Overview
This lecture covers fundamental concepts in physics including physical quantities, motion, forces, Newton's laws, momentum, deformation of materials, and moments.
Physical Quantities: Scalars and Vectors
- Physical quantities consist of a numerical magnitude and a unit.
- Scalars have only magnitude (e.g., distance, speed, mass, energy).
- Vectors have magnitude and direction (e.g., displacement, velocity, force, momentum).
Motion: Distance, Displacement, Speed, and Velocity
- Distance is the total length of path traveled (scalar, meter).
- Displacement is the directed distance from start to end (vector, meter).
- Speed is total distance moved per unit time (scalar, m/s).
- Velocity is speed in a given direction (vector, m/s).
Acceleration and Equations of Motion
- Acceleration is change in velocity per unit time (vector, m/s²).
- Equation: a = (v - u)/t, where a = acceleration, v = final velocity, u = initial velocity, t = time.
- If velocity is constant, acceleration is zero.
Graphical Representation of Motion
- Distance-time graph: gradient represents speed.
- Velocity-time graph: gradient represents acceleration; area under graph is distance moved.
- Horizontal line on distance-time graph = object at rest; constant gradient = constant speed.
- Curved line indicates changing speed (acceleration/deceleration).
Forces and Their Effects
- Force is a vector (unit: Newton).
- Contact forces: push, pull, friction, tension, normal reaction.
- Non-contact forces: gravity, electrostatic, magnetic.
- The resultant force is the single force with the same effect as all combined forces.
Types of Friction and Stopping Distances
- Static friction prevents motion, kinetic friction opposes sliding, fluid friction (drag) opposes motion in fluids.
- Stopping distance = reaction distance + braking distance.
- Factors: speed, mass, road conditions, reaction time, braking efficiency.
Newton's Laws of Motion
- First Law: Balanced forces mean object is at rest or moves with constant velocity.
- Second Law: Resultant force causes acceleration, F = m × a.
- Third Law: Every action has an equal and opposite reaction.
Momentum and Its Conservation
- Momentum: p = m × v (vector, kg m/s).
- Total momentum is conserved in collisions and explosions.
- Change in momentum over time relates to force: F = (mv - mu)/t.
Deformation of Materials: Hooke’s Law
- Extension of spring is proportional to applied force up to elastic limit (Hooke’s Law).
- Beyond elastic limit, deformation becomes plastic and irreversible.
- Elastic deformation: returns to original length; plastic deformation: permanent change.
Moments and Equilibrium
- Moment = force × perpendicular distance from pivot (unit: Nm).
- Clockwise and anticlockwise moments can balance for equilibrium.
- Principle of moments: total clockwise moment = total anticlockwise moment for objects in equilibrium.
Center of Gravity
- The point where the total weight of an object acts.
- Uniform objects have center of gravity at their geometric center.
Key Terms & Definitions
- Scalar — Quantity with magnitude only.
- Vector — Quantity with both magnitude and direction.
- Resultant force — Net force acting on an object.
- Momentum — Product of mass and velocity; tendency to keep moving.
- Moment — Turning effect of a force about a pivot.
Action Items / Next Steps
- Practice sketching and interpreting distance-time and velocity-time graphs.
- Complete assigned problems on applying Newton's laws and conservation of momentum.
- Review Hooke’s Law and graph interpretations for different materials.