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AP Physics C 1.1 Newtonian Mechanics 199 Views


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AP Physics C: Newtonian Mechanics Drill 1, Problem 1. Which of Newton's equations of motion would have to be modified to account for the airplane's uniform speed of 220 meters per second?

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Transcript

00:03

Here's your shmoop du jour, brought to you by Airplanes.

00:07

In the event of an emergency, your Physics textbook MAY be used as a flotation device.

00:12

A high-school student on a long plane journey thinks about doing a projectile experiment

00:17

using some of the airplane food... just from a spoon, not her stomach...but her

00:21

better judgment takes over and she refrains.

00:24

If she had launched some food and wanted to estimate the launch angle required to hit

00:29

a target, which of Newton's equations of motion would

00:32

have to be modified to account for the airplane's uniform speed of 220 meters per second?

00:40

Initial and final velocities are u and v respectively, d is displacement, a is acceleration,

00:48

and t is time.

00:50

And here are the potential answers...

00:58

This problem might be intimidating at first... but before diving right into the answer choices,

01:02

let's think about the bigger picture here.

01:03

Ok. A high school student is on a plane, and is thinking about physics.

01:08

What's wrong with this picture? It's already amazing.

01:11

Specifically, she's thinking about the physics of launching some of her less than palatable

01:15

food across the aisle to hit her victim.

01:17

But we're told the plane is moving forward at a rate of 220 meters per second.

01:22

How do we account for this movement, to ensure that the victim, and only the victim gets

01:27

beaned by... beans?

01:30

Well this problem is actually testing us on our ability to isolate systems.

01:35

In other words, we have to determine whether or not the airplane's movement actually affects

01:38

the food projectile's flight, and the answer is....no.

01:42

Because the girl is sitting in the plane, which is moving at 220 meters per second,

01:46

the girl is also moving at 220 meters per second.

01:49

Boy, she's speedy.

01:50

And oh by the way, the beans are moving at 220 meters per second too.

01:54

Because her launching food takes place all in the plane, it means that the catapult and

01:59

her projectile, before she fires it, is also moving at 220 meters per second.

02:07

Keep in mind that all this "movement" is relative to the earth.

02:11

If we think about the airplane as an isolated system, with the girl, her weapons of destruction,

02:16

and her victim all contained inside the system...

02:19

...we can see that they are all stationary relative to each other because they are all

02:23

moving at 220 meters per second.

02:26

If this doesn't make sense, think about people on earth.

02:29

Every moment, we are rotating around the earth's axis at 1000 miles an hour,

02:32

around the sun at around 67,000 miles an hour,

02:35

and around our galaxy at 483,000 miles an hour.

02:39

That's.... fast.

02:41

We don't feel a thing because we're always moving this fast relative to space...

02:45

and everyone else, but relative to the earth, we aren't moving at all...

02:49

It's a good thing too, since we'd find it tough to play baseball if the balls couldn't

02:53

keep up with the earth.

02:55

With all this said, we can come full circle and answer the question.

02:58

Since the airplane is an isolated system, the movement of the airplane doesn't affect

03:03

the trajectory of the projectile.

03:04

The answer is (E), we don't have to modify any of the basic kinematics equations to accommodate

03:10

for the airplane's movement.

03:12

Well hey, sounds like the stars are aligned... heads up in Seat 17F...

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