A student conducts an experiment to estimate the acceleration g of free fall by dropping a
heavy metal ball from a high wall into a sandpit below. Describe how this student can estimate
the acceleration g of free fall. Your description should include:
• the measurements taken
• the instruments used to take the measurements
• how the measurements are used to estimate g
• an explanation of why the value for g is only an estimate
Measurements:
height (of wall)
time (of fall)
Instruments:
ruler / tape (measure)
stopwatch / timer / clock /video
gradient of s-t2 graph x 2
From s= ut + 1/2 at^2
Any two from: g is an estimate because
air resistance / drag ignored
parallax problems with ‘landing time’
starting / stopping the clock
Terminal velocity
Wrap elastic bands around the tube of viscous liquid ( glycerol) at set intervals measured by the
ruler.
Drop the ball into the tube and record the time it reaches each band (it will help to
use a lap feature on the stopwatch here).
Repeat 4 times to reduce the effect of random errors and use the strong magnet to
remove the ball bearing from the bottom of the tube.
Calculations
● Calculate the time taken to travel between consecutive bands and calculate the
average of this time for each experiment.
● Use the equation speed = distance/time to find the average velocity of the bearing
between each set of bands.
● Plot a graph of velocity against time. The velocity to which the graph tends to is the
terminal velocity.
Safety
Note s
● Use a viscous liquid that doesn’t cause skin irritation.
● Using a taller tube allows the bearing to travel at its terminal velocity for longer.
● Using larger intervals for the bands reduces the percentage uncertainty in both the
distance and time between the bands.
● Terminal velocity occurs when the weight of the bearing is equal to the drag force
due to the fluid, as there is no resultant force on the bearing, it travels at a constant
velocity.
Resistivity of a wire
Measure the diameter of the
wire at 3 points along its length using the
micrometer, and calculate the mean
diameter.
Set up the apparatus Circuit diagram with crocodile clips, voltmeter, ammeter and potentiometer ( to keep current constant)
Adjust length to 10 cm using the crocodile
clips and metre ruler.
Read and record the current (I) on the
ammeter and the voltage (V) on the voltmeter. Calculate the resistance (R) by using
R=V/I and record this value.
Switch the circuit off in between readings to prevent heating of components which
could affect their resistance.
Increase l by 10 cm and repeat the above two steps, increasing l by 10 cm each time
up to 80 cm.
Repeat the experiment twice more, then calculate the mean resistance for each
length.
Calculations
πd2
● Calculate cross sectional area of the wire by using A = where d is the wire’s 4
diameter.
● Plot a graph of the mean resistance against length and draw a line of best fit, the
resistivity will be the gradient multiplied by the cross-sectional area of the wire.
RA
ρ = R = ρ
l
then divide through by A to find .
L
A
R/L is the gradient of the graph hence resistivity is found by the gradient multiplied by
the cross sectional area.
Safety
● Disconnect the crocodile clips in between measurements to avoid the wire heating up
and causing burns if touched. If the current rises too high, reduce the voltage using
the variable power supply.
● If the wire is taut, safety goggles should be worn in case it snaps.*
I V characteristics
Set up the circuit as shown where ‘component’ is the
filament lamp, copper block or diode.
Vary the voltage across the component by changing the
resistance of the variable resistor, using a wide range of
voltages.
For each voltage record the current 3 times and
calculate the mean current.
Make sure to switch off the circuit in between readings to prevent heating of
components.
Repeat for all 3 components.
Calculations
● Plot a graph of mean current against voltage (an I-V characteristic graph) for each
component.
● Compare the shapes of each graph and consider the reasons behind the difference
between the filament lamp and copper block characteristic graph.
Source of error:
The voltmeter will not have infinite resistance and the ammeter will not have 0
resistance (ie. they won’t be ideal) therefore the voltages and currents displayed may
be slightly inaccurate.
● The equipment used (except from the components) and the temperature of the area
should be controlled as these can affect the results.
● To reduce uncertainty take more readings at more voltages and use ammeters and
voltmeters with greater resolution.
PAG 5.2 speed of sound
- Set signal generator to 900Hz. Using oscilloscope on a time-base setting. Determine frequency by counting number N of full waves on the screen and the number of decisions for N waves. Call uncertainly
- Have a meter rule clamped parallel to the air column. Adjust the spot ion of the air column until intensity is at a maximum which suggests resonance. Measure the resonance length and uncertainty. Keep signal generator at the same frequency and adjust position until another resonance is found and measure length.
- Resonating column of air slightly protrudes at the top causing a systematic error so to find wl it would be l(1) +c = 1/4 L and l(2) + c = 3/4 L so to cancel out c you do l(2) - l(1) = L/2 so double it to get wavelength. Use v = fL to find speed
- For graphical method use different frequency by adjusting signal generator and then measure wl for each frequency. Plot wl against 1/f so that the gradient is v. the line shouldn’t be straight and go through the origin.
Experiment by de Broglie to show electrons wave nature. Compare wavelength of electron to a car
Electrons accelerated by a high pd in a vacuum. Electrons fired at a polycrystalline graphite target. Rings are observed due to diffraction between spacing of atoms. Wavelength approximately is equal o the spacing of atoms. Increase in accelerating p.d. decreases spacing of rings. Multiple layers of graphite atoms means diffraction in all directions therefore rings are observed. Safety: avoid touching terminals or use insulated connections. Wl= planck constant/ momentum. Wl of electron is much bigger than wl of car as car has a much higher momentum.
How to determine g using electromagnet and a trapdoor
Use a meter rule to measure h. Place the rule in retort stand. Use set squares to improve accuracy. Use a data logger to record time which is connected to the trap door which starts when electromagnet is turned off and releases the ball and stops when the ball hits the trap door. Include the diameter of the ball in height measurement. Use instruments with a high resolution of millimeters and milliseconds.
Measuring wavelength using young’s double slit with uncertainties
- Waves must be coherent and it must be monochromatic or use a filter and a single slit first make sure to diffraction the same source through both slits and close to the source
- Measurements include distance between slits using a vernier caliper. Distance between centre of fringes using a vernier caliper. Distance between slits and source using a tape measure.
- Uncertainty. Vernier caliper is (0.1mm) and tape measure is (1mm) most cases distance between slits is source of largest uncertainty.
A metal ball is rolled off the edge of a horizontal laboratory bench. The initial horizontal velocity
of the ball is v. The ball travels a horizontal distance x before it hits the level floor.
Use your knowledge of projectile motion to suggest the relationship between v and x. Describe
how an experiment can be safely conducted to test this relationship and how the data can be
analysed
Ruler used to determine x
Average readings to determine x
x recorded for various v
Suitable method for consistent v or
varying v e.g.
Released from same point on a
track
Ejected from a spring device with
different compressions
Suitable method of determining point of
impact e.g.
trial run to get eye in approximate
correct position
carbon paper so that ball makes a
mark on paper
scale in frame of video recording
tray of sand to catch ball
Suitable instrument used to determine
v (light–gate / motion sensor / video
techniques) or suitable description of
inference of v from other
measurements such as energy
released from spring of known k and x
Ensuring the initial velocity of ball is
horizontal
Analysis
Horizontal velocity is constant
Time of fall is independent of v
/horizontal velocity
Suggested relationship: e.g. x ∝ v,x
d.p. to V2
, etc
Plot a graph of x against v or graph
consistent with candidate’s suggested
relationship
If relationship is correct, then a straight
line through the origin.
Suggested relationship supported by
correct physics or algebra.
Correct relationship supported by
physics.
