Physics flash cards

Topic 2

Distance

• A scalar quantity which measures how far two locations are apart from each other along a certain path.

Displacement

• A vector quantity defined by the length and direction of the line segment joining the initial and final positions of an object.

Speed

• Change of distance to time

Velocity

• Change of displacement to time

Acceleration

• Rate of change of velocity

Displacement- time graph

Velocity- time graph

Acceleration- time graph

Projectile motion

• An object is said to undergo projectile motion when it follows a curved path due to the influence of gravity.

• with no air resistance

  • The horizontal component of velocity is constant

  • The vertical component of velocity accelerates downwards at 9.81m/s^2

  • The projectile reaches its maximum height when its vertical velocity is zero

  • The trajectory is symmetric

Changes of projectile motion with air resistance

• The maximum height of the projectile is lower

• The range of the projectile is shorter

• The trajectory is not symmetric

Terminal- velocity

• Net force acting on a body is zero

Translational equilibrium

• A body is said to be in translational equilibrium if it the net force acting on the body is zero. This means the body is either at rest or travels at constant velocity. For example:

  • Mass hanging at rest

  • Elevator moving upwards at constant velocity

  • Parachutist reaching terminal velocity

Newton’s first law

• Newton’s First Law (Law of Inertia) states that a body remains at rest or travels with constant speed along a straight line unless acted upon by an external force. (Net force = 0)

Newton’s second law

• Newton’s Second Law states that net force is directly proportional to acceleration and to mass. (F=ma)

Newton’s third law

• Newton’s Third Law states that if a body A exerts a force on body B, then body B exerts a force of the same magnitude but in the opposite direction of body A.

  This pair of forces is called an action-reaction pair, which must act on two different bodies.

Static friction

• Static friction is that which stops objects from beginning to move

Dynamic friction

• Kinetic friction is that which slows objects down when they are moving

Kinetic energy

• Kinetic energy (KE) is the energy of a body due to its motion and is given by the equation

Gravitational potential energy

• The gravitational potential energy (GPE) of an object changes with its height and is given by the equation

Elastic potential energy

• Elastic energy is potential energy stored as a result of the deformation of an elastic object such as the stretching of a spring and is given by the equation

Work done

• Work done measures the transfer of energy due to a force and is a scalar quantity.

  The work done W by a force F on an object is given by the equation

  

• In a force- displacement graph, work is the area under the graph

Power as energy transfer

• Power (P) is the work done or the energy output per time given by the equation:

  

• For constant force acting on an object with constant velocity, the power is given by the equation: P=Fv.

Conservation of energy

• Energy can neither be created nor destroyed; it can only be changed from one form to another. For example:

  • An electrical heater transforms electrical energy to thermal energy.

  • A falling object transforms potential energy to kinetic energy.

  Total energy of an isolated body remains constant. In other words, ΔKE+ΔPE=0

Efficiency

• Efficiency is the ratio of useful energy output to energy input as a percentage

Momentum

• The linear momentum (p) is a vector with the same direction as the velocity of an object.

  The change of momentum of an object is called impulse.

Impulse

• Area of a force- time graph

Conservation of linear momentum

• The law of conservation of linear momentum states that the sum of initial momentum is equal to the sum of final momentum in a closed system and can be given by the equation

Elastic collision

• Momentum conserved

• Kinetic energy conserved

Inelastic collision

• Momentum conserved

• Kinetic energy not conserved

Explosion

• Momentum conserved

• Kinetic energy not conserved

Topic 3

Temperature

• Temperature is a measure of average kinetic energy inside a body

• Measured in kelvin and absolute zero is -273 C

Internal energy

• Internal energy is the sum of total kinetic energy (total thermal energy) and total potential energy.

• Kinetic energy is energy associated with the random/translational rotational motions of molecules.

• Potential energy is associated with forces between molecules.

Specific heat capacity

• Defined by the amount of heat needed to raise the temperature of 1kg of the substance by 1K.

Phase change (KE, PE)

Specific latent heat of fusion

• The amount of heat required to change 1kg of a substance from solid to liquid without any change in temperature

Specific latent heat of vaporisation

• The amount of heat required to change 1kg of a substance from liquid to gas without any change in temperature.

Pressure

• Pressure is defined as the normal force per unit area

Equation for ideal gas

Kinetic model of an ideal gas

• Assumptions:

  • The collisions between molecules are perfectly elastic.

  • The molecules are identical spheres.

  • The volume of molecules is negligible compared to the volume of the gas.

  • Molecules do not interact with each other except when they are in constant.

• Implications:

  • Absolute temperature is directly proportional to the average KE and average speed of the molecules of an ideal gas.

Mole

• Like the word “dozon”, a mole is a unit of quantity. It is used to measure the number of atoms or molecules.

• A mole of any material contains 6.022*10^23 atoms or molecules. The value 6.022*10^23 is called the Avogadro constant.

• The number of moles of a substance can be calculated by dividing the number of molecules of that substance by the Avogadro constant.

Molar Mass

• The molar mass is the mass of 1 mole of any element or compound.

• Different materials/elements have different molar masses which can be found as the Mr in the periodic table.

Differences between real and ideal gases

• The ideal gas is based on a list of assumptions stated previously. However, in real gases, such assumptions may not be true.

  • Forces exist between gas molecules in real gases (intermolecular forces).

  • The volume of molecules is not negligible compared to the volume of gas in real gases.

• Real gases may behave similarly to ideal gases under high temperatures and low pressure.

Topic 4

Simple harmonic motion/oscillations

• Oscillations are periodic motions which center around an equilibrium position.

• An object undergoes SHM if it experiences a force which is proportional and opposite of the displacement from its equilibrium position.

Period of a pendulum

Period of a mass- spring

Oscillation displacement

• Displacement of the oscillating object at a specific time from its equilibrium position

Oscillation amplitude

• Maximum displacement of the oscillating object

Oscillation Period

• Time taken for one complete oscillation (in seconds)

Oscillation Frequency

• Number of times the object oscillates per unit time (usually one second)

  f=1

Phase difference

• The difference between two SHMs with the same frequency in terms of their relative position in a cycle measured in radian

Conditions for simple hamonic motion

• When the body is displaced from equilibrium, there must exist a restoring force (a force that wants to pull the body back to equilibrium).

• The magnitude of the restoring force must be proportional to the displacement of the body and acts towards the equilibrium.

Travelling waves

• A travelling wave is a continuous disturbance in a medium characterized by repeating oscillations.

  • Energy is transferred by waves.

  • Matter is not transferred by waves.

  • The direction of a wave is defined by the direction of the energy transfer.

Waves speed

Transverse waves

• The direction of oscillation is perpendicular to the direction of the wave

• ex. Water waves

• A point with maximum positive displacement is called a crest.

• A point with minimum displacement is called a trough.

Longitudinal wave

• The direction of oscillation is parallel to the direction of the wave

• Ex. sound waves

• A region where particles are closed to each other is called a compression.

• A region where particles are furthest apart from each other is called a rarefaction.

Nature of electromagnetic waves

• All EM waves travel in vacuum at the same speed of 3*10^8m/s.

• EM waves are transverse waves.

• 

Nature of sound waves

• The speed of sound in 20 degrees Celsius dry air is approximately 343.2m/s.

• Sound waves are longitudinal waves.

• 

Wave fronts

• Lines joining points which vibrate in phase.

• Can be straight lines or curves.

• The distance between successive wavefronts is the wavelength of the wave.

Rays

• Lines which indicate the direction of wave propagation.

• Rays are perpendicular to wavefronts.

Amplitude

• The amplitude and intensity of a wave depends on its energy.

• The intensity of a wave is proportional to the square of its amplitude (I∝A^2)

Superposition

• The left shows constructive interference (superposition) where the two waves add up (e.g. 1+1=2). The right shows deconstructive interference (superposition) where the two waves cancel each other (e.g. 1+(-1)=0).

Polarization

• Light is a transverse wave (polarization only occur to transverse waves).

• The polarization of light refers to the orientation of the oscillation in the underlying electric field.

• Light is plane polarized if the electric field oscillates in one plane.

Polarizers

• Sheet of material that polarizes light

• Unpolarized light passes through and intensity is reduced by 50%

Analyzer

• Polarized light passes through a polarizer, intensity will be reduced by a factor dependent on the orientation of the polarizer. This property allows us to deduce the polariation of light by using the polarizer

• A polarizer used for this purpose is called an analyzer

Malu’s Law

• relates the incident intensity and transmitted light passing through a polarizer and an analyzer

• 

Reflection

• Angle of incidence = angle of relfection

• Reflection of waves from a fixed end is inverted

• Reflection of waves from a free end is not inverted

Refraction

• The change in direction of a wave when it transmits from one medium to another

• The angle of incidence and the angle of the refraction can be determined by Snell’s law

• 

• n1 and n2 are related by

• 

• Where v1 and v2 are the speed of the wave in each medium

Critical angle

• If the angle of incidence is equal to the critical angle, most of the light travles along the surface and some is reflected back.

• If the angle of incidence is greater than the critical angle all the light is reflected back

• 

• It must come from an optically denser medium to an optically less dense medium

Single- slit diffraction

• A wide slit will create a narrower diffraction pattern than a narrow slit

• There is one max and then the other peaks are smaller

Diffraction around objects

• Sound waves are able to bend around objects and you are therefore able to hear what is going on even though you can’t see it.

Interference patterns

• Maximums form at constructive interference, when the path difference is a full wavelength apart

• Minimums form at deconstructive interference, when the path difference is a multiple of half wavelengths

Double slit interference

• Double slit interference has the same pattern as single slit, however each peak is divided into smaller peaks and it has more minimums

Standing waves

• Energy is not transferred by standing waves

• When a standing wave hits a wall, it is reflected back identically by reflected

• Superposition is two identical waves that are the same at the same time so it becomes double

Boundary conditions

• Antinodes are open ends and nodes are closed ends

• For a closed and open end pipe the wavelength is 4L/n where n is the harmonic

• For a string with two closed ends the wavelength is s2L/n and same for a pipe with two open ends

Nodes and antinodes

• Fixed positions also known as minimum are nodes and the positions of largest displacement is antinodes.

• The distance between nodes are half a wavelength

Difference between Standing and Travelling waves