Physics, Kinematics (1 of 12) What is Free Fall? An Explanation

Summary

In 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

00:00

🌌 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.

05:00

📚 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.

10:01

📢 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.

Keywords

💡Freefall

Freefall refers to the motion of an object under the sole influence of gravity, moving in a straight line towards the Earth's surface. In the video, freefall is described as either dropping an object straight down or projecting it straight up, with the object eventually falling back down. The concept is central to the video's theme, as it sets the stage for discussing one-dimensional vertical motion and the absence of air resistance.

💡One-dimensional motion

One-dimensional motion is a type of motion where an object moves along a single axis, either the x-axis or the y-axis. In the context of the video, the focus is on vertical motion, which is one-dimensional in the y-direction. This simplifies the analysis of the object's motion, as it eliminates the need to consider movement in the horizontal plane.

💡Air resistance

Air resistance, also known as drag, is the force that opposes the motion of an object through the air. The video emphasizes that in the case of freefall, air resistance is typically ignored, allowing for a simpler analysis of the motion. This assumption is common in physics problems where the effects of air resistance are negligible compared to the force of gravity.

💡Acceleration due to gravity

The acceleration due to gravity, often denoted as 'g', is the rate at which objects accelerate when in freefall near the Earth's surface. The video specifies this value as 9.81 m/s², which is a standard approximation used in physics. This acceleration is constant for freefall scenarios on Earth and is a critical factor in calculating the motion of falling objects.

💡Initial velocity

Initial velocity is the speed of an object at the beginning of its motion. In the video, it is mentioned that for objects dropped straight down, the initial velocity is zero, as they start from rest. This is an important variable in the kinematic equations used to analyze motion, and its value is assumed or given in problems involving freefall.

💡Final velocity

Final velocity is the speed of an object at the end of its motion. In the context of the video, when an object is thrown straight up, its final velocity at the peak of its trajectory is zero, as it momentarily stops before falling back down. This concept is used to simplify equations when dealing with vertical motion in freefall.

💡Kinematic equations

Kinematic equations are formulas used to describe the motion of an object. The video outlines several of these equations, which relate variables like initial velocity, final velocity, acceleration, time, and displacement. These equations are essential for solving problems involving freefall motion, as they allow for the calculation of unknown variables given the others.

💡Displacement

Displacement is the change in position of an object. In the video, displacement is used to describe the vertical distance an object falls, which is a vector quantity. The script mentions that for freefall problems, displacement is typically a negative value because the object is moving downward, which is the opposite of the upward positive direction.

💡Time

Time is a critical variable in kinematic equations, representing the duration of motion. The video provides an example problem where the goal is to find the time it takes for an iPhone to fall 8.75 meters to the ground. Time is an unknown in this problem, and the video demonstrates how to use the kinematic equations to solve for it, given the other known variables.

💡Vector

A vector is a quantity that has both magnitude and direction. In the video, displacement and velocity are described as vectors because they involve both how far and in which direction an object moves. The script clarifies that when dealing with vectors, the direction is crucial, and it affects the sign of the values used in calculations, such as positive for upward and negative for downward motion.

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

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