PH4 definitions

Question 1

Period T for a point describing a circle

Answer 1

Time taken for one complete circuit.

Question 2

Frequency f

Answer 2

The number of circuits or cycles per second.

Question 3

Angular velocity, w

Answer 3

For an object describing a circle at uniform speed, the angular velocity w is equal to the angle pheta swept out by the radius in time t divided by t . (w= pheta /t) UNIT: [rad] s-1

Question 4

Simple harmonic motion (shm)

Answer 4

Shm occurs when an object moves such that its acceleration is always directed toward a fixed point and proportional to its distance from the fixed point. (a = –(w^2)x)

Question 5

Period T for an oscillating body

Answer 5

The time taken for one complete cycle.

Question 6

Amplitude A of an oscillating object

Answer 6

The maximum value of the object’s displacement (from its equilibrium position).

Question 7

Phase

Answer 7

The phase of an oscillation is the angle (wt + E) in the equation x = A sin (w t +E). [E is called the phase constant.]
UNIT: rad

Question 8

Free oscillations

[Natural oscillations]

Answer 8

Free oscillations occur when an oscillatory system (such as a mass on a spring, or a pendulum) is displaced and released.
[The frequency of the free oscillations is called the system’s natural frequency.]

Question 9

Damping

Answer 9

Damping is the dying away, due to resistive forces, of the amplitude of free oscillations.

Question 10

Critical damping

Answer 10

Critical damping is the case when the resistive forces on the system are just large enough to prevent oscillations occurring at all when the system is displaced and released.

Question 11

Forced oscillations

Answer 11

These occur when a sinusoidally varying ‘driving’ force is applied to an oscillatory system, causing it to oscillate with the frequency of the applied force.

Question 12

Resonance

Answer 12

If, in forced vibrations, the frequency of the applied force is equal to the natural frequency of the system (e.g. mass on spring), the amplitude of the resulting oscillations is large. This is resonance.

Question 13

Momentum

Answer 13

The momentum of an object is its mass multiplied by its velocity.
(p = mv). It is a vector. UNIT: kg m s-1

Question 14

Newton’s laws of motion: 1st law

Answer 14

An object continues moving at constant speed in a straight line, or remains at rest, unless acted upon by a resultant force.

Question 15

Newton’s laws of motion: 2nd law

Answer 15

The rate of change of momentum of an object is proportional to the resultant force acting on it, and takes place in the direction of that force.

Question 16

Newton’s laws of motion: 3rd law

Answer 16

If a body A exerts a force on a body B, then B exerts an equal and opposite force on A.

Question 17

The principle of conservation of momentum

Answer 17

The vector sum of the momenta of bodies in a system stays constant even if forces act between the bodies, provided there is no external resultant force.

Question 18

Elastic collision.

Answer 18

A collision in which there is no change in total kinetic energy.

Question 19

Inelastic collision.

Answer 19

A collision in which kinetic energy is lost.

Question 20

Boyle’s law

Answer 20

For a fixed mass of gas at constant temperature [unless its density is very high], the pressure varies inversely as the volume. (pV = k).

Question 21

Ideal gas

Answer 21

An ideal gas strictly obeys the equation of state
pV = nRT, in which n is the number of moles, T is the kelvin temperature and R is the molar gas constant. R = 8.31 J mol-1K-1. Except at very high densities a real gas approximates well to an ideal gas.

Question 22

The mole

Answer 22

The mole is the S.I. unit of ‘amount of substance’, n. It is the amount containing as many particles (e.g. molecules) as there are atoms in 12 g of carbon12.

Question 23

Avogadro constant NA

Answer 23

This is the number of particles per mole. (NA=6.021023 mol-1).

Question 24

Internal energy, U, of a system

Answer 24

This is the sum of the kinetic and potential energies of the particles of the system.

Question 25

Heat

Answer 25

This is energy flow from a region at higher temperature to a region at lower temperature, due to the temperature difference. In thermodynamics we deal with heat going into or out of a system. It makes no sense to speak of heat in a system.

Question 26

Work

Answer 26

If the system is a gas, in a cylinder fitted with a piston, the gas does work of amount pV when it exerts a pressure p and pushes the piston out a small way, so the gas volume increases by V. Work, like heat, is energy in transit from (or to) the system.

Question 27

First law of thermodynamics

Answer 27

The increase, dU, in internal energy of a system is dU = Q – W in which Q is the heat entering the system and W is the work done by the system. Any of the terms in the equation can be positive or negative, e.g. if 100 J of heat is lost from a system Q = –100 J.

Question 28

Specific heat capacity c.

Answer 28

The heat required, per kilogram, per degree Celsius or Kelvin, to raise the temperature of a substance. UNIT: J kg-1 K-1 or J kg-1°C-1

Question 29

Newton’s law of gravitation.

Answer 29

The gravitational force between two particles is proportional to the product of their masses, m1 and m2, and inversely proportional to their separation squared, r2. F = G m1m2/r2 in which G is the gravitational constant. G = 6.67  10-11N m2 kg-2.

Question 30

Coulomb’s Law

Answer 30

The electrostatic force, F, between two small bodies is proportional to the product of their charges, Q1 and Q2, and inversely proportional to their separation squared, r2. F = Q1Q2/40r2 in which 0 is the permittivity of free space. 0 = 8.85  10-12Fm-1.

Question 31

Electric field strength E.

Answer 31

The force experienced per unit charge by a small positive charge placed in the field. Unit: V m-1 or N C-1.

Question 32

Gravitational field strength g.

Answer 32

The force experienced per unit mass by a mass placed in the field. Unit: m s-2 or N kg-1.

Question 33

Electric potential VE.

Answer 33

Electric potential at a point is the work done per unit charge in bringing a positive charge from infinity to that point. Unit: V. [= JC-1]

Question 34

Gravitational potential Vg.

Answer 34

Gravitational potential at a point is the work done per unit mass in bringing a mass from infinity to that point. Unit: Jkg-1.

Question 35

Kepler’s laws of planetary motion: 1

Answer 35

Each planet moves in an ellipse with the Sun at one focus.

Question 36

Kepler’s laws of planetary motion: 2

Answer 36

The line joining a planet to the centre of the Sun sweeps out equal areas in equal times.

Question 37

Kepler’s laws of planetary motion: 3

Answer 37

T^2, the square of the period of the planet’s motion, is proportional to r3, in which r is the semi-major axis of its ellipse. [For orbits which are nearly circular, r may be taken as the mean distance of the planet from the Sun.]

Question 38

Dark matter

Answer 38

Matter which we can’t see, or detect by any sort of radiation, but whose existence we infer from its gravitational effects.

Question 39

Radial velocity of a star [in the context of Doppler shift]

Answer 39

This is the component of a star’s velocity along the line joining it and an observer on the Earth.