Overview
This lecture covers IGCSE Physics Chapter 3: Forces and Motion, focusing on different types of forces, gravity, the relationship between force, mass, and acceleration, momentum, impulse, and distinguishing scalar from vector quantities.
Types of Forces
- A force is a push or pull that affects the motion of objects; measured in Newtons (N).
- Types of forces: weight (gravitational force), contact force, friction, air resistance (for air), drag (for liquids), and upthrust (in fluids).
- Unbalanced forces change an object's speed or direction.
- Newton's First Law states an object remains at rest or moves at constant speed unless acted upon by a resultant force.
- Resultant force is the sum of all forces, considering their directions.
Gravitational Force and Weight
- Gravity pulls objects toward Earth, causing acceleration known as free fall (acceleration due to gravity, g ≈ 9.8 m/s² on Earth).
- Weight (W) is the gravitational force acting on a mass: W = mg.
- Mass is the amount of matter (kg), weight depends on location (varies with gravitational field strength, g).
- Gravitational field strength is the force per kg at a location (Earth: 9.8 N/kg, Moon: 1.6 N/kg, Jupiter: 23 N/kg).
Free Fall, Air Resistance, and Terminal Velocity
- All objects fall at the same acceleration in a vacuum; air resistance causes lighter objects to fall slower.
- When falling, initial acceleration is due to weight, but as speed increases, air resistance increases.
- Terminal velocity is reached when weight equals air resistance and speed becomes constant.
- Opening a parachute increases air resistance, slowing down descent.
Circular Motion and Forces
- To move in a circle, an object needs a force toward the center (centripetal force).
- The centripetal force increases with greater mass, speed, or smaller radius.
- Direction of circular motion force is always toward the center.
Newton’s Second Law: Force, Mass, and Acceleration
- Newton’s Second Law: F = ma (force = mass × acceleration).
- More mass or acceleration requires greater force.
- Resultant force is the sum of all forces (subtract forces in opposite directions).
Momentum and Impulse
- Momentum (p) measures the motion of an object: p = mv (mass × velocity).
- Impulse is the change in momentum: impulse = force × time = change in momentum.
- Increasing contact time (e.g., using airbags or cushions) reduces the force during impacts.
Conservation of Momentum
- In collisions, total momentum before equals total momentum after (conservation of momentum).
- Momentum lost by one object is gained by another.
Scalars and Vectors
- Scalar quantities have magnitude only (e.g., mass, time, speed).
- Vector quantities have magnitude and direction (e.g., force, velocity, acceleration).
- Resultant force calculation requires considering directions of all forces.
Key Terms & Definitions
- Force — a push or pull acting on an object (measured in Newtons, N).
- Weight — gravitational force acting on a mass (W = mg).
- Mass — measure of matter in an object (kg); does not change with location.
- Gravitational field strength (g) — force per kg at a point (N/kg).
- Terminal velocity — constant speed reached when air resistance equals weight.
- Momentum (p) — quantity of motion; p = mv.
- Impulse — change in momentum; impulse = F × t.
- Scalar quantity — a measurement with size only.
- Vector quantity — a measurement with size and direction.
- Resultant force — net force from combining all acting forces.
Action Items / Next Steps
- Review and practice problems on calculating weight, resultant force, momentum, and impulse.
- Ensure understanding of Newton’s laws and their applications.
- Prepare for next lesson on "moment."