Physics, Kinematics (1 of 12) What is Free Fall? An Explanation
Summary
TLDRIn this educational video, the concept of freefall motion is explored, focusing on one-dimensional vertical motion. The video distinguishes between two scenarios of freefall: dropping an object straight down and projecting an object straight up. It emphasizes the significance of considering gravitational acceleration (9.81 m/s² on Earth) and disregarding air resistance. The tutorial proceeds with a practical example of calculating the time it takes for an iPhone to fall 8.75 meters, illustrating the kinematic equations involved and the importance of consistent sign usage. The video concludes with a call to action for viewers to engage with the content by liking, commenting, and subscribing.
Takeaways
- 📚 Freefall is a type of one-dimensional motion that can be vertical and either involve an object falling straight down or being projected straight up and then falling back down.
- 🌐 In freefall problems, air resistance is typically neglected, simplifying the calculations to focus on the effects of gravity alone.
- 🌍 The acceleration due to gravity near Earth's surface is approximately 9.81 m/s², often rounded to 10 m/s² for simplicity in calculations.
- 📉 Freefall motion is characterized by an initial velocity of 0 m/s when an object is dropped and a final velocity of 0 m/s when an object reaches its peak after being thrown upwards.
- 🔢 The kinematic equations used to solve freefall problems involve variables such as initial velocity, final velocity, change in position, acceleration, and time.
- ⏱️ For an object in freefall, the time it takes to reach the ground from a certain height can be calculated using the kinematic equation \( \Delta y = \frac{1}{2} a t^2 \), where \( \Delta y \) is the change in position, \( a \) is the acceleration due to gravity, and \( t \) is the time.
- 🔄 The motion in freefall is unidirectional (in the y-direction), with no movement in the x-direction, simplifying the problem to a one-dimensional issue.
- 📉 When solving freefall problems, it's crucial to be consistent with the use of signs, where downward is considered the negative direction, and upward is positive.
- 📈 The final velocity of an object thrown upwards is the same as its initial velocity but in the opposite direction due to the symmetry of the motion.
- 🎯 In the provided example, the time it takes for an iPhone to fall 8.75 meters from a window is calculated to be approximately 1.34 seconds, illustrating how to apply the kinematic equations to real-world scenarios.
Q & A
What are the two types of freefall motion discussed in the video?
-The two types of freefall motion discussed are when an object falls straight down and when an object is projected straight up and then falls back down.
What is the significance of air resistance in the context of freefall problems?
-In freefall problems, air resistance is typically ignored, which is why freefall is often defined as motion with no air resistance affecting the object.
What is the standard acceleration due to gravity on Earth's surface in freefall problems?
-The standard acceleration due to gravity on Earth's surface is 9.81 m/s^2, which is often approximated as 10 m/s^2 for simplicity in calculations.
How does the acceleration due to gravity on the Moon compare to that on Earth?
-On the Moon, the acceleration due to gravity is about one-sixth of that on Earth, which is approximately 1.61 to 1.62 m/s^2.
What is the initial velocity when an object is dropped in freefall?
-The initial velocity of an object when it is dropped in freefall is 0 m/s, as it starts from rest.
What is the final velocity of an object at the peak of its trajectory when thrown straight up?
-The final velocity of an object at the peak of its trajectory when thrown straight up is 0 m/s, as it momentarily stops before falling back down.
How does the direction of velocity change when an object thrown straight up falls back down?
-When an object thrown straight up falls back down, its velocity direction changes from positive (upward) to negative (downward), but the speed remains the same.
What is the relationship between the time it takes for an object to rise and the time it takes to fall back down in freefall?
-In freefall, the time it takes for an object to rise to the peak of its trajectory is equal to the time it takes to fall back down.
What kinematic equation is used to solve for time in freefall problems when the initial velocity is zero?
-When the initial velocity is zero, the kinematic equation used to solve for time in freefall problems is Δy = -1/2 * a * t^2, where Δy is the change in position, a is the acceleration due to gravity, and t is the time.
How is the time calculated in the example problem where an iPhone is dropped from a window 8.75 meters above the ground?
-In the example, the time it takes for the iPhone to reach the ground is calculated using the rearranged kinematic equation t = √(2 * Δy / a), where Δy is -8.75 meters and a is -9.81 m/s^2, resulting in a time of approximately 1.34 seconds.
Outlines
🌌 Introduction to Freefall Motion
This paragraph introduces the concept of freefall motion, which is a type of one-dimensional, vertical motion. The speaker explains that freefall can occur when an object is dropped straight down or when it is projected straight up and then falls back down. It is emphasized that in freefall scenarios, air resistance is typically neglected, and the only acceleration considered is due to gravity, which is approximately 9.81 m/s² on Earth. The speaker also mentions that the acceleration due to gravity varies in different parts of the solar system, such as on the moon where it is about one-sixth of Earth's gravity. The paragraph sets the stage for understanding the differences between freefall and horizontal motion and prepares for an example problem to be solved later in the video.
📚 Solving Freefall Problems with Kinematic Equations
The second paragraph delves into solving freefall problems using kinematic equations. The speaker outlines the steps for setting up a freefall problem, starting with drawing a simple diagram to visualize the motion. The variables involved in the kinematic equations are discussed, including initial velocity, final velocity, change in position, acceleration, and time. The speaker points out that for a freefall problem, the initial velocity is zero, and the acceleration due to gravity is -9.81 m/s². The paragraph focuses on selecting the appropriate kinematic equation to solve for time, given that the initial velocity and acceleration are known, and the change in position is provided. The speaker simplifies the chosen equation by accounting for the zero initial velocity and then rearranges it to solve for time. The process involves isolating the time variable and applying the values to find the time it takes for an object to fall a certain distance, which in the example provided is 1.34 seconds.
📢 Conclusion and Additional Resources
In the final paragraph, the speaker concludes the video by summarizing the key points discussed about freefall motion. They reiterate the importance of understanding the differences between freefall and horizontal motion and the need to consider factors like air resistance and gravitational acceleration. The speaker also encourages viewers to practice solving freefall problems by providing links to additional resources. They invite viewers to engage with the content by liking the video, leaving comments, and subscribing to the channel for more educational content on physics, chemistry, and math.
Mindmap
Keywords
💡Freefall
💡One-dimensional motion
💡Air resistance
💡Acceleration due to gravity
💡Initial velocity
💡Final velocity
💡Kinematic equations
💡Displacement
💡Time
💡Vector
Highlights
Introduction to freefall and one-dimensional vertical motion
Freefall defined as motion with no air resistance and only in the y-direction
Two types of freefall: falling straight down or projecting straight up
Acceleration due to gravity (g) is 9.81 m/s^2 on Earth
Difference in acceleration due to gravity on the Moon compared to Earth
Consistency in using signs for acceleration in freefall problems
Initial velocity is zero when an object is dropped straight down
Final velocity is zero at the peak of an object's trajectory when thrown straight up
The symmetry in time for an object thrown straight up and falling back down
Setting up a problem with a sketch and identifying known and unknown variables
Choosing the appropriate kinematic equation based on given variables
Simplification of the kinematic equation for freefall with zero initial velocity
Solving for time in freefall using the rearranged kinematic equation
Calculating the time it takes for an iPhone to fall 8.75 meters
Emphasizing the importance of signs in calculations to avoid taking the square root of a negative number
Final answer: iPhone takes 1.34 seconds to fall 8.75 meters
Summary of the process for solving freefall problems
Encouragement for viewers to practice freefall problems with provided resources
Transcripts
okay in today's video I'm going to go
over a brief explanation of Freefall
which we might also call onedimensional
vertical motion and talk a little bit
about some things we need to keep in
mind when we're doing these problems and
how it's different from onedimensional
horizontal motion and then we'll do an
example at the end of the video okay
freef fall I would say there are two
different kinds of freef fall two
different ways we think about freef fall
one is when something Falls straight
down that's one kind of free fall you
drop something something Falls just
straight down the other kind of free
fall is when you project something
straight up and then it comes back
straight down so there's two things drop
something straight down or shoot
something or project something straight
up and it comes straight down in both
cases there's no change in position in
the X direction there's only Motion in
the y direction in both
cases when we talk about freef fall we
often we say there's no air resistance
that's generally what we mean by freef
fall there's no air resistance so we can
ignore air
resistance also because it's freef fall
and we're on Earth the acceleration
which in this case because it's the
acceleration due to gravity we
abbreviate with a G is 9.81 m/s squared
when you drop something and there's no
air resistance it has an acceleration of
9.81 m/s squared that's when you're on
Earth and near Earth's surface other
places in the solar system other places
the acceler due to gravity is different
like on the moon I believe it's one six
of that which I think is 1.61 and 1.62
m/s squared but on Earth it's a constant
9.81 m/s squared sometimes approximated
as
10 now in both cases whether it's 9.81
or 10 the object is falling down and the
acceleration is pulling it down it's
accelerating I shouldn't say the
acceleration pulling down the
acceleration is in the negative
direction or is accelerating the
negative Direction so therefore we say
it's negative 9.81 when you do freefall
problems when you do kinematic problems
you always want to be consistent with
your signs and use your signs minus 9.81
m/ second now as I said there's two
kinds of problems one is when
something's dropped straight down when
you drop something straight down the
initial velocity because you're holding
it in your hand or it's being held is 0
meters per second and it won't often say
that in the problem explicitly it'll
just say Johnny drop something
and you have to remember the initial
velocity in that case is 0 m/ second and
you know
that okay when you throw something
straight
up the final velocity when it gets to
the top of its path before it starts
returning back down the final velocity
is 0 m/ second if you remember that
that'll help you simplify some of your
equations okay also so that's the
velocity at the top also when you throw
something straight up or you project
something straight up let's say for
example it leaves your hand with a speed
of 5 m/ second or it leaves the thing
that's projecting at 5 meters per second
when it's moving up up is in a positive
direction it's call that 5 MERS per
second excuse me positive 5 met per
second is the velocity well when it
comes back down to your hand or comes
back down to the same place from which
it was projected the speed is going to
be the same it'll have the same speed 5
m/ Second 5 m/ second but now it's
traveling the Direction so the velocity
is going to be -5
m/s all right now also when you project
something straight up the time it takes
for it to get to the top of its path
will be equal to the time it takes for
it to come back down okay these are some
things you need to keep in mind that
will help simplify some of the problems
help you have a better conceptual
understanding and we'll do an example
right now and we'll use some of this and
I'll put some links at the end of this
video for some additional problems to
practice free
fall
okay Richard this is the first problem
it's mad as s so he takes your iPhone
drops it out the window the bedroom
window is 8.75 meters above the ground
we want to know the time it takes for
the iPhone to reach the ground now for
freef fall motions I think the best
thing to do the first thing I always do
is I like to draw a picture so I draw a
very simple sketch horizontal motion I
don't know I you don't draw a picture
freef fall motion I do so I have this
place where it's being dropped from
here's where it's being dropped here's
the ground surface I Dr draw the object
I just draw a circle or a square I'm not
going to draw rich or n this iPhone
takes too long and I mark down that it's
8.75 M this distance is 8.75 meters now
you want to keep in mind you don't
always have to draw an XY coordinate
system but you want to keep in mind that
where the thing starts we generally
designate as zero so this is going to
fall downward mean the change in
position is going to be negative 8.75
the distance is 8.75 but the change in
position because position is a vector is
uh our changing position is a vector is
8.75 so we drew our picture we set it up
the second thing we do is we write down
all five of the variables that are
included in the kinematic equations
initial velocity final velocity change
in position so this is freef fall I put
Delta y as opposed to Delta X doesn't
really matter of course but we talking
about y motion Motion in the y direction
acceleration I just leave that as a I
don't put a g could put a g and then the
time what do we know well we're given
that the distance is 8.75 we need to
remember that this is actually the
change in position so it has to be minus
8.75 m/ second not meters per second
meters okay the next thing we know is
it's dropped and that means the initial
velocity is zero as we said in the
previous slide we're given also the
acceleration now you're actually given
it but you know it is- 9.81 m/s squared
it doesn't actually say that in the
problem that's another thing this the
initial velocity and the acceleration
are not given in the problem but you
need to recognize that you know the
initial velocity and you know the
acceleration when you have something
dropped all right we want to find the
time and we're not given the final
velocity we're not going to find the
final velocity all right so that's
basically how we set it up now I'm going
to take all this with us to the next
slide we're going to get out our
kinematic equation we need to figure out
which of the kinematic equations we're
going to use now you'll know notice
we're looking for time and if you had
some experience now with kinematic
equations you'll notice this equation
doesn't have time in it so we can't
solve for the time now the other three
equations all have time time and time in
them but in order to use one of these
equations we have to know the other
three variables once again you're given
three variables you're given three of
the variables you're solving for the
fourth each equation has three four
variables in it if you're given three of
those four then you can solve for the
fourth okay so that means the first
equation has the time in it so we're
going to solve for the time except we
don't know the final velocity the final
velocity in this we don't know the final
velocity therefore we don't know all
three of the other variables we don't
know the final velocity the second
equation also has time but also has
final velocity we can't use that
equation this equation right
here we're we're looking for the time it
has the time in it we know the other
three variables we know the change in
position minus
8.75 we know the initial velocity zero
and we know the acceleration is - 9.8 so
we're going to take this slide with our
diagram and the equation and the
information we're given and we're going
to use this equation to solve this
problem okay now another thing you
should recognize which will simplify
this equation is the initial velocity
zero and this term right here is initial
velocity times the time well the initial
velocity zero the initial velocity times
the time is zero and therefore this
equation simplifies to the change in
position is equal to 12 h^2 and this is
kind of an important equation for freef
fall because the distance that something
Falls is equal to2 times the
acceleration times the time squar and
the acceleration is a constant so it's
just you just mostly depends on the
time all right now we're solving for the
time so we're going to rearrange this
equation solve for time which means
we're going to have to multiply both
sides by two take the square root of
both sides and we get that the time is
equal to 2 * the change in position 2 *
Delta y divided by the acceleration now
we simply plug the values in 2 * minus
8.75
M divided by the acceleration and take
the square that now you'll notice I just
want to point out because the signs you
can't take the square root of a negative
number but you'll notice we have a
negative on the top and a negative on
the bottom that's going to be a positive
if you leave one a negative off you'll
have the right number under heel but
it'll have the wrong sign and you try to
take the square root of negative number
can't take the square root of a negative
number so therefore in this case when we
do all of this we have our time have our
signs all figured out get that the time
it takes this object to fall or the
iPhone to fall 8.75 meters is
1.34 seconds okay so there you go at the
beginning we talked about the things we
need to take into consideration and keep
in mind when we're doing free-for-all
problems there's two kinds you throw
something straight up and it comes
straight back down or you drop something
in both cases there's no Motion in the X
Direction We did an example we drew a
simple
diagram and we wrote down all five of
the variables we filled in what we knew
and what we didn't know we knew three we
wanted to find a fourth we chose the
right equation we simplified it we
rearranged it for the variable we're
solving for we plug the values in get
the answer with the correct unit and
there you go okay keep those things in
mind I don't think it's that complicated
it's basically the same thing as
onedimensional horizontal motion thank
you very much for watching I'll put some
links to some further practice problems
here thanks for watching I hope you
found it helpful if you did you can do
all of the following three things give
me a thumbs up for this video leave me a
nice comment in the comment section and
therefore subscribe to my channel and
get all of my excellent physics
chemistry and math videos thank you very
much
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