Unit 1.1 - Basic Physics Flashcards
What includes the 7 basic units?
The SI system
What does the SI system stand for?
Systems Internationale d’Unites
What can all units be derived from?
The 7 basic units of the SI system
What’s good about the 7 basic units of the SI system?
-Have been agreed internationally
-Do not vary over time
-All can be reproduced by observing physical phenomena
What can all the 7 basic units be observed using?
Physical phenomena
Which one of the 7 basic units of the SI system is an exception? Why?
Mass
-the kilogram is based on a prototype kept in Paris (a lump of metal - “the big K”)
-the only one with a prefix in its name (for historical reasons)
What is the kilogram based on?
A prototype kept in Paris - the lump of metal known as the “big K”
Which is the only basic unit with a prefix in its name and why?
The kilogram (kilo) for historical reasons
Mass symbol, basic unit and symbol
m, kilogram, kg
Length symbol, basic unit and symbol
l, metre, m
Time symbol, basic unit and symbol
t, second, s
Temperature symbol, basic unit and symbol
T, Kelvin, K
Electric current symbol, basic unit and symbol
l, ampere, A
Amount of substance symbol, basic unit and symbol
n, mole, mol
Electric current symbol
l
What’s the new way we’re calculating the kg as the unit of the big k has previously changed over time?
Pure silicon with 1 isotope - can calculate the amount of atoms in it
Why do we use Kelvin for temperature as the basic unit?
It goes down to absolute zero
What IS a metre?
The length travelled by light in a vacuum during 1 ÷ the speed of light
What are the derived units made up of?
The basic units
Area symbol, derived unit name and symbol
A, square metre, m2
Volume symbol, derived unit name and symbol
V, cubic metre, m3
Density symbol, derived unit name and symbol
d, kilogram per cubic metre, kgm-3
Velocity symbol, derived unit name and symbol
v, metre per second, ms-1
Acceleration symbol, derived unit name and symbol
a, metre per second squared, ms-2
Momentum symbol, derived unit name and symbol
p, kilogram metre per second, kgms-1
Show why area and volume’s units are what they are
Area - m x m = m2
Volume - m x m x m =m3
What’s p the symbol for?
Momentum
What’s l the symbol for?
Length OR electric current
What’s v the symbol for?
Velocity (big V is volume)
What is kilogram metre per second?
Momentum
What is metres per second squared?
Acceleration
What is kilogram per cubic metre?
Density
What do amperes measure?
Electric current
Why are some units given a special name?
They appear complicated if the basic units are shown
Force symbol, derived unit name and symbol
F, newton, N
Pressure symbol, derived unit name and symbol
P, pascal, Pa
Energy symbol, derived unit name and symbol
E, joule, J
Work symbol, derived unit name and symbol
E, joule, J
Power symbol, derived unit name and symbol
P, watt, W
Frequency symbol, derived unit name and symbol
f, Hertz, Hz
Electric charge symbol, derived unit name and symbol
Q, coulomb, C
Resistance symbol, derived unit name and symbol
R, ohm, Ω
Electromotive force symbol, derived unit name and symbol
E, volt, V
Potential difference symbol, derived unit name and symbol
V, volt, V
Coulomb (C) quantity name and symbol
Electric charge (Q)
Ohm quantity name and symbol
Resistance (R)
Hertz (Hz) quantity name and symbol
Frequency (f)
Pascal (Pa) quantity name and symbol
Pressure (P)
Newton (N) quantity name an symbol
Force (F)
Volt (v) quantity name and symbol
-Electromotive force (E)
-Potential difference (V)
Why do we use prefixes?
More convenient and quicker to use than standard form
Which multiples are used in Physics?
Multiples of 1000 generally (usually in 3 times table)
Homogeneity
The equation is the same throughout - same units on both sides
What’s the term for an equation being the same throughout?
Homogeneity
What do we use to represent a physical quantity? (give an example)
2kg
2 - numerical magnitude
Kg - unit abbreviation
What does a quantity stand for when it’s represented by a symbol? Give an example
A number AND a unit
E.g - m for mass
What do we call the process of finding the unit of quantity in an equation?
Quantity algebra
Quantity algebra
Finding out the unit of the quantity
How can we test to see if an incorrect mathematical equation has been used?
The unit for the required quantity shows up as being wrong
Which quantity do we need to be careful with in quantity algebra?
Mass with kg
If given n grams, we need to change it to kg - always go with the base unit!
Force symbol and worded equation (including unit symbols)
F = ma
Force (N) = mass (kg) x acceleration (ms-2)
Force’s derived unit expressed as its base units
N = kgms-2
Energy’s derived unit expressed as its base units
E = kgm2s-2
Power’s derived units expressed as base units
P = kgm2s-3
Work done symbol and worded equation (including unit symbols)
E = fd
Work done/energy (J) = force (N) x distance (m)
Kinetic energy symbol and worded equation (including unit symbols)
E = 0.5mv^2
Kinetic energy (J) = 0.5 x mass (kg) x (velocity (ms-1)^2
Potential energy symbol and worded equation (including unit symbols)
E = mgh
Potential energy (J) = mass (kg) x g (ms-2) x height (m)
Power symbol and worded equation (including unit symbols)
P = E
—
t
Power (W) = Energy (E)
————
Time (s)
What do all energy equations (what are they?) come down to and why?
Work done, kinetic energy and potential energy
All come down to the same base units
All measured in joules (J)
Frequency symbol and actual basic units
Hz, s-1
Do numbers have units?
No
what do we do if units are squared?
Square each individual base unit too
What is weight measured in?
Newtons
What doesn’t homogeneity show us?
Whether any of the units are constants
How do you work out the unit of a constant?
Make it the subject and literally work it out as if they were normal numbers (e.g - divided, the indices minus)
Then, cancel the relevant things and whatever’s left is the unit
Scalar
Only size
Vector
Size AND direction
8 examples of scalar quantities
Mass
Speed
Current
Distance
Mole
Density
Time
Pressure
5 examples of vector quantities
Force
Velocity
Acceleration
Displacement
Weight
Is displacement a vector or scalar measurement?
Vector
Is force a vector or scalar measurement?
Vector
Is pressure a vector is scalar measurement?
Scalar
Is current a vector or scalar measurement?
Scalar
How do we add scalar quantities?
Very easy - literally just add them for the total
What do we call the vector sum?
Resultant
What do we have to write for the resultant if directions are involved?
Say which direction it’s to
What does a free body diagram show?
The object as a point with all of the forces acting on it labelled in their respective directions
Directions and names of the forces for a free body diagram of an aeroplane
Up - lift
Down - weight
Right - thrust
Left - drag
What is the resultant if lift=weight?
The difference between Thrust and Drag (easy)
When do vectors start getting a little more complicated?
When the left and right (e.g - thrust and drag) and up and down (e.g - lift and weight) forces are not equal and we cannot simply calculate the difference between them
What would happen to a plane if its thust force is higher than its drag and its lift force is higher than its weight?
It travels upwards, to the right, which is the resultant
What can we do to vectors and why is this useful?
Move them independently to form right angled triangles so that we can use…
-Pythagoras theorem for the size of resultant
-Tan function for the direction of the resultant
What do we use for calculating the size of the resultant?
Pythagoras theorem
What do we use for calculating the direction of the resultant?
The Tan function (trigonometry) - remember you can move vectors independently to form right angled triangles!
How do we find out the horizontal and vertical components of a vector?
Use trigonometry!
What do we do if we get 2 non horizontal or vertical vectors?
Resolve the vector by finding out the vertical and horizontal components that make it up
What units do we use for the horizontal and vertical components?
The same as the resultant!
How do we get the initial velocity?
We need to know the vertical and horizontal velocities
What do we need to say about our direction of a vector if there’s no diagram?
What direction it is from the horizontal (eg. up from the horizontal)
What do need to put if we’re calculating a vector?
Both the resultant and the direction
Define work
W=Fd
(Distance travelled in the direction of the force)
What do we do to forces in the same direction?
Add them together
Coplanar vectors
Vectors which are acting on the same plane as each other
Vectors which are acting on the same plane as each other
Coplanar vectors
What type of quantity is weight?
Vector
What’s the word we should use for size when referring to scalar and vector?
Magnitude
Density
The amount of mass/matter that fits in a given volume
Density formula
P=m
—
V
(mass divided by volume)
What’s the symbol for density and what is this?
P
(rho, the greek “r”)
Density unit
kgm-3
Volume unit
m^3
Mass unit
kg
What is density usually part of? Give an example
Another question, for example…
To calculate the mass of water passing through a turbine to produce electricity
-density of water and volume per second would give the mass per second
What quantities would we need to calculate the mass of water passing through a turbine?
-Density of water
-Volume per second
1cm^3 in m^3
1cm^3 = 1
—
100^3
1x10-6m^3
1ml in cm^3
1cm^3
1cm^3 in ml
1ml
1gcm^-3 to kgm^-3
1000 kgm^-3
(so weirdly, if the answer is in gcm-3, you MULTIPLY by 1000 to get it in kgcm-3)
How do you convert from gcm-3 to kgm-3?
Multiply by 1000
What can you do with two quantities that have the same units (proven to be homogeneous)?
Add/subtract them
Additional scalar quantities from the textbook
Volume
Area
Work
Energy (all forms)
Power
Resistance
Refractive index
Temperature
Potential
Electric charge
Additional vector quantity from textbook
Momentum
What’s the word for finding the components of a force?
Resolving
If in a density question, the volume is given in cm3 and the density in kgm-3, what can we do?
Either…
-Convert the volume using 1cm3 = 1x10-6m-3
-Convert the density using 10gcm-3 = 1000kgm-3
Equilibrium
The turning effect of a force
What happens when a force acts at a distance from a pivot?
Its effect is amplified - given a mechanical advantage
When is the effect on a lever amplified?
When a force acts at a distance from a pivot
What type of advantage is given when the distance from the pivot has increased?
Mechanical
What is the principal behind levers?
Equilibrium
What is equilibrium the principal behind?
Levers
Moment
The size of the force multiplied by the perpendicular distance to the line of action of the force from the pivot
Fx
Moment equation
Fx
Moments symbol
(They don’t have one)
Do moments have a direction?
Yes, but don’t call it a vector!
What is the formula of a moment if the force acts at an angle?
Fxcos0 (theta)
What are the conditions for equilibrium?
The vector sum of all the forces = 0
The vector sum of all the moments about the same point = 0
What are the conditions that the vector sum of all of the forces =0 and the vector sum of all the moments =0?
Conditions for equilibrium
What would happen to an object if the condition for equilibrium that the vector sum of all the moments = 0 was not true?
It would rotate at an accelerating rate
When would an object rotate at an accelerating rate?
If the condition for equilibrium that the vector sum of the moments = 0 was untrue
If there are no moments, where are all the forces acting?
On the centre of gravity
What ones it mean if all the forces are acting on the centre of gravity?
There are no moments
Centre of gravity
The point at which the whole weight of the object may be considered to act
What is the point at which the whole weight of an object may be considered to act?
Its centre of gravity
If a finger were holding up a ruler, what would the two forces be?
Reaction force of finger
Weight of ruler
When a ruler balancing on a finger is in rotational equilibrium, what does every particle of the ruler have?
A corresponding particle an equal distance from the midpoint, on the other side
When is an object in rotational equilibrium?
When the object is not rotating or rotating in 1 direction at a constant rate
If an object is not rotating or is rotating in 1 direction at a constant rate, what is it in?
Rotational equilibrium
Write an equation for the moments when an object is in equilibrium
Σclockwise moments = Σanticlockwise moments
Σ meaning
Capital sigma - “sum of”
What is an object in if Σclockwise moments = Σanticlockwise moments?
Rotational equilibrium
Where is the centre of gravity located?
-A point here the sum of moments is zero
-Where the whole weight of the body acts
Where does an object balance?
On the pivot
Pivot
Where the object balances
What complicates a question where we’d otherwise only have to multiply the size of the force by the perpendicular distance to the line of action of the force/pivot?
The objects’ mass
How do we calculate an objects’ weight?
Mass x gravitational field strength
Normal reaction force
Acts at right angles to the surface when an object at rest exerts a force on the surface
(Upwards from the ground arrow)
What’s the name of the force that acts at right angles to the surface when an object at rest exerts a force on the surface?
Normal reaction force
What is the normal reaction force equal to?
The downward forces
In our balancing beam example, where would the normal reaction force come from?
The pivot
What is a cue that the weight of the object can be labelled from the middle?
‘Uniform’ object, as this means that the weight is the same throughout
What can we confidently do when an object is described as ‘uniform’?
Label its weight in the middle as this means that the weight is the same throughout
How do we know whether an object will topple over or not?
If the COG is within the footprint, it’s stable.
If not, it’ll topple.
Moments unit
NM (newton metres)
What does a body NOT have when in equilibrium?
A resultant moment or force
What does the equilibrium condition that the vector sum of the moments is zero actually mean and what’s a phrase for this?
Clockwise moments = anti-clockwise moments
Principle of moments
Principle of moments
The vector sum of all the moments = 0
(clockwise moments = anti-clockwise moments)
Principle of moments
The vector sum of all the moments = 0
(clockwise moments = anti-clockwise moments)
What are we assuming if we label the centre of gravity of a shape in the middle?
That it’s of uniform density
How do we calculate the centre of gravity of a more complex shape?
Hang it up- the CoG will fall directly underneath how it hangs.
Repeat hanging until all the lines cross to make a clear point
What do we use to represent g in W=mg to calculate weight?
9.81Nkg-1 (on data booklet)
What does a closed triangle of vectors represent?
That the forces are in equilibrium and balance, therefore the object’s moving at a constant velocity
If a rope is being used to pull a boat, is it better to use a longer or shorter rope? Why?
A long rope, as there’s a greater component in the direction of motion
Where does an object’s weight act?
Vertically down towards the centre of the Earth
Principle of moments
For an object in rotational equilibrium, the sum of the clockwise moments is equal to the sum of the anti-clockwise moments
What do you call the rule that covers - for an object in rotational equilibrium, the sum of the clockwise moments is equal to the sum of the anti-clockwise moments?
Principle of moments
How do we know which force is the one balancing out another?
On the opposite side to that one
How do we wok out the weight of an object based off of the other forces present?
-Add the vertical upwards components (don’t have to include any diagonals/resultants)
-Downward weight force is equal to this
What do we do if explaining why something such as pi or a number has no unit?
Follow the same method for working out what somethings unit is
What do we do if there are two unknown forces and were asked to take moments from a “suitable point”?
Take moments from the pivot underneath the opposite to the force being calculated in order to eliminate its affect
What’s it important to consider when calculating moments about a point?
Its weight
Which angle do we use for FxCos0 with moments about a point?
The angle from the direction it’s pointing (e.g. - upwards or downwards)
If a particular force moves close to the pivot, what happens to the value of the pivot?
Increases in order to counteract the increased downward moment as the distance has increased
How do you calculate the angle at which a cylinder would topple?
Tan (O) = d
—
l
When would the moments about the pivot be zero and why?
Directly above or directly underneath, as the perpendicular distance from it is zero
In which direction does weight act?
Vertically down towards the centre of the Earth, every time!!
How do you get the final uncertainty of a value in a table?
Range
——— = absolute uncertainty
2
Absolute uncertainty
—————————— x100 = % uncertainty
mean
% uncertainty of value = final answer
What should always be included at the end of a homogeneity question?
LHS=RHS
Is a force greater with a bigger or smaller angle?
Smaller angle (think of the equation)
Why would a block that’s being tilted right return to its upright position ?
The weight will still give a counter clockwise moment, which will cause the block to right itself
What does increasing the angle at which a projectile is thrown do to the horizontal velocity?
Decreases it
What’s vital to do in a moments question involving angles?
Work out all the sides as use these as distances
(We want perpendicular distances)
Which quantity could be negative when using equations for uniformly accelerated motion and under which circumstance?
Acceleration
When moving in the opposite direction to that set as positive
How do you go from cm2 to m2?
Divide by 100^2
