Difference between revisions of "A bug in my high school physics intuition"

From stacky wiki
Jump to: navigation, search
(Created page with "I think of myself as having pretty good physical intuition, at least for plain old mechanics. Some systems are hard to get a grip on (e.g. why does a hard-boiled egg stand up whe...")
 
Line 1: Line 1:
I think of myself as having pretty good physical intuition, at least for plain old mechanics. Some systems are hard to get a grip on (e.g. why does a hard-boiled egg stand up when you spin it?), but at least I recognize when I'm confused about them. However, I recently got some simple mechanics really wrong, and didn't feel the slightest bit of confusion about it. So I went on a quest to update my intuition so that the same failure mode never happens again.
+
__NOTOC__
 +
 
 +
= This is obviously a draft. How did you even get here? =
 +
 
 +
You're sitting on a bicycle or in a rocket ship, stationary. This gets boring, so you accelerate to 1 m/s. After cruising for a while you get bored again, so you accelerate some more, up to 2 m/s. What took more energy, getting from 0 m/s to 1 m/s, or getting from 1 m/s to 2 m/s?
  
Here's the motivating question.
 
<blockquote>
 
You're sitting on a bicycle or in a rocket ship, stationary. This gets boring after a while, so you accelerate to 1 m/s. After cruising for a while you get bored again, so you accelerate some more, up to 2 m/s. What took more energy, getting from 0 m/s to 1 m/s, or getting from 1 m/s to 2 m/s?
 
</blockquote>
 
The problem is that there are two intuitively compelling answers, which severely conflict.
 
 
:'''Answer 1.''' Once you're cruising at 1 m/s, we may as well use the inertial reference frame which is moving at 1 m/s, in which you are stationary. Then the second spurt of acceleration corresponds to speeding up from 0 to 1 m/s, which is exactly what the first spurt did in the stationary reference frame. So the two take '''the same amount of energy'''.
 
:'''Answer 1.''' Once you're cruising at 1 m/s, we may as well use the inertial reference frame which is moving at 1 m/s, in which you are stationary. Then the second spurt of acceleration corresponds to speeding up from 0 to 1 m/s, which is exactly what the first spurt did in the stationary reference frame. So the two take '''the same amount of energy'''.
 
:'''Answer 2.''' Just measure the kinetic energy differences. Kinetic energy is proportional to the square of speed, so the first spurt of acceleration took $1^2 - 0^2 = 1$ unit of energy and the second one took $2^2 - 1^1 = 3$ units of energy, which is '''3 times as much energy'''.
 
:'''Answer 2.''' Just measure the kinetic energy differences. Kinetic energy is proportional to the square of speed, so the first spurt of acceleration took $1^2 - 0^2 = 1$ unit of energy and the second one took $2^2 - 1^1 = 3$ units of energy, which is '''3 times as much energy'''.
 +
 +
The problem is that both of these answers seem intuitively compelling. For a similar problem, I thought of one answer but not the other, and it seemed so clear that it wasn't even worth double checking. After I realized what had happened, I started on a quest to update my intuition so that the same failure mode doesn't happen again. I haven't entirely succeeded, but I was at least able to console myself that many people with very good physical intuition also found this confusing. Let's review some possible explanations.
 +
 +
 +
== "You can't ignore friction because bicycles can't work without it" ==
 +
 +
"... and I don't own a rocket ship, so your question doesn't make sense." Nobody I talked to really proposed this answer, but I did run into a small number (~0.6) of lawyerly personalities who, when confronted with cognitive dissonance, immediately look for a loophole to relieve the pressure. We know better than to do that; this section is just here to give the other parts of your brain a few more seconds to find their own resolution for the paradox rather than anchoring to one of the resolutions proposed below. If you find a good one that I haven't listed below, please send it to me or add it to the discussion page.
 +
 +
 +
== "It depends on your reference frame" ==
 +
 +
Maybe the energy expended depends on reference frame. After all, different reference frames disagree about how much total energy an object has. This is a fine first step, but it doesn't stand up to scrutiny at all. We would have noticed long ago if it were possible for the physicist on the bench to observe an operation consuming 3 gallons of gas while the physicist on her leisurely stroll of 1 m/s observes the same operation to consume only 1 gallon of gas. '''Answer 1 and Answer 2 can't both be correct.'''
 +
 +
 +
== "Answer 2 is correct" ==
 +
 +
<!-- work is force times distance, not force times time -->
 +
 +
 +
== "Answer 1 is also correct" ==
 +
 +
<!-- if you're in a rocket ship. '''You have to think about what you're pushing off of.''' -->
 +
 +
 +
== What's right about Answer 1? ==
 +
<!-- the work done by the thing that produces the impulse is independent of reference frame (at least for speeds << c), but how much work it does on each piece is not. -->

Revision as of 06:47, 19 September 2013


This is obviously a draft. How did you even get here?

You're sitting on a bicycle or in a rocket ship, stationary. This gets boring, so you accelerate to 1 m/s. After cruising for a while you get bored again, so you accelerate some more, up to 2 m/s. What took more energy, getting from 0 m/s to 1 m/s, or getting from 1 m/s to 2 m/s?

Answer 1. Once you're cruising at 1 m/s, we may as well use the inertial reference frame which is moving at 1 m/s, in which you are stationary. Then the second spurt of acceleration corresponds to speeding up from 0 to 1 m/s, which is exactly what the first spurt did in the stationary reference frame. So the two take the same amount of energy.
Answer 2. Just measure the kinetic energy differences. Kinetic energy is proportional to the square of speed, so the first spurt of acceleration took $1^2 - 0^2 = 1$ unit of energy and the second one took $2^2 - 1^1 = 3$ units of energy, which is 3 times as much energy.

The problem is that both of these answers seem intuitively compelling. For a similar problem, I thought of one answer but not the other, and it seemed so clear that it wasn't even worth double checking. After I realized what had happened, I started on a quest to update my intuition so that the same failure mode doesn't happen again. I haven't entirely succeeded, but I was at least able to console myself that many people with very good physical intuition also found this confusing. Let's review some possible explanations.


"You can't ignore friction because bicycles can't work without it"

"... and I don't own a rocket ship, so your question doesn't make sense." Nobody I talked to really proposed this answer, but I did run into a small number (~0.6) of lawyerly personalities who, when confronted with cognitive dissonance, immediately look for a loophole to relieve the pressure. We know better than to do that; this section is just here to give the other parts of your brain a few more seconds to find their own resolution for the paradox rather than anchoring to one of the resolutions proposed below. If you find a good one that I haven't listed below, please send it to me or add it to the discussion page.


"It depends on your reference frame"

Maybe the energy expended depends on reference frame. After all, different reference frames disagree about how much total energy an object has. This is a fine first step, but it doesn't stand up to scrutiny at all. We would have noticed long ago if it were possible for the physicist on the bench to observe an operation consuming 3 gallons of gas while the physicist on her leisurely stroll of 1 m/s observes the same operation to consume only 1 gallon of gas. Answer 1 and Answer 2 can't both be correct.


"Answer 2 is correct"

"Answer 1 is also correct"

What's right about Answer 1?