# Moving perspectives

Navigation bar: [carnival] [river] [superball] [collide] [barnpole] [driving] [carousel] [slingshot] [gravity] [bungee] [hispeed] "Go back" to return.

### How might the following illustrations be used to pose puzzles for students in this class or that?

The illustrations below are generally not accompanied by a narrative. That's because we think they offer direct experience, as well as opportunities to take data for analysis, to students in courses that range from conceptual and introductory physics through modern physics and beyond. Hence you supply the narrative. However, we do suggest some possible questions and mention that:

```A useful mnemonic for relative motion in the Newtonian world is: vAC=vAB+vBC, where e.g. vAB is the vector velocity of object A with respect to object B. Note always that: vAB=-vBA and in sums "a common middle letter cancels out".```

What follows are not just moving illustrations that anyone can enjoy. They are data streams. Each panel contains on the order of fifty thousand data points. These include afterimages from previous panels as if the display has a long-duration phosphor. Hence rates of change may also be determined from one panel alone. The information in these panels can be used to explore the physical phenomena being examined in a quantitative way, as described in the note below. Thus they are experimental data from a virtual world. As with experimental data from the real world, they can be used to examine the fit to models being illustrated in a given course, as well as deviations from those models that might give rise to better models downstream.

Note: You can freeze the animations by hitting escape on some browsers, so that measurements can be made from the captured stills e.g. by saving them to disk (e.g. after a [Ctrl-PrtSc] copy to Window's Clipboard) and then loading them into the free scientific image analysis program ImageJ. It's good practice for doing detective work on real world videos as well. To reanimate the figures here after hitting escape, you can reload the page. Some more interactive empirical observation exercises may be found here and here.

Empirical observation exercises (EOE's) for instance play an increasing role in AP Physics courses and assessments. Some exercises adapted in 2015 from these "moving perspectives" may be found here. You and your students may also enjoy our JS/HTML5 electron-microscope simulator here. Earlier interactive EOE's may be found here and here.

## The Ferris wheel toss

The above animation illustrates a Ferris wheel problem that Professor Bernard Feldman used on a kinematics test long ago. It basically shows simultaneous views of a ball tossed up and then caught by a Ferris wheel rider from one inertial and two non-inertial points of view. Although Newton's predictions are easier to track from the inertial point of view, it turns out that they still work locally in accelerated frames and curved spacetime if we consider "geometric accelerations and forces" that act on every ounce of an object's being and can be made to disappear by a suitable vantage point change. (Disclaimer: Please stick with only computer versions of this experiment.)

### Possible questions:

• If this illustration is taking place on earth in real time*, what's the Ferris wheel's radius?
• Alternatively, what's the projectile's elevation change between toss and apogee?
• What are measurement uncertainties in these values, and how can you lower them?
• What's the rider's speed, and the map-frame speed/angle of projectile launch?
• At what speed/angle does the rider toss the ball from atop the Ferris wheel's arc?
• What forces do map-frame observers see acting on the in-flight projectile?
• What forces do those non-inertial observers see acting on the in-flight projectile?
• What might be learned from the still image of a real Ferris wheel below?

* The animation should run at ~6 frames per second or a wheel rotation rate of ~4.5 rpm.

## Landlubbers and river rats

The animation above shows the Golden Eagle Ferry working its way back and forth across the Mississippi between St. Charles County in Missouri and Calhoun County in Illinois. As shown at right in the marked up screencap from this animation, the transverse vector velocity of the boat with respect to the land (vBL) equals the 45 degree upstream vector velocity of the boat with respect to the water (vBW) plus the downsteam vector velocity of the water with respect to the land (vWL). Thus as suggested above, the expression vBW+vWL equals the quantity vBL obtained by removing "W+vW" bracketed by identical subscripts W from the middle of the former. In this particular example, the downstream motion of the water in effect cancels out the upstream component of the boat's motion in the water.

### Possible questions:

• How much does a round trip on the Golden Eagle Ferry cost?
• How many logs per minute are floating down the animated river?
• How might you capture and analyze an inset like that at right?
• From whose perspective is the boat moving fastest?
• Whose perspective is correct?
• What might be learned from the snapshot of the real ferry below?

## A super ball rocket

Above you'll see map-frame and ball-frame perspectives of a collision between a massive ball and a light ball that have opposing velocities. The result is a light ball with much more kinetic energy than it had before collision. But why?

### Possible questions:

• How can afterimages in animation panels be used to measure velocity?
• In the frame of the big ball, how does the small ball's velocity change on collision and why?
• In the map frame, why does the small ball leave at a higher speed than it arrived?
• If the big ball dropped from 2 meters high, how high will the small ball bounce?
• Can you put experimental constraints on the mass ratio between the two balls?

## Collision in 2D

Here is a glancing collision between two objects, seen from the intial rest frame of the blue object as well as from the (green) center of mass frame.

### Possible questions:

• Is the collision elastic, perfectly inelastic, or something else?
• What can you say about the relative mass of the two objects?

## The farmer and the pole vaulter

This is the classic story about a farmer hoping to capture a pole vaulter running at half lightspeed in his barn as the vaulter passes through. The farmer locks the rear barn door and opens the front, realizing that the length-contracted pole will allow him to shut the door even though the pole would otherwise barely fit. The pole vaulter, who sees the barn length-contracted, figures there is no way the farmer can shut the door with both ends of the pole (blue) in the barn (red). Who is right?

### Possible questions:

• Which happens first: Pole's nose hits back wall or pole's tail enters barn?
• What does this exercise say about the simultaneity of spatially-separated events?
• If the pole is 4.5 meters long, what is the elapsed-time between frames?
• When does the shock from the barn back-wall collision reach the pole's trailing end?
• Can the back-wall be unmoved AND the pole uncompressed during this event?

## Proper versus coordinate acceleration and force...

The following animations illustrate acceleration and/or force vectors from more than one perspective. The illustrations include those coordinate-dependent geometric accelerations and "fictitious" forces that act on every ounce of an object, and that allow Newton's laws to remain locally useful at low speed in accelerated frames as well as in curved spacetime. You'll see further that in high speed situations, without curved spacetime or accelerated frames, that coordinate acceleration and the proper-acceleration felt by a traveler are also not one and the same. Thus the coordinate acceleration of objects in moving and/or accelerated frames includes geometric (frame-dependent) effects that distinguish it from the proper-acceleration caused by the sum of external physical forces.

A draft paper on this topic may be found here.

### ...while driving to work

Map and car frame perspectives of proper (red) and geometric (blue) accelerations
for an automobile driving from one stop sign to the next.

This illustrates how geometric or "inertial" forces (blue), that are felt in accelerated frames and which act on every ounce of one's being, vanish entirely when one sees the same motion from an unaccelerated perspective. On the other hand the physical forces (red) e.g. of the car on the driver, which act at the point of contact rather than on every ounce of one's being, cannot be made to vanish. At low speeds, these "proper forces" also do not change in magnitude or direction from one frame to the next.

#### Possible questions:

• In the map frame, where is the force that pushes you back into your seat on acceleration?
• While pressing on the brakes, from the map perspective what causes your seat belt to stretch forward?

### ...on a carousel

Map and spin frame perspectives of proper (red) and geometric (blue) accelerations
for an object released from a carousel.

In this example, the "fictitious" geometric force experienced by the carousel rider in the spin frame is correctly described as "centrifugal" or radially-outward. After release of the object, describing the straight line motion in the spin frame is even more complicated even though the map frame perspective sees no forces operating at all!

#### Possible questions:

• In the map frame, where is the force that pushes you toward the rim of the carousel?
• In the spin frame, what part of your body does that blue outward force act on?

### ...on a rotating sling

Map and spin frame perspectives of centripetal (red) and geometric (blue) forces
on an object released after being spun around on a massless rope,
along with the reaction force on the rope's anchor point (magenta)
caused by the centripetal force on the object.

Before the object is released, the blue geometric force is purely centrifugal (pointing radially outward), while after release the geometric force is a sum of centrifugal and Coriolis components. Note that after release in the spin frame that the centrifugal component (light blue) is always radial, while the Coriolis component (green) is always perpendicular to the projectile's velocity in the spin frame.

#### Possible questions:

• In the map frame after projectile release, where are the centrifugal and Coriolis forces?
• In the spin frame, what parts of the projectile do those centrifugal and Coriolis forces act on?
• Before release, is there a force on the rope equal and opposite to that of the rope on the projectile?

### ...rolling off a cliff

Rain and shell frame perspectives of proper (red) and geometric (blue) accelerations
for an object that rolls off a cliff.

Note that the rain frame perspective is more like that of a trampoline jumper who tops out just as the ball reaches the edge of the cliff, than it is like the perspective of a raindrop per se. There is of course also a downside to the rain frame perspective, namely that sooner or later the planet you're on gets in the way.

#### Possible questions:

• In the rain frame, where is the force on the rolling ball due to gravity?
• In the shell frame, on what part of the ball does that blue downward acceleration act?
• Which of these two frames is best called "locally inertial"?

### ...on the pipeline bungy

Here are two animated perspectives on a bungee jumping problem that involves conversion of mechanical energy from gravitational potential to kinetic to spring potential and then back. This conversion allows one to predict end points of the trajectory and the way in which speed depends on position. More detailed analytical solution of Newton's second law equations, in the map frame, was done using differential equation routines in this Mathematica worksheet. How would the same event play out if the bungee jump took place inside a large interstellar spaceship undergoing 1 gee acceleration?

#### Possible questions:

• Why does the jumper "float like a lazy fly ball" with the bungee cord unstretched.
• How does the character of the oscillation change when the cord is stretched?
• Where does the gravity force come into the problem from the jumper's perspective?
• Hint: To the jumper, g may enter as a non-force property of the bridge...

### ...on high speed roundtrips

Map and traveler perspectives of a roundtrip
at constant 1 gee proper-acceleration (red arrow in the traveler frame)
between the sun (yellow) and a hypothetical star (cyan) 6.9 lightyears away.
Proxima Centauri (orange) 4 lightyears from the sun is shown in orange toward the upper left.

Moving perspectives get even more interesting when high speeds are involved. Times elapsed between events depend on the motion of your clock. Distances between stationary (co-moving) objects may change from some vantage points. Even the sequence of events that are not on the same time-like world line becomes a matter of perspective!

In the animation above, from each perspective a year should elapse about every two seconds or every 100/17.4 frames. However unlike most earlier examples, the panels in the map frame at left are not just translated or rotated versions of those in the traveler frame at right. To begin with, after each round trip ship-pilots on this shuttle-run will have aged only half as much as their starfleet academy classmates stationed on earth. This is time-dilation in action.

To illustrate this, we've chosen to sacrifice panel-by-panel simultaneity. Since traveler and map see the sequence of events differently, there was no unbiased way to keep them in sync anyways. Snapshots associated with individual events can still be compared even if they happen at different times.

Major new differences between panels still abound. These include the distance changes between co-moving stars, seen in the traveler frame. This is length-contraction in action.

Returning to the acceleration theme of this series, note also that the coordinate acceleration (green) seen in the map frame is only significant in the year before and after each launch. The proper-acceleration (red) felt by the traveler is significant throughout the voyage.

We've further added the trace of a light signal initiated from each launch point, but 0.886 map years after launch. This light pulse reaches the traveler at the voyage midpoint to remind them to begin deceleration. Aside: An interesting fact about 1 gee constant proper acceleration is that a year's head start from rest is enough to keep a photon from ever catching you, that is until you either take your foot off the accelerator or you run out of gas. Thus even if the traveler was heading to another galaxy that reminder pulse would have to be sent within the first year.

The light pulse radii are clearly different in the two frames. They illustrate different perspectives on simultaneity. For instance, note in the map frame that Proxima Centauri sees the turnaround pulse before the destination star does, but that the converse is true in the traveler frame. If one could warn the other that it was coming before it got there, of course, we would be able to test who was correct. The large distances involved make this impossible, so that alas the answer to which of two separated events come first, in this case, remains hopelessly dependent on one's point of view. Thankfully both observers do agree about the sequence of events along any time-like world line e.g. during any individual object's history.

Here are (2+1)D views of a similar trip: map frame, traveler frame.

#### Possible questions:

• Can you estimate model lightspeed from the data here?
• What is 1 gee of acceleration in lightyears per year squared?
• What's happening in the traveler frame at voyage midpoints?
• The ship's headstart on the reminder pulse seems bigger to the traveler. Why?
• What is the simplest equation able to quantitatively model these observations?
• Why and how does the apparent distance between stars change in the traveler frame?

Note: In addition to differences between proper and coordinate acceleration at high speeds, proper-velocity (momentum per unit mass w) may be more useful than coordinate speed v. The coordinate speed of such objects is generally near lightspeed, while proper-velocity tells us how rapidly they are covering ground on traveling-object clocks. This is important for example if, like some cosmic ray particles, the traveling objects have a finite lifetime. Proper velocity also clues us in to the object's momentum, which has no upper bound.

For example, a 45 GeV electron accelerated by the Large Electron-Positron Collider (LEP) at Cern in 1989 would have had a Lorentz factor γ of about 88,000 (90 GeV divided by the electron rest mass of 511 keV). Its coordinate speed v would have been about sixty four trillionths shy of lightspeed c at 1 lightsecond per map second. On the other hand, its proper-speed would have been w = γv ~88,000 lightseconds per traveler second. By comparison the coordinate speed of a 250 GeV electron in the proposed International Linear Collider[9] (ILC) will remain near c, while its proper-speed will significantly increase to ~489,000 lightseconds per traveler second.

Proper-velocity is also useful for comparing relative velocities along a line at high speed. In this case wAC = γABγBC(vAB+vBC) where A, B and C refer to different objects or frames of reference. For example wAC refers to the proper-speed of object A with respect to object C. Thus in calculating the relative proper-speed, Lorentz factors multiply when coordinate speeds add. Hence each of two electrons (A and C) in a head-on collision at 45 GeV in the lab frame (B) would see the other coming toward them at vAC~c and wAC = 88,0002(1+1) ~ 1.55×1010 lightseconds per traveler second. Thus colliders can explore collisions with much much higher momentum than can fixed-target accelerators.

### Corollary links...

#### ...related to information physics

This page is http://www.umsl.edu/~fraundorfp/p001htw/relativeMotion.html. Although there are many contributors, the person responsible for errors is P. Fraundorf. This site is hosted by the Department of Physics and Astronomy at UM-StL. MindQuilts site page requests ~2000/day approaching a million per year. Requests for a "stat-counter linked subset of pages" since 4/7/2005: .