November 2012 SAT Physics Subject Test Discussion

Test Preparation SAT Subject Test Preparation
101

<p>The last question was definitely 1 and 2, the first one had to do with the velocities being different (closer to the planet goes faster, which is true), the second had to do with the areas being equal which is also true, but the last said that gravity acted equally on both the near and far positions, which is false.</p>

<p>I said 17 m/s for the tornado one, it got 680m closer in 40 seconds, so 680/40= 17.</p>

<p>I said 60% efficiency.</p>

<p>What about the one with the block sliding into a rough patch until stopping? It started with 36 joules of energy…</p>

102

<p>And I think the force for that one with scientific notation would’ve been 5x10^6. It ended up as (2x10^11)/(4x10^4) I think, so it would be (1/2)x10^7 which is the same as 5x10^6. I may be wrong though…</p>

103

<p>^I agree with 5x10^6</p>

104

<p>And the heat engine one was that 300j was exhausted into a reservoir, or something along those lines, which would make it 60%.</p>

105

<p>Actually it’s 40%. The formula is 1 - (heat out/ heat in). 1- (300/500) = .4</p>

106

<p>guys… efficiency isnt exhaust over heat absorbed…</p>

107

<p>when they said “exhausted” does it mean wasted? or was it the work done by the engine?</p>

108

<p>Anyone remember the one converting from watts to joules?
Was that a trick problem or what
What was the answer?</p>

109

<p>^I think that’s why our answers are switching between 40% and 60%. The way they stated it wasn’t entirely clear, I assumed they meant that 300 was used (transferred to a reservoir, not released in heat), which is 60%, but I could see how I may have misinterpreted the question.</p>

<p>Anyone know the rough patch energy one?</p>

110

<p>Anyone remember

  1. Watts to joule conversion
  2. White color changing to different colors (I put 1 and 2. Idk what 1 was. 2 was prism)</p>
111

<p>I also said 1 and 2 for the color one, but that was an educated(ish) guess=)</p>

112

<p>Joule = Watt*second
But I think the question gave us Kelvins, which are a unit of temperature, not energy…</p>

113

<p>question about collisions dealing with kinetic energy,momentum, etc</p>

<p>Here is the answer to that: <a href=“https://www.■■■■■■■■.com/questions/369?title=Physics+SAT+II+Question[/url]”>https://www.■■■■■■■■.com/questions/369?title=Physics+SAT+II+Question&lt;/a&gt;&lt;/p&gt;

114

<p>Momentum definitely was not conserved in case 3—initially, momentum was only in the x direction so Py=0
For case 1, both balls had equal and opposite components in the y direction, so net momentum Py was still 0.
Case 2 is the special case of equal masses colliding elastically head on, and momentum is DEFINITELY conserved by definition; Py is also 0 here. Case 3 had the two going off at right angles, with one ball going down. This creates some momentum Py≠0 so momentum is not conserved.</p>

115

<p>I second @blerb on the momentum, I also said it was only conserved in 1 and 2.</p>

116

<p>For the scientific notation one, I got 1 x 10^-7…I’m assuming I got it wrong, since two people on here say it was something else. This is what I get for not going to a school that offers AP Physics and being forced to take Honors Physics with a teacher who didn’t realize that we students actually understood basic algebra and trigonometry enough to do real problems in mechanics… :(</p>

117

<p>What? Momentum is ALWAYS— I repeat ALWAYS— conserved unless a net external force is acting on the system (friction, pushing, etc…).</p>

118

<p>Also, why is the .6c one greater than .6c but less than c? According to special relativity, shouldn’t the observed speeds by both observers be equal? Or did I just teach myself that wrong too?</p>

119

<p>when the two observers are speeding towards each other, the oncoming person looks faster than him or herself. as an observer outside of the motions like when “on Earth,” the two people are going in pop direction at an equal pace. </p>

<p>so, to the observers themselves, the other person seems faster. so it is greater than 0.6c. but it has to be less than c because nothing can be as fast as or faster than light.</p>

120

<p>

I agree–momentum is conserved in the entire system. But momentum is a vector quantity, so when separated into its x and y components, they must remain equal. Pi=Pf in the x direction, and Pi=Pf in the y direction–in this case, Pi=Pf=0 for the y direction.</p>