

Levers: A rigid bar that pivots on a fixed point called a fulcrum. Classes of levers are defined by the relative positions of the fulcrum, load (resistance), and effort (force). Mechanical Advantage (MA) = Load / Effort Velocity Ratio (VR) = Distance moved by effort / Distance moved by load Pulleys: A wheel with a grooved rim around which a rope, cable, or belt passes. Used to lift objects or transmit force. MA = Number of rope segments supporting the load (for a block and tackle system) VR = Number of rope segments supporting the load (for a block and tackle system)

Inclined Plane: A sloping surface used to raise objects. MA = Length of the incline / Height of the incline VR = Length of the incline / Height of the incline Wedge: Two inclined planes joined back-to-back. Used for splitting or separating objects. MA = Length of the wedge / Thickness of the wedge The VR is not usually calculated for wedges.

Screw: An inclined plane wrapped around a cylinder. Used to fasten objects or apply force. MA = Circumference of the screw (2πr) / Pitch (distance between threads) VR = Circumference of the screw (2πr) / Pitch (distance between threads)

Wheel and Axle: Two cylinders of different radii rotating together. MA = Radius of the wheel / Radius of the axle VR = Radius of the wheel / Radius of the axle Example 3: A builder uses a lever to lift a heavy stone. The fulcrum is placed 0.5 m from the load, and the effort is applied 2 m from the fulcrum. If the load is 200 N, what is the effort required?

Solution: Load = 200 N Load arm = 0.5 m Effort arm = 2 m Using the principle of moments (Load x Load arm = Effort x Effort arm): 200 N x 0.5 m = Effort x 2 m Effort = (200 N x 0.5 m) / 2 m = 50 N 2.3 Properties of Materials Understanding material properties is vital in selecting the correct material for a specific engineering application.

Hardness: Resistance to scratching or indentation.

Toughness: Ability to absorb energy and plastically deform without fracturing.

Malleability: Ability to be deformed under compression without fracturing (e.g., hammering into thin sheets).

Ductility: Ability to be drawn into wires without fracturing.

Elasticity: Ability to return to its original shape after being deformed when the applied force is removed.

Tensile Strength: Resistance to being pulled apart by a tensile force.

Example: When building a bridge in South Africa, which material property is most important to consider for the support cables, and why?

Solution: Tensile strength is the most important property to consider. The support cables must be able to withstand the large tensile forces pulling on them due to the weight of the bridge and traffic.