7.4 - free fall
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do heavy objects and light objects fall at different rates to each other?
they fall the same rate, regardless of their relative weights
how did galileo test if falling objects speed up as they fall?
the inclined plane test
what is galileo’s inclined plate test?
used a dripping water clock (with volume of trips as a measure of time)
measure the time taken for a ball to travel equal distances down the slope from rest
his measurements found that the ball gained speed when going down the slope, therefore objects accelerate whilst falling
what did galileo use to time his inclined plane test?
a dripping water clock
what does galileo’s inclined plane test measure the time taken for?
a ball to roll equal distances down the slope from rest
why does galileo’s inclined plane test measure the time taken for a ball to travel equal distances down the slope from rest?
to see how the ball gains speed whilst rolling, proving that objects accelerate whilst falling
what does the ball in galileo’s inclined plane test start from?
rest
why does the ball in galileo’s inclined plane test start from rest?
otherwise it’d be hard to accurately compare the acceleration due to gravity of balls in different repeat tests if they don’t have an equal initial velocity, and as the easiest initial velocity to induce is zero, the ball is released from rest
how does a dripping water clock work?
it uses the volume of the water dops as a measure of time
what was the findings of galileo’s inclined plane test?
the ball gained speed when going down the slope, therefore objects accelerate whilst falling
the steeper the slope, the greater the acceleration, therefore an object falling vertically accelerates
how does slope steepness affect acceleration?
the steeper the slope, the greater the acceleration
the steeper the slope, the greater / smaller the acceleration?
the steeper the slope, the greater the acceleration
the shallower the slope, the greater / smaller the acceleration?
the shallower the slope, the smaller the acceleration
which falling direction has maximum acceleration?
vertical
how did galileo prove that objects falling vertically accelerate?
using his inclined plane test, galileo proved that a ball rolling down the slope accelerates. it accelerates more the steeper the slope is, therefore galileo proved that objects falling vertically will accelerate
how can you investigate the free fall of a ball?
by making a multi-flash photo / video clip of the ball’s flight as it falls after being released from rest
what does investigating the free fall of a ball show you?
the acceleration due to gravity
when investigating free fall, what is a multi-flash photo / video taken of?
the ball’s flight as it falls after being released from rest
when investigating free fall, why is the ball released from rest?
otherwise it’d be hard to accurately compare the acceleration due to gravity of balls in different repeat tests if they don’t have an equal initial velocity, and as the easiest initial velocity to induce is zero, the ball is released from rest
free fall apparatus here
free fall apparatus apparatus
what is in the free fall apparatus?
here
ball released from rest
camera
vertical metre ruler
how do you obtain a multi-flash photo?
use an ordinary camera with a slow shutter speed in a dark room illuminated by a stroboscope
what is the point of the vertical ruler in the free fall apparatus?
to provide a scale
what is the flash rate of a stroboscope in a free fall apparatus?
constant rate of about 20 flashes per sec
what is the point of the dark room in the free fall apparatus?
so you can see the flashes of the stroboscope
what is the point of the stroboscope in the free fall apparatus?
idk
how do you use a multi-flash video clip to investigate free fall?
take a video, then rerun the clip at slow speed with the time displayed to mimic multi-flash photographs at different points of ball travel
what is the benefit of using a multi-flash photograph?
you can measure the time and distance directly from each photograph
what is the disadvantage of using a multi-flash video?
you have to manually find the images at each even time interval
does a multi-flash camera suffer human error? why?
no, because it takes pictures at even time intervals automatically / captures the entire free fall in video
what can be measured directly from a multi-shot picture?
time of descent of the ball
distance fallen by the ball from rest
what do the multi-shot photographs show?
that a ball in free fall speeds up as it falls, therefore an object in free fall will accelerates due to the force of gravity
how does a multi-shot camera show that a ball in free fall speeds up as it falls?
because the ball travels further between successive images
what else could be used to take a multi-shot video clip?
a digital camera or a similar device
what is included in a table of free fall measurements?
number of flashes (of the stroboscope) after start
time of descent
distance fallen
here
independent value is number of flashes?
how can you tell if the acceleration is constant from free fall measurements?
s = u t + ½ a t2
u = 0
s = ½ a t2
compare to straight line equation
plot an s against t2 graph
if the graph is a straight line through the origin, and its acceleration (2 x gradient) ~ 10 m s-2, the object is in free fall
here
what will a free fall practical tell you?
if an object (ball) is in free fall, i.e., accelerating due to the earth’s gravitational force
what SUVAT equation do we use when analysing free fall measurements?
s = u t + ½ a t2
s = ½ a t2
equation for an object in free fall that considers how displacement varies with time, where initial speed = 0
graph for free fall here
displacement against time squared graph for free fall
derive s = ½ a t2
s = u t + ½ a t2
u = 0
s = ½ a t2
what is the equation s = ½ a t2 compared to?
the equation of a straight line
what is s = ½ a t2 compared to the equation of a straight line?
s = ½ a t2
y = mx + c
y = displacement
gradient = ( ½ x acceleration)
x = t2
y-int (c) = 0, i.e., through the orign
what graph would the results from a free fall investigation give?
a straight line with positive gradient through the origin
why is the graph from the results of a free fall investigation give a straight line with positive gradient through the origin?
constant acceleration (s = u t + ½ a t2) when initial velocity is zero, s = ½ a t2
compare to equation of a straight line, y = mx + c
therefore :
linear (as it compares to equation of a straight line)
positive gradient (as ½ a, gradient function, is positive)
goes through the origin (because c = 0)
what is g?
gravitational field strength, acceleration due to the earth’s gravitational field, 9.81 ms-2
what is gravitational field strength?
acceleration due to the earth’s gravitation field, 9.81 ms-2
what is the value for g?
9.81 ms-2
what is the gravitational field strength?
g
how do you find the acceleration on a free fall graph?
gradient x 2
why is the acceleration on a free fall graph the gradient x 2?
the graph’s equation is s = ½ a t2, where ½ a is the gradient, and on a displacement-time graph, acceleration is the gradient
if grad = a / 2, then a = grad x 2
on a free fall graph, what should your calculated value of g be?
~ 10 ms-2
if the accepted value of g is 9.8 ms-2 , why would you expect your free fall graph to give a value of 10 ms-2 for g?
because of human or systematic error
what is free fall?
when an object falling vertically has no external forces acting on it other than the force of gravity, meaning the object is accelerating at a constant rate due to the earth’s gravitation field (accelerating at a rate of 9.81 ms-2)
what can be used to calculate free fall situations?
SUVAT
can SUVAT be used in free fall situations?
yes
why can SUVAT be used in free fall situations?
because the object in free fall is accelerating constantly due to the earth’s gravitational field, and SUVAT applies to situations in constant acceleration
what can be used to calculate situations where objects are thrown vertically upwards?
SUVAT
can SUVAT be used in situations where objects are thrown vertically upwards?
yes
why can SUVAT be used in situations where objects are thrown vertically upwards?
because the object, when falling, is accelerating due to the earth’s gravitational field, therefore the object is constantly accelerating and SUVAT can be used
for situations where objects are thrown vertically upwards, which direction is resolved positively?
upwards
for situations where objects are thrown vertically upwards, which direction is resolved negatively?
downwards
how else can you investigate free fall?
by using an electronic timer or motion sensor