NSSCO Physics (6118) flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

SI unit of force

The newton (N) is the force that gives a 1 kg mass an acceleration of 1 m/s^2; in base units 1 N = 1 kg m s^-2.

Scalar versus vector

A scalar has magnitude only, such as mass, speed, time and energy. A vector has both magnitude and direction, such as force, velocity, displacement and acceleration.

Density formula

rho = m/V, mass per unit volume in kg m^-3. Measure mass with a balance; find volume of a regular solid from its dimensions, or of an irregular solid by displacement.

Micrometer screw gauge precision

Reads to 0.01 mm (0.001 cm) and is used for the diameter of thin wire or the thickness of a sheet. Record the zero reading and subtract it from each measurement.

Equation for constant acceleration

v = u + at links final velocity, initial velocity, acceleration and time; it applies only when acceleration is uniform, not when the acceleration changes during the motion.

Newton's first law

An object stays at rest, or keeps moving in a straight line at constant speed, unless a resultant external force acts on it. It is also called the law of inertia.

Conservation of momentum

With no external resultant force, total momentum before a collision or explosion equals total momentum after: m1u1 + m2u2 = m1v1 + m2v2.

Terminal velocity

The constant speed reached when the upward drag on a falling object equals its weight; the resultant force is then zero, so the acceleration is zero.

Work done

W = F x d, where d is the distance moved along the direction of the force; it is measured in joules, and 1 J = 1 N m.

Kinetic energy formula

KE = 0.5mv^2; the energy of motion depends on mass and on the square of the speed, so doubling the speed increases kinetic energy four times.

Pressure in a liquid

p = rho g h: pressure at depth h depends on liquid density, gravitational field strength and depth. It acts equally in all directions and is independent of container shape.

Archimedes' principle

The upthrust on an object in a fluid equals the weight of fluid it displaces. An object floats when upthrust equals its weight and sinks when its weight is greater.

Specific heat capacity

The energy needed to raise the temperature of 1 kg of a substance by 1 K. Q = mc(delta theta); water's value is about 4200 J kg^-1 K^-1.

Specific latent heat

The energy needed to change the state of 1 kg of a substance at constant temperature. Q = mL; the latent heat of vaporisation is larger than the latent heat of fusion.

Boyle's law

For a fixed mass of gas at constant temperature, pressure multiplied by volume is constant: p1V1 = p2V2. Doubling the pressure therefore halves the volume.

Heat transfer methods

Conduction in solids by vibrating particles and free electrons; convection in fluids by density differences; radiation by infrared waves that need no medium and pass through vacuum.

Wave equation

v = f lambda: wave speed equals frequency multiplied by wavelength. During refraction the frequency stays constant, but the speed and the wavelength both change.

Transverse and longitudinal waves

Transverse waves oscillate perpendicular to the direction of travel, like light and water ripples. Longitudinal waves oscillate parallel to travel, forming compressions and rarefactions, like sound.

Diffraction

The spreading of waves as they pass through a gap or around an edge. It is most noticeable when the gap width is about the same as the wavelength.

Law of reflection

The angle of incidence equals the angle of reflection, both measured from the normal; the incident ray, the reflected ray and the normal lie in the same plane.

Refractive index

n = sin i / sin r for light entering a medium, and n = c/v, the ratio of the speed of light in vacuum to that in the medium; n is always at least 1.

Critical angle

The angle of incidence in the denser medium for which the refracted ray grazes the boundary, so the angle of refraction is 90 degrees. Beyond it, total internal reflection occurs.

Speed of sound

About 340 m/s in air at room temperature. Sound travels faster in liquids and solids than in gases, and it cannot travel through a vacuum.

Real and virtual images

A real image forms where light rays actually meet, is inverted and can be shown on a screen. A virtual image cannot be projected onto a screen and is upright.

Ohm's law

The current through a metallic conductor is directly proportional to the potential difference across it, provided the temperature stays constant: V = IR.

Resistors in parallel

1/R = 1/R1 + 1/R2 + ...; the total resistance is less than the smallest single resistor, and the supply current splits between the parallel branches.

Electrical power

P = VI = I^2R = V^2/R; the energy transferred is E = Pt, often measured in kilowatt-hours for household electricity bills.

Fleming's left-hand rule

For a current-carrying conductor in a magnetic field: first finger = field, second finger = current, thumb = force or motion. Field and current must be perpendicular.

Alpha particle

A helium nucleus with 2 protons and 2 neutrons. It is strongly ionising but has a short range, being stopped by paper or a few centimetres of air.

Half-life

The time taken for half the nuclei in a radioactive sample to decay, or for the activity to halve. Temperature, pressure and chemical bonding do not change it.

Fission and fusion

Fission splits a heavy nucleus such as uranium-235 into lighter nuclei, releasing energy. Fusion joins light nuclei such as hydrogen isotopes, releasing more energy per kilogram.

Isotopes

Atoms of the same element with the same number of protons but different numbers of neutrons, so they share a proton number but have different nucleon numbers.