#!/usr/bin/python

# A pound sign (hash) # is used as a comment so that Python ignores it

# The top line tells the computer what language we are using #!/usr/bin/python

#####YOU NEED TO EDIT THIS VALUE###############################################

height = 0.72 # height in metres of the electromagnet above the tin foil switch

#######YOU DON'T NEED TO EDIT ANYTHING BELOW HERE (but you can, it's fun!)#####

############################IGNORE THESE BITS##################################

##### We tell it what external libraries we will be using.

# In our case, we are using the PiFace###########

import pifacecommon # Read the pifacecommon library

import pifacedigitalio # Read the pifacedigitalio library

pifacedigitalio.init() # Initialise the PiFace making it ready for use

pifacedigital = pifacedigitalio.PiFaceDigital()

# PiFace now accessed from the pifacedigital variable

################################################ End of PiFace additions

# We are going to need to use the time library

import time

#################################################End of imported libraries#####

# Check tinfoil switch is not already pressed and exit if it is

if ( pifacedigital.input_pins[0].value == 1 ):

print ("Warning. Tin Foil switch is pressed")

print ("Exiting.")

exit()

################################################################

####################COIL ENERGISING#############################

input = "n" # We don't want the countdown to start just yet

# so we set a variable start_the_program to n

print "Energising the coils."

# Prompt the user to hit y to start the countdown

while (input != "y"): # While input not equal to y

input = raw_input("Press y and hit return to Energise the coil.\n")

pifacedigital.relays[0].turn_on()

print ("Coil energised.")

print ("Attach one (or more) ball bearings now.")

################################################################

#################### COUNTDOWN #################################

input = "n"

while (input != "y"): # While input not equal to y

input = raw_input("Press y and hit return to begin the countdown.\n")

print ("5");

time.sleep(1);

print ("4");

time.sleep(1);

print ("3");

time.sleep(1);

print ("2");

time.sleep(1);

print ("1");

time.sleep(1);

print ("freefall");

################################################################

################## DROP THE BALL ###############################

# Switch off the relay and drop the ball bearing

pifacedigital.relays[0].turn_off()

# So we need to measure the time now

start_time = time.time()

while (pifacedigital.input_pins[0].value == 0): # While switch 0 is not closed

time.sleep(0.0001) # sleep for 10,000th of a second

#Ball has landed at this point

# Measure the time that switch 0 closed

end_time = time.time()

# Calculate elapsed time

elapsed_time = end_time - start_time

# %2.6f is a non-integer number with 2 figures before, 6 after decimal point

print("Elapsed time was %2.6f seconds" % (elapsed_time))

# Switch off relay to preserve battery power for next time

pifacedigital.relays[0].turn_off()

######## MATHS BIT ############################################################

# Distance travelled (height) = initial velocity * time + 0.5 (acceleration * time squared)

# s = ut + 0.5*g*(t ^ 2)

# s = 0 + 0.5 gt^2

# gt^2 = 2s

# g = 2s / t^2

# Acceleration due to gravity = ( 2 * height ) / (time * time)

acceleration_due_to_gravity = ( 2.0 * height ) / ((elapsed_time)**2)

print ("I calculate the acceleration due to gravity to be %1.3f (ms^-2)\n" % acceleration_due_to_gravity)

# Things to think about:

# 0: Should we repeat the experiment and take the mean of our readings?

# 1: Does the ball bearing release at the same time as the relay switches off?

# 2: Does the tin-foil slow down the ball bearing before the switch closes?

# 3: Does the Pi read fast enough?

# 4: Air resistance?

# 5: Would it be different if we were dropping a tennis ball?

# 6: Would it be different on the Moon?

# 7: A better way to measure acceleration due to gravity?