Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Define and apply Newton's three laws of motion.
• Define and calculate static and kinetic frictional forces.
• Draw free-body diagrams for various scenarios.
• Solve problems involving objects on inclined planes, including friction.
• Explain and apply the concept of terminal velocity.

Lesson summary

This week, we delve deeper into Newton's Laws of Motion and their applications. Understanding these laws is crucial, not just for acing your Physical Sciences exam, but also for understanding the world around you. From car safety features to the mechanics of sports, from understanding why a bakkie struggles more than a sedan to stop on a dirt road, to the design of bridges and buildings, Newton's Laws are fundamental. As South Africans, we often encounter situations where a grasp of mechanics is beneficial – be it understanding road safety, optimising energy consumption in agriculture, or even improving sporting performance.

Lesson notes

2.1 Newton's First Law: The Law of Inertia Statement: An object continues in a state of rest or uniform (constant velocity) unless it is acted upon by a resultant/net force.

Explanation: Inertia is the tendency of an object to resist changes in its state of motion. A heavier object has more inertia. Importantly, Newton's First Law implies that forces are required to change motion, not to maintain motion.

Example: A cricket ball will continue to travel at the same speed and direction after being hit, unless affected by air resistance or gravity. A stationary taxi won't move unless a force is applied (the engine and wheels provide this force). 2.2 Newton's Second Law: The Law of Acceleration Statement: When a resultant/net force acts on an object, the object will accelerate in the direction of the force. The acceleration is directly proportional to the net force and inversely proportional to the mass of the object.

Mathematical Representation: F net = ma, where F net is the net force (in Newtons, N), m is the mass (in kilograms, kg), and a is the acceleration (in meters per second squared, m/s 2 ). This is a vector equation; direction matters!

Explanation: The bigger the force, the bigger the acceleration. The bigger the mass, the smaller the acceleration for the same force. The net force is the vector sum of all forces acting on the object.

Example 1: Imagine a bakkie (mass = 1500 kg) accelerating at 2 m/s². The net force acting on it is F net = (1500 kg)(2 m/s²) = 3000

N. Example 2: A learner pushes a 5 kg box with a force of 10 N on a smooth surface. The acceleration of the box is a = F net /m = 10 N / 5 kg = 2 m/s². 2.3 Newton's Third Law: The Law of Action and Reaction Statement: When one object exerts a force on another object, the second object exerts an equal in magnitude but opposite in direction force on the first.

Explanation: Forces always come in pairs. These forces act on different objects. The action and reaction forces are of the same type. They are NOT the net force, as they act on different objects.

Example: When you walk, your feet push backward on the Earth (action). The Earth, in turn, pushes forward on your feet (reaction), propelling you forward. A rocket expels exhaust gases downward (action). The gases exert an equal and opposite force upwards on the rocket (reaction), propelling it upward. 2.4 Frictional Forces Definition: Friction is a force that opposes motion between two surfaces in contact.

Types: Static Friction (f s ): The force that opposes the start of motion. It varies in magnitude up to a maximum value. f s ≤ μ s N, where μ s is the coefficient of static friction (dimensionless) and N is the normal force (the force exerted by a surface perpendicular to the object in contact with it).

Kinetic Friction (f k ): The force that opposes motion when an object is already moving. f k = μ k N, where μ k is the coefficient of kinetic friction (dimensionless). μ k is usually less than μ s .

Explanation: Friction arises from the microscopic roughness of surfaces. Static friction prevents motion until the applied force overcomes the maximum static friction. Once the object is moving, kinetic friction acts. Coefficients of friction depend on the materials in contact (e.g., rubber on concrete has a high coefficient, ice on ice has a low coefficient).

Example: A heavy crate rests on a factory floor. The static friction prevents it from moving until you apply a large enough force. Once moving, the kinetic friction opposes its motion, requiring you to continually apply force to maintain its velocity. 2.5 Inclined Planes Analysis: When dealing with objects on inclined planes, it's essential to resolve forces into components parallel and perpendicular to the plane. Gravity (mg) acts vertically downwards. The component of gravity parallel to the plane is mg sin θ, where θ is the angle of inclination. This component causes the object to slide down the plane. The component of gravity perpendicular to the plane is mg cos θ. This component is balanced by the normal force (N = mg cos θ).

Friction on Inclined Planes: If friction is present, it acts parallel to the plane and opposes the motion (or attempted motion). The frictional force is calculated as described above (f s or f k ).

Example: A farmer pushes a plough up a hill. The force they need to apply must overcome both the component of gravity pulling the plough down the hill (mg sin θ) and any frictional forces. 2.6 Terminal Velocity Definition: Terminal velocity is the constant speed that a freely falling object eventually reaches when the force of air resistance (drag) equals the force of gravity.

Explanation: When an object falls, gravity accelerates it downwards. As its speed increases, the air resistance acting upwards also increases. Eventually, the air resistance equals the weight of the object (mg). At this point, the net force is zero, and the object stops accelerating, falling at a constant velocity - the terminal velocity.