Acceleration

0:00- [Instructor] Let's look at three cases of a car moving to the right, and let's see if you can see the difference. Here's the first one, here's the second one, and here's the third one. Did you notice any difference? Okay, let's slow down a little bit and look at the snapshots of the car at regular time intervals.

0:18Let's say we take a snapshot of the car every second, for example, okay? Here's the first one again. We are taking snapshots every second. What do you notice? Well, look, the distance that the car is traveling every second, it's increasing, meaning the car is speeding up.

0:35So this is an example where the car is speeding up. What do you notice in the second case? Ooh! We notice that now the distance that the car is traveling every second is decreasing. This means the car is slowing down. And what about the last case? Notice the distance that the car is traveling every second is the same, it's not changing.

0:57So, here, the car is traveling at a steady rate. Now, whenever an object is speeding up or slowing down, or in general, whenever object's velocity changes, we say that object is accelerating. So in both, in the first two cases, the car was accelerating, but in the last case, the car is not accelerating.

1:18But what exactly is acceleration? Well, let's see. Well, we think of acceleration as a measure of how quickly the velocity changes. So, for example, in our daily life, we might hear things like, you know, race cars, for example, they can go from 0 to 60 miles per hour in 2.3 seconds.

1:38Now, look, this is a measure of how quickly the velocity changes. So if we go back to our example of, say, the speeding up, let's take some numbers. Let's say the car starts at zero, and let's say that time is zero. And after one second, the car's velocity becomes three meters per second, then it becomes six meters per second, then it becomes nine meters per second.

2:03Notice the car is speeding up. But what you notice is that, this object, this car is moving three meters per second faster after each second. That is its acceleration. And a cool thing about acceleration is that, just like velocity, it also has a direction. How do we know the direction of the acceleration?

2:27Well, whenever an object is speeding up, it's going faster, its acceleration is in the same direction as the velocity. So in this particular case, the velocity is to the right, so its acceleration would also be to the right. So whenever things are speeding up, whenever things are going faster, acceleration has the same direction as velocity.

2:47Now let's look at the second case, when the car slowed down. Now look, what do we notice? We notice that its velocity is decreasing. In fact, the car is going in such a way that, you know, its velocity is becoming three meters per second slower, it's traveling three meters per second slower after each second.

3:09Again, that represents the acceleration. And what about its direction now? Well, whenever an object is slowing down, its acceleration is in the opposite direction of the velocity. So, here, the velocity is to the right and this object is slowing down, so the acceleration must be in the opposite direction, so the acceleration is to the left.

3:30Okay, what about the last case? Well, in the last case, the object's velocity is the same. It's neither speeding up nor slowing down, it has a constant velocity. What about its acceleration now? Well, the velocity is not changing at all, therefore it is not accelerating.

3:47In other words, its acceleration is zero. What if the car was traveling really fast? Let's say it was zooming by at 300 meters per second! Well, again, look, the velocity is not changing, and so the acceleration is zero. And this is important, I still get confused between acceleration and velocity, I should think that if something's going very fast, it must be accelerating very highly as well.

4:12But that's not true. Something can be moving very fast, but if its velocity is not changing, acceleration is zero. So acceleration is not about how fast something is going, it's a measure of how quickly its velocity is changing. Velocity must change in order to have an acceleration.

4:28All right, let's test our understanding and see if we can predict the direction of the acceleration for a few cases. So, in the first case, we throw a ball up. So as it goes up, it slows down, right? So can you think about the direction of the acceleration of that ball?

4:40Why don't you pause the video and think about it? Okay, we know the ball is going up, which means its velocity is upwards. Velocity is in the same direction and which the ball is moving, okay? Now we also know that when things are slowing down, its acceleration is in the opposite direction, which means, if the velocity's upwards, its acceleration must be downward.

4:59So we need to be very careful, the ball's acceleration was downwards. Okay, here's the next example. We have an airplane that is cruising to the left at a constant velocity. What's the direction of the acceleration? Okay, well, it's moving at a constant velocity, which means there is no change in velocity.

5:20If there's no change in velocity, there is no acceleration at all, acceleration is zero. There is no acceleration, there is no direction, it was a trick question. (laughs softly) Okay, here's the last question. Now we have a bike that's moving to the left and it's getting faster, meaning it's speeding up.

5:37What's the direction of the acceleration? Okay. All right. We know that the velocity of the bike is to the left, because it's moving to the left, and if the object is speeding up, then its acceleration is in the same direction as the velocity. So since the velocity is to the left, the acceleration must also be to the left.

5:56Okay, before we wind up, one last question we could have is, could an object have an acceleration if it has a constant speed, what do you think? Well, let's see. Consider a car that's moving in a circle at a constant speed, so here we have the speed to be a constant.

6:12Is it accelerating or not? At first we might think it's not accelerating, it's not speeding up, not slowing down. But what about its velocity? Remember velocity is speed with direction, and if you look at its direction, it is continuously changing, so the velocity is definitely changing.

6:29Remember, acceleration is about changing velocity. If the velocity changes, there must be acceleration. So, look, this object, this car is accelerating. We'll not worry too much about the direction of the accelerations over here, but what's important is that it's possible for the speed to be constant and the velocity to be changing if the direction is changing.

6:48So even if an object's direction is changing, it is accelerating. Okay so, acceleration is a measure of how quickly the velocity changes. And if that velocity is changing due to increasing speed, for example, if an object is moving faster, then the acceleration and velocity will be in the same direction.

7:08On the other hand, if the velocity is changing due to a decrease in speed, if the object is moving slower, then the acceleration will be in the opposite direction of the velocity. And finally, if the speed is a constant, along with the direction, if both the speed and direction are constant, then the velocity doesn't change at all.

7:26In that case, there is no acceleration. So in that case, the acceleration would be zero.