Velocity Time Graphs Video Lecture - Class 9

science shorts gives you speed time
graphs and acceleration for those of you
doing OCR science GCSE this will be
topic p3 AQA will be p2 and Edexcel will
be topic 15 so in the last video we
found Bob on its way to Google's HQ to
do some architectural reconfiguring and
we made a distance time graph of his
journey and found that we can calculate
an average speed by dividing distance by
time or by calculating the gradient of
the graph if you haven't watched that
yet please click the link in the
description so now Bob needs to choose
it before the fuzz arrive will document
his escape by drawing a speed time graph
he first drives off at a steady speed of
15 meters per second for 10 seconds now
to draw this on a speed time graph we
draw a horizontal line at 15 meters a
second all the way to 10 seconds this is
where people get confused though because
it's a flat line people often go into
distance time mode and so think that the
person isn't actually moving because as
we all know on a distance time graph a
horizontal line means that it's actually
not moving so be careful about that make
sure you take off your distance time hat
and put on your speed time hat when
dealing with these ok so now the police
are on Bob's tail pushing the nose
button in his tank helps him to
accelerate to 20 m/s in 5 seconds let's
show this on the graph so we go from 15
meters a second to 20 meters a second in
5 seconds like this but oh no calamity a
little bunny has hopped onto the road in
front Bob at being the caring type slams
on the brakes which brings in to a halt
in 2 seconds flat stopping just short of
bun-buns
sadly for Bob that allows the coppers to
have the drop on him and sorry mate
there just ain't no room for vigilantes
in this world so what can we now do with
this graph then well kind of like the
distance time graphs we can calculate
the gradients at different points but we
aren't calculating speed this time but
the acceleration acceleration is equals
to change in speed divided by time or
change in speed over time you take the
final speed subtract the initial speed
and divide by how long that took
acceleration is measured in meters per
second squared which makes sense seen as
though it's a change in speed meters per
second divided by time again so that's
meters per second per second and we can
write that as meters per second squared
right your turn now pause the video and
calculate Bob's acceleration for the
three parts of his journey let's see how
you got on of course 1 Bob's traveling
at a steady speed his acceleration is
zero meters per second squared why
because his change in speed is zero
he's not getting any faster he's not
getting any slower ok next here his
change in speed is 20 takeaway 15 so
that gives us 5 meters per second and
that takes him 5 seconds so his
acceleration is 5 divided by 5 equals 1
meters per second squared what about the
last bit then if you're on the ball you
would have noticed that if we do final
speed takeaway initial speed we get zero
takeaway 20 and that gives us minus 20
meters per second if we divide that by 2
seconds we get minus 10 meters per
second squared that's something to be
aware of that a negative acceleration
means that the person is decelerating or
slowing down by the way in later units
you'll find that that's not always the
case but for now it'll do but that's not
all it turns out that calculating the
area under the graph gives us the
distance traveled spookie to do this you
must split the area into rectangles and
triangles the few you have the better
for example I'd split up this graph like
this you multiply the signs of a
rectangle to find its area and multiply
the sides of a triangle and then divide
by 2 to find its area then add them all
up ok all yours
pause the video here and calculate the
distance he traveled
let's see how you got on with that so
here are all the areas for my triangles
and rectangles add them all up and we
get this distance here well done if you
got that right so don't forget to like
this video and leave a comment for what
science topic you'd like to see sketch
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