A Level Physics Mistakes Collection & Notes
Advanced Subsidiary
Force and Motion
Controlled Variable: things you don’t change (ensure fair test)
Independent Variable: things you would change
Dependent Variable: things you would like to measure
Ideal calculation format,
1. Show formula
2. Substitution
3. Answer
Mistakes
Largest prefix, Pento, smallest Prefix, femto
When doing unit calculations (reducing S.I units to base units), use [] symbol as the quantity
When calculation, always use full precision (just literally copy all the values)
For an object is projected from the surface, to find the maximum height, need to half the time
of flight ( _ seconds go up, _ seconds go down)
The decrease in mass of fuel of a car would cause the result lap time decrease
o Not because the maximum speed of the car has increased
o The maximum acceleration greater, (by F = ma if m decreases, a increases)
Increasing the height from the ground could reduce the percentage uncertainty in the measure
of time as the time travelled increases
To reduce the percentage uncertainty two ways
o Use smaller divisions of the measurement (smaller divisions is a better term)
o Increase the magnitude of the measurement (larger denominator)
If divisions too large, less precise
Scales and distances are large compared with the absolute uncertainty so absolute uncertainty
too small to be shown
To increase the initial speed, use springs in different compressions
For base units, it is Ampere not charge….
o The 6 are mole (amount of substance), second (Time), metre (length), kilograms
(mass), Ampere (current), kelvin (thermodynamic temperature)
Why the pair of forces are not an example of Newton’s Third Law
- Both forces act on the same object
- The types of forces are different
Factors affecting braking distance
Factors Reason
Roughness of the road The rougher the road more frictional force, the deceleration
was higher, and the braking distance becomes shorter
The function of the brakes in the The better the function, the deceleration applied by the brake
car can be higher and the braking distance becomes shorter
, The initial speed of the car The higher the initial speed, the longer time needed to stop
the car, the braking distance would be larger
The mass of the car The larger the mass, for F=ma, the smaller the deceleration,
more time is needed to stop the car the therefore the braking
distance becomes larger
If the question ask you about factors affecting stopping distance, mention thinking distance too
To explain free fall
Explain free fall (terminal velocity concept)
- Immediately after jumping
- The object falls in an acceleration of 9.81ms -2
- It experiences a gravitational force
- Before the terminal velocity is reached
- The acceleration decreases
- VELOCITY SQUARED IS DIRECTLY PROPORTIONAL TO AIR RESISTANCE
- Means that if the velocity increases, the air resistance increases
- The net force decreases since the air resistance get larger, but weight remains constant
- At terminal velocity
- The object moves at constant velocity
- Air resistances increase to an amount that is equal to the weight
- Net force is zero
- No acceleration
Projectile
- Time is independent with the horizontal velocity
Moment
- When there is rotation of the bike, the height of the front wheel decreases
At which angle the wheelie bin starts to topple clockwise (without calculation)
- When line of force of the weight falls to the right of the wheel
Energy
- When object rising at a constant speed
o KE constant
o GPE increases at a constant rate (linearly)
Pressure
The equation p= ρgh is not affect by the surface area
Lower position of pressure in a fluid > upper position of pressure in a fluid
Lower - upper= upthrust
Upthrust - weight = Resultant force
Different densities of material would cause the fluid to provide different buoyancy forces
different volumes of material with the same mass occupies different space in fluid and
therefore the upthrust will be different (equal to the weight of the fluid it displaces)
, The Upthrust is always constant as the even the pressure in the lower position due to P=pgh
increases, the pressure in the higher position also increases in the same rate.
Weight > Upthrust / the object sinks, the density of the object is higher than the liquid
If the object floats, the weight of the object is equal to upthrust
In liquid, there would be three forces: Drag force (similar to air resistance as it varies with velocity),
weight and upthrust (the force that made the object float)
If the object is denser than the liquid, there would be terminal velocity (as upthrust less than
the weight)
o V increases cause drag increase till net force=0, terminal velocity reached
If the object is less denser than the liquid (upthrust larger than the weight, both constant)
o V decreases and cause drag decrease too (net force decrease), but till rest, (net force=
upthrust-weight, upwards), accelerate back up
Weight = Upthrust / the top of the object just touches
Note that pressure is a scalar quantity
Materials
Mistakes
- Divide force by mass to find acceleration
𝐹2
- There is an equation 𝐸 =
2𝑘
- If ask about proportionality between m and E, use the above equation instead of 𝐹 = 𝑚𝑔𝑥
as for that it has two variables, 𝑚 2 ∝ 𝐸
(Read spring graph notes)
Tensile deformation = (extension)
Compressive deformation = (compression)
Stiffness= spring constant
Area of the Force-Extension Graph= Elastic Potential energy (Note that if its area is a triangle
so that need to divide by 2)
When spring in parallel not only say extension in each spring is halved, force in each spring is
also halved
Newton meter could measure force
Force= extension x spring constant
Physical Properties of deforming materials
, (Details in spring graph notes)
Metal wire
- Obeys Hooke’s Law until the limit of proportionality is reached
- The loading and unloading graphs would be the same below the limit of proportionality, but beyond
the limit of proportionality, the unloading curve is parallel but not identical with the loading curve.
- The material has permanent deformation after the removal of force if the material exceeds the elastic
limit
Rubber
- Does not obey Hooke’s law
- The loading and unloading graphs are different
- The Material is elastic as it returns to its original shape and size after force is removed (elastic)
- The area under the force- extension graph is work done
- (good for aeroplane as some KE of the plane is transferred to thermal energy, it would not bounce
and minimise the risk of passengers)
Polythene
- Does not obey Hooke’s law
- Loading curve is different from unloading curve
- The material suffers plastic deformation
Young’s Modulus only suitable when it is below limit of proportionality
Young’s modulus vs springs constant
- springs constant changes with length
- young’s modulus does not change
Determining the spring constant
- If the spring is compressed
- to compress the spring, the net force must zero to let it stay at rest,
- the force for both ends would be the same
- IN THIS CASE, we need to neglect the force of one end of the spring
(Just think asking you the force of the spring is 10N, and the normal reaction force is 10N, so in the
body has net force 0, it balances the applied force)
