Acceleration | Motion | Physics class 9 | Khan Academy
Summary
TLDRThe video explains the concept of motion by comparing three cars moving in different ways. The first car moves at a constant speed, illustrating uniform motion. The second car accelerates, meaning its speed increases over time. The third car decelerates, showing decreasing speed. The video then defines acceleration as the rate of change of velocity, giving examples and calculations to clarify the concept. It also emphasizes that acceleration is about changing velocity, not speed, and uses real-world scenarios to show how acceleration and deceleration work in physics.
Takeaways
- 🚗 The first car moves with a constant velocity, showing uniform motion where the displacement is the same every second.
- 🚗 The second car's displacement increases each second, indicating that its velocity is increasing, which is called acceleration.
- 🚗 The third car’s displacement decreases every second, meaning it is slowing down, showing deceleration or negative acceleration.
- 📉 In the case of uniform motion, the velocity remains constant, so the acceleration is zero.
- ⚡ Acceleration is not about moving fast but about changing velocity; a car can move fast but have zero acceleration if the velocity doesn't change.
- 📏 Acceleration is defined as the rate of change of velocity over time. Mathematically, it's the change in velocity divided by the time taken for that change.
- 🔢 For a car that accelerates from 5 m/s to 17 m/s in 3 seconds, the acceleration is calculated as 4 m/s², meaning the velocity increases by 4 m/s every second.
- ⚖️ Acceleration is measured in meters per second squared (m/s²), which reflects how much the velocity changes each second.
- 🛑 In deceleration, like when a car slows down from 20 m/s with a deceleration of 3 m/s² for 4 seconds, its final velocity becomes 8 m/s.
- 🔄 Deceleration (negative acceleration) results in a reduction of velocity by a constant amount per second.
Q & A
What is uniform motion?
-Uniform motion occurs when an object travels the same distance every second, indicating that its velocity remains constant.
How can we identify when an object is accelerating?
-An object is accelerating when its velocity is increasing, meaning that its displacement is growing more each second.
What does it mean when an object is decelerating?
-Decelerating means that an object's velocity is decreasing, or in other words, the object is slowing down. This is also referred to as negative acceleration.
What is the key concept of acceleration in physics?
-Acceleration in physics refers to the rate at which an object's velocity changes over time. This includes both increasing and decreasing velocity.
How do we calculate acceleration?
-Acceleration is calculated by dividing the change in velocity by the time taken for that change. The formula is: acceleration = (final velocity - initial velocity) / time.
What does it mean if the acceleration of an object is zero?
-If the acceleration of an object is zero, it means that the object’s velocity is constant, and there is no change in speed or direction.
How can we interpret an acceleration of 4 m/s²?
-An acceleration of 4 m/s² means that the object's velocity increases by 4 meters per second every second.
Why can acceleration be confusing when related to speed?
-Acceleration can be confusing because it doesn't necessarily refer to how fast something is moving. Instead, it refers to how quickly the velocity changes. Even a fast-moving object can have zero acceleration if its velocity remains constant.
What is the significance of negative acceleration?
-Negative acceleration, or deceleration, means that an object is losing velocity over time. It describes the process of slowing down.
In the example of a car decelerating at 3 m/s², what is its final velocity after 4 seconds if it starts at 20 m/s?
-If a car decelerates at 3 m/s² for 4 seconds, starting from 20 m/s, its final velocity will be 8 m/s.
Outlines
🚗 Three Cars in Motion: Understanding Their Movement
The video introduces three cars moving to the right and asks viewers to identify any differences in their movement. Initially, it seems like all the cars are moving the same way. To better understand, pictures are taken at one-second intervals, revealing differences. The first car shows constant displacement, indicating a constant velocity or uniform motion. The second car's displacement increases, showing acceleration, while the third car's displacement decreases, showing deceleration or slowing down. This demonstrates how velocity changes in different cases, and introduces the concept of acceleration and deceleration, also called negative acceleration.
🏎️ Understanding Acceleration and Its Mathematical Definition
This section clarifies that acceleration is not about moving fast but about how much the velocity changes over time. The video emphasizes that even if a car moves quickly, if its velocity remains constant, its acceleration is zero. The script explains how acceleration is calculated using the formula: acceleration equals the change in velocity divided by the time taken. An example is provided where a car accelerates from 5 m/s to 17 m/s over 3 seconds, resulting in an acceleration of 4 m/s². The idea of velocity increasing by a certain amount each second is illustrated to help explain the concept of m/s².
🚙 Applying Deceleration: An Example
The final paragraph demonstrates deceleration with a car initially moving at 20 m/s that decelerates at 3 m/s² over 4 seconds. Using the acceleration formula, it's calculated that the car's velocity after 4 seconds is 8 m/s. The example is further explained by showing how the velocity decreases by 3 m/s every second, confirming the accuracy of the calculation. This reinforces the concept of deceleration as negative acceleration and how it affects an object's motion.
Mindmap
Keywords
💡Uniform Motion
💡Acceleration
💡Deceleration
💡Displacement
💡Velocity
💡Rate of Change
💡Time Interval
💡Constant Velocity
💡Negative Acceleration
💡Meters per Second Squared (m/s²)
Highlights
Introduction to observing three different car motions: constant velocity, increasing velocity (acceleration), and decreasing velocity (deceleration).
Uniform motion: When a car's displacement is the same every second, its velocity is constant.
Acceleration: When the displacement of the car increases every second, indicating increasing velocity.
Deceleration: When the displacement of the car decreases every second, indicating decreasing velocity.
Clarification that acceleration is about changing velocity, not simply moving fast.
Acceleration is defined as the rate of change of velocity, showing how quickly velocity changes over time.
Formula for acceleration: Acceleration = (Change in velocity) / (Time taken for the change).
Example: A car’s velocity changes from 5 m/s to 17 m/s in 3 seconds. The acceleration is calculated as 4 m/s².
Explanation of the unit of acceleration: meters per second squared (m/s²), meaning how much velocity changes per second.
A second example of deceleration: A car moving at 20 m/s decelerates at 3 m/s² over 4 seconds.
Deceleration is treated as negative acceleration in the formula.
Final velocity calculation for deceleration example: The car slows down from 20 m/s to 8 m/s after 4 seconds.
Breaking down the calculation: The car loses 3 m/s of velocity every second, which is subtracted step by step.
Recap: Acceleration and deceleration depend on changes in velocity, not the speed of the object.
Importance of understanding the difference between velocity and acceleration in physics, where acceleration focuses on the change in velocity over time.
Transcripts
let's look at a video of three cars
moving and let's see if we find any
difference here we go here's the first
car here's the second
car and here's the third car did you
find any difference in the way they were
moving no right I mean I I just saw them
moving to the right what was the
difference this time we will look at it
one more time but we will take pictures
of it after every one second okay so
you'll see here we go
every 1 second we're taking its picture
okay this is what it looks like look at
the next one every 1 second we're taking
a picture o it's different you can see
the difference over here right so let's
look at the pictures what difference do
you notice well you notice that in the
first picture in the first uh motion you
saw that every second the car is
traveling the same distance so what can
we say about its velocity well if it's
traveling the same distance or its
displacement is the same every second
then its velocity must be a constant see
right therefore it's a constant velocity
or same velocity such a motion is called
uniform motion its velocity remains a
constant but in the second case what do
you notice you notice that it is its
displacement is increasing it's getting
more and more displaced every second ooh
this must be going faster a car must be
going faster in other words this is the
case where his velocity is increasing
this is what we call
acceleration or we would say the car is
accelerating okay what can you say about
the last case the last case is exactly
the opposite here the displacement every
second is decreasing can you see oh that
means it's slowing down my car must be
slowing down therefore this is
decreasing velocity we call it the car
is
decelerating okay so in front of your
eyes you can clearly see that in these
two cases the velocity is either
increasing or decreasing whenever the
velocity of an any object is increasing
inreasing or decreasing we say that
object is
accelerating decelerating can also be
thought of as negative acceleration so
in general the word we use is
acceleration in physics okay so we say
this is positive acceleration in general
we call this the negative acceleration
in general okay and um what about this
one what can you say about the
acceleration over here well since its
velocity is the same it's not
accelerating right in other words we
would say its acceleration is zero okay
so you understand what acceleration is
you say something is accelerating when
its velocity is changing it's increasing
or decreasing now I want to clarify that
acceleration can be confusing I mean
look at this first image again all right
if that car was going to the right very
fast and somebody asked you hey is this
car accelerating we might say yeah yeah
it's going very fast but you can see
right in front of your eyes acceleration
is not about going fast acceleration is
about changing that velocity okay so
even if this car is is going very fast
to the right the fact that its velocity
is not
changing that's why its acceleration is
zero so what you need for acceleration
is not that it should go with a very
fast velocity but you need to change
your velocity and that is the most
confusing thing about acceleration okay
so we need to get some practice to
understand whether something is
accelerating or not all right anyways
now that we have some idea some
intuition behind it let's now think
about how do we actually Define
acceleration okay how do we calculate it
so how do we Define acceleration we
Define it as rate of change of velocity
in other words it is a measure of how
quickly your velocity is changing I'll
take an example and it'll make more
sense but as of now let's write it
mathematically so let's say that you
have a car who initial velocity is U and
then it accelerates and let's say its
final velocity becomes V okay um let's
assume that it has increased for the
sake of example okay and let's say it
takes time T to go from U to V
then we Define acceleration as look at
this change in velocity divided by the
time taken for that change so can you
see now how quickly your velocity
changes and how how this definition
makes sense if if that velocity changed
very quickly in a short amount of time
in a much smaller time you'll have a
much bigger acceleration okay that's the
whole idea behind uh acceleration how
quickly your velocity changes so let's
take an example now let's take some
numbers so let's say the initial
velocity was 5 m/s
and it went from 5 m/s to 17 m/s and it
took 3 seconds to do that how much is
the acceleration can you pause and first
see if you can find the acceleration
yourself using the
formula okay let's substitute so v17 U
is 5 so vus U becomes 12 17 - 5 is 12 12
m/s and the time taken is 3 seconds so
you can see 12 m/s represents the change
in velocity and 3 seconds represents how
much time it took for that change change
okay and so what is 12 ID 3 12 3 is 4 so
we will say the acceleration is 4 m per
second per second what does that mean it
means the car's velocity Changed by 4
m/s every second think about it every
second in this case it gained 4
m/s does that make sense but let's let's
count in our head okay so initially it
was five after 1 second it gains a four
5 + 4 becomes 9 so after 1 second it
becomes 9
m/s after one more second it gains
another four 9 + 4 is 13 after one more
second it GS another four 13 + 4 is
17 that's what this means you
understanding it this means that every
second it's gain in its velocity is
increasing by 4 m/s that's the whole
idea behind this okay and finally we the
way we write it 4 m/s per second can
also be written as 4 m/s squar so you
can see the unit for acceleration is m/s
square and I know meter per second
Square sounds very weird like what is it
meter per second square but remember the
actual unit the the one that makes sense
to me is saying m/s per second how much
velocity it's gaining per second does
that make sense so that's what this
whole idea thing or whole thing means
okay all right now that we understand
the formula for the acceleration um
let's take one more
example this time we are given a car
moving at 20 m/s decelerates at 3 m/s
squared find its velocity after 4
seconds can you pause the video and try
to figure this out
yourself okay let's do this so let's try
to draw the situation we have a car
that's initially moving at 20 m/s then
we are given it decelerates at 3 m/s
Square deceleration is like negative
acceleration so acceleration is - 3 m/s
square and the time taken is 4 seconds
which means it's slowing down so now the
question is find its velocity after 4
seconds so after slowing down what will
be its new velocity we can also we can
already predict that its new velocity
should be smaller because it's slowing
down should be less than 20 but exactly
how much well we can use acceleration is
change in velocity divide by time rate
of change of velocity and we can plug in
we know - 3 m squ acceleration is equal
to V - 20/ by 4 and if you just um
rearrange this uh we multiply by four on
both sides so I'll get - 12 m/s = vus 20
if I add 20 on both sides now I finally
get V is equal to 8 m/s - 12 + 20 is 8
so my final velocity is 8
m/s and again you can check you can
check does it make sense see it is
saying that it is decelerating at 3 m/s
Square meaning it is losing velocity of
3 m/ second every second every second
you have to remove three from here so
first time you remove three you get 17
remove three one more time second time
you get 14 remove three the third time
you get 11 remove three the fourth time
you get eight so at the end of 4 seconds
you get eight which makes perfect sense
Voir Plus de Vidéos Connexes
5.0 / 5 (0 votes)