Understanding Forces, Weight, and Energy

Forces

• Forces are either contact forces needing physical touch or non-contact forces like magnetism, electrostatic forces, and gravity.
• Normal contact force examples: pushing a door, while friction, air resistance, and tension are other contact forces.
• Forces are vectors with direction and magnitude, shown with arrows.
• Resultant force uses vector addition, with opposite directions as negative.
• Right angle vectors use the Pythagorean theorem to find resultant force.
• Balanced forces mean zero resultant force; objects maintain constant velocity as per Newton's first law.
• Scalar quantities have magnitude only; vectors have magnitude and direction.
• Displacement includes direction, velocity is speed with direction.

Weight and Energy

• Weight, a gravitational force, equals mass (kg) times gravitational field strength (g), about 9.8 N/kg on Earth.
• 1 kg mass on Earth weighs about 10 Newtons.
• Lifting at constant speed needs a force equal to its weight.
• Work done: force times distance moved, equals energy transferred.
• Gravitational potential energy (GPE) gained/energy gained when lifting an object: mgh.

Hooke's Law

• Hooke's Law: Force (F) = spring constant (k) * extension (e), valid for elastic deformation.
• Spring constant (N/m) indicates stiffness.
• Force and extension are directly proportional.
• Hanging masses from a spring and measuring extension gives a straight line through the origin.
• Align ruler zero mark with the spring bottom and measure at eye level to prevent Parallax error.
• Energy stored in a spring: 1/2 * ke^2.
• Released spring: kinetic energy gained equals stored energy in a closed system.

Turning Forces

• Moment calculation: force * perpendicular distance to pivot, in Newton-meters (Nm).
• Balanced moments: clockwise = anticlockwise, maintaining equilibrium, known as the principle of moments.
• Gears apply moments; small gears turn larger ones to increase moment production.

Pressure

• Pressure: force concentration, calculated as force / area, in N/m² or Pascals (Pa).
• Water pressure increases with depth: p = hρg (h=depth, ρ=density).
• Gas pressure stems from gas particle collisions, increased by adding gas, reducing volume, or raising temperature.
• Atmospheric density and pressure decrease with altitude.

Speed, Velocity, and Acceleration

• Speed and velocity are in meters per second (m/s), velocity includes direction.
• Typical speeds: walking at 1.5 m/s, running at 3 m/s, cycling at 6 m/s, driving at 13-30 m/s.
• Speed/velocity calculation: distance/displacement over time.
• Distance-time graph gradient: speed/velocity; curves use tangents.
• Velocity-time graph gradient: acceleration (m/s²).
• Area under velocity-time graph: distance traveled; area below 0 m/s is negative displacement.

Suvat Equations

• Suvat equations relate displacement (s), initial velocity (u), final velocity (v), acceleration (a), and time (t).
• Only one equation is provided on the formula sheet.
• Initial velocity (u) is zero if starting at rest.
• Acceleration (a) same as G in free fall .
• List variables with known and unknown values, choose and rearrange equation.

Newton's Law of Motion

• Newton's first law: objects maintain constant motion unless a resultant force acts; inertia describes this.
• Newton's second law: Resultant Force (F) = mass (m) * acceleration (a); F=ma.
• Proportional relationship between Force and Acceleration can be shown in a practical
• Newton's third law: Every action Force has an equal, opposite reaction Force (e.g., Earth pulls down on an object, and the object pulls up on the Earth).

Stopping Distance and Momentum

• Overall stopping distance: thinking distance + braking distance.
• Doubling speed doubles thinking distance, quadruples braking distance due to kinetic energy increase.
• Thinking distance factors: distractions, alcohol, drugs; braking distance factors: brakes, tires, road, weather.
• Momentum: how hard it is to stop something, mass * velocity.
• Collision: kinetic energy may not be conserved, but total momentum is.
• Momentum Formula: M1U1 + M2U2 = M1V1 + M2V2.
• Force equals rate of change of momentum.
• Seatbelts, airbags, and crumple zones increase impact time, reducing the force felt.

Waves

• Waves transfer energy via oscillations/vibrations, without transferring matter.
• Longitudinal waves: oscillations are parallel to energy direction (e.g., sound, seismic P waves, with compressions and rarefactions).
• Transverse waves: oscillations are perpendicular (e.g., water waves, seismic S waves, light, EM waves).
• Waveform: displacement vs. distance or time; amplitude is max displacement.
• Wavelength (λ): length of one complete wave (meters).
• Time period (T): time for one complete wave (seconds).
• Frequency (f): waves per second (Hertz); frequency = 1 / time period.

Wave Equation

• Wave Equation: V = F λ (wave speed (v) = frequency (f) * wavelength (λ)).
• Ripple tank measures wavelength, frequency, and speed of waves.
• Wave travel is measured through a tray.
• Sound speed measured by claps next to a microphone connected to an oscilloscope, timing echoes off a wall.

Soundwaves and Ultrasound

• Human hearing range: 20 Hz-20 kHz.
• Ultrasound: Frequencies above 20kHz.
• Sound reflects off of the material boundaries.
• Sound reflection times from different layers of the material are Measured and processed to determine the location of the boundary
• Used in medical (scanning babies) and sonar (waves in water) applications.
• P waves transmit through liquids; S waves cannot.
• Angle of incidence = angle of reflection.

EM Waves

• EM waves travel without a medium.
• Produced when electrons lose energy.
• Higher frequency corresponds to more energy, shorter wavelength, and increased absorption by electrons.
• UV/X-rays/gamma rays can cause electrons to leave atoms, ionizing them and damaging DNA.

Refraction

• Medium change = speed change (e.g., air to glass slows speed).
• Wavelength decreases in refraction.
• The Ray movement changes refraction.

Lenses

• Curved glass blocks that refract rays.
• Convex lenses converge rays.
• Concave lenses diverge rays.
• Convex lenses: parallel rays converge at the principal focus.
• The intersection of the extensions of diverging rays forms the virtual image where the object is closely placed.
• Concave lenses always give virtual images.
• Magnification: image height / object height; > 1 means magnified.

Black Body

• Absorbs and emits all wavelengths.
• Absorbing more leads to increased temp & rate increases to match.

Magnetism

• Permanent magnet: aligned molecules create a magnetic field, exerting force due to particles/electrons.
• Magnets have north/south poles.
• Iron mini compasses visualize the field.
• Magnetic field lines never cross, exit north, enter south, forming complete loops.
• Induced magnets temporarily align in a magnetic field.
• Iron is attracted to a magnet

Magnetism and Wire experiments

• Magnets attract/repel each other depending on poles.
• Current-carrying wire: has a concentric magnetic field, use the right-hand rule, B denotes magnetic field.
• Motor effect: Force depends on current (I), Length, (L), and Magnetic Field (B).
• Electric motors: use the motor effect with coil sides pushing in opposite directions to turn.
• Loudspeakers: use backwards and forwards motor.
• Induced/generator effect: wire moved in a magnetic field generates current.
• Dynamos do the opposite.

Transformers

• Transformers change voltages in grid wires.
• Step-up Transformer: increases voltage before grid entry to reduce current and energy loss.
• Electrical power: V * I.
• Power in = power out.
• No electricity in the core, but AC flows through the wires.
• The core contains primary and secondary coils.
• Alternating current produces a magnetic field; the iron core guides it.
• The ratio of turns in the coil = Ratio of the Voltage