You're driving along in your truck at 70 miles per hour [when] your friend sticks his hand outside your truck
We're given some constants here. This is a drag coefficient
0.7 and this is the density of air the density of the fluid that you're traveling through that is the
[Drag] [Force] on his hand when it's turned
we're asked to find the drag force on his hand when it's turned sideways like this [if] the direction is this way the direction of
Travel is [this] way sideways and then in part B
When it's perpendicular like this, so we know that whenever his hand is turned like this. This is going to there's going to be
fewer air Molecules
Exchanging momentum with [his] hand right and so the [cross-sectional] area that is of concern is this area right [here]
the area
That is exchanging momentum
So whenever it's like this it's exchanging less momentum with with them because there's fewer molecules
Hitting it then when it's like this there's more molecules
exchanging more momentum with it, right
Okay, so that makes sense that this number is is a factor it's it's a multiplicative factor because if this goes up then that
Means the Drag Force goes up the density is is also proportional because the density goes up then
the Force goes up
And that makes sense because if the air is is less Dense
Right like if it's if you're you know up on a mountain or something
Then there there's less molecules to exchange momentum with your hand
you can imagine the extreme case where you're in space where there's no air where there would be no force on your hand or
If your drop if you're you know traveling in a boat?
and you
Stick your hand down into the water and you know that even if you're traveling at a very low speed that your hand is going
To get pushed back dramatically because the density of water is a thousand kilograms per Meter Cube
So it would be a result a lot much larger Drag force
Okay, and then this coefficient here this C is the drag coefficient
It's highly variable from anywhere from [0.4] to [1.0]. And so we've taken the average of the two
It also depends on [velocity]
But we're moving at constant velocity so [we] won't [have] to deal with those factors and and this this this constant is dependent on
the shape of the body and [how] the body you know interacts with the the flow of the air and so forth, so
Okay, so that's basically and then of course we realize that the faster the car is moving the object through the [flue] we realize [that]
The Force will be much more if it's moving faster through the food, okay?
so [we] first thing we need to do is convert everything from English to SI units so [that] 70 miles per hour and
These these areas [I've] calculated [the] area when it's like this to be [4.25] inches
I just did an approximation and in this area whenever it's like this to be [thirty] inches squared
[so] we need to convert that the meter squared and then the velocity to meters per second, and I've done that previously for time sake
70 miles per hour thirty one point three meters [per] second [4.25] inches squared is this and 30 inches squared is this so?
Part a that's right. Let's let's get calculating so
Fd, and we'll call this [Fd] 1 is equal to 1/2 C is 0.7
[Roe] is one point two nine
The area of the first one 4.25 inches squared is this point zero zero two seven meters squared point zero zero two
Seven and then times the velocity squared 31 point three
meters per second squared so thirty one
point three
squared
We calculate and we will obtain
one Point one nine, Newton's
Which is approximately equal to point two seven pounds?
So about a four pound so it's it's not very dangerous to do these things, but it is dangerous
Let me say this way don't do it because if there's oncoming traffic you get your arm torn off
That's that's the dangerous part, so if there is not on coming traffic than it's not you know. That's [silly] a dangerous thing but
you never know these things happen and
they have happened in history, so
Don't become a stat
point seven
1.29 same number and then this number changes this number is a little bit bigger. We see point zero one nine four
Thirty one point three we calculate this number is eight point five eight newtons [and]
[that's] equal [to] one point nine three pounds
Okay, so these numbers seem. You know about right based on my experience, so
Okay, see you next time dry Force
