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Posts Tagged ‘gravity’

10 physics misconceptions, explained poorly

Posted in Physics, tagged , , , , , , on November 14, 2013| 4 Comments »

[This blog post was written by a guest columnist, a D-student in freshman physics who will remain anonymous]

10.         It’s winter because we’re far from the Sun

Everyone knows that it’s cold in January because, well, we’re farther from the Sun that usual.  The orbit of the Earth is elliptical, so in the Summer we’re closer to the Sun, like Mercury.  I have no idea why the seasons are reversed in Australia…maybe it’s because they’re upside-down?

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9.            Force is non-reciprocal

I tug on a rope with a force of 100 N.  On the other end of the rope is a football player; let’s say Greg Olsen (TE for the Carolina Panthers, of course, but you knew that I’m sure).  With what force is Greg Olsen pulling on the rope?  It must be much more than 100 N, because a football player is stronger than me.

8.            Areas and volumes have the same conversion factors as linear units

If 100 cm = 1 m, then 100 cm2 = 1 m2.  This is so obvious it doesn’t merit comment.  Another way to look at it is that a meter and a square meter are, basically, the same thing.

7.            Acceleration is the same as speed

Acceleration is, like, how fast you’re going.  So if I throw a ball straight up, at the top of its arc, its speed is zero, so its acceleration must be zero.  Can I have some of those Cheetos?

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Best comic ever?

6.            Weight and mass are the same

I was asked in lab the other day to find the weight of a brass cylinder.  So I did:  I weighed it, and got that its weight was 250 g.  I was then asked to find the force due to gravity on the object, but I don’t know how to do that.  Oh, I have to go; I’m rushing Phi Upsilon.

5.            There’s a magical force that appears whenever you move in a circle

So, I was driving the Tail of the Dragon on my scooter the other day, and almost got pulled off the road because of centrifugal force.  That’s another kind of force; you know, like gravity, friction, drag, spring force…centrifugal force.  It appears whenever you move in a circle.  It’s directed outward.  It is a repulsive force, the opposite of gravity.

4.            Objects have a memory of circular motion

If you spin a circle with a ball in your hand, then let go, the ball will spiral outward (obviously) because by the 1st Law objects in motion stay in the same kind of motion that they had before: circularly moving objects keep moving in a circle, etc.  I might then wonder why my scooter didn’t keep going in a circle in spite of centrifugal force, but luckily I don’t ever experience cognitive dissonance.

3.            There’s no gravity in space

Here’s a spoiler in case you didn’t see Gravity with Sandra Bullock and George Clooney.  In the scene where Sandra Bullock is knotted up in some ropes, she tries to hold on to George Clooney, but lets go.  Of course then George Clooney plummets towards the Earth, because of gravity.  They must have been right at the invisible border between space and not-space, where gravity suddenly drops to zero.

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2.            g stands for “gravity”

The formula for weight is w = mg, which stands for mass times gravity.  g is gravity.  It’s like a force or something.  I have no idea why my instructor winces every time I say this.

1.            No net force means no movement

This is the most obvious one of all.  On one of our homework problems, there were only two forces acting on a box: 50 N up, and 50 N down.  The net force is clearly zero.  So the box cannot be moving!  Therefore v = 0 (duh!)  But my professor marked this wrong.  She said that v might be 50,000 m/s for all we know.  That makes no sense!  Physics is too hard.

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If you enjoyed this post, you may also enjoy my book Why Is There Anything? which is available for the Kindle on Amazon.com.

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I am also currently collaborating on a multi-volume novel of speculative hard science fiction and futuristic deep-space horror called Sargasso Nova.  Publication of the first installment will be January 2015; further details will be released on Facebook, Twitter, or via email: SargassoNova (at) gmail.com.

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Truck pulls space shuttle: so what?

Posted in Uncategorized, tagged , , , on January 1, 2013| Leave a Comment »

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A recent Toyota commercial begins, “In space, the shuttle Endeavor is practically weightless.”

Do we really have to go over this again?

The fact that the word “practically” is in there indicates that the copy writers don’t have a clue about physics at all.

If they had just said that Endeavor is weightless, I’d be more forgiving.  Such a statement could mean that Endeavor was millions of miles from the solar system, in deep space, and therefore (almost) weightless.  Or it could (more plausibly) mean that Endeavor was in orbit, and that its apparent weight was zero, and they were just confusing weight with apparent weight (like most non-physicists do).

But the Madison Avenue geniuses said Endeavor was “practically weightless.”

Sigh.

This implies, of course, that in space you have weight, but it has been reduced—by being in space, apparently.  The acceleration due to gravity, g, does decrease as you leave the Earth, but as I’ve already discussed, it doesn’t go down enough to approach zero—not unless you go ridiculously far from any other massive object.

Now, a commercial with stupid physics wouldn’t normally get me to reblog about a topic I’ve already covered.  But it gets worse: the Toyota people double-down on their ignorance, and pile BS onto their BS.  The whole point of the commercial is that their truck can pull the space shuttle.

Gee, really?  Well guess what—a mini-Cooper could have pulled the space shuttle, too, given enough time.  So could I.  So could Mr. Burns.  You see, Newton’s 2nd Law says that a net force causes an acceleration, so any net force will cause (some) acceleration.  Sure, it might be small, but in the absence of friction it will eventually get the job done.

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…and so, ad infinitum.

I once saw a video of a flea pulling a hockey puck along the ice, even though the puck (around 160 g) had a mass over 700,000 times bigger than the flea (around 220 μg).  It took some time, but the puck eventually moved noticeably.  (Sorry, I couldn’t find the video on the internet.)

Well, what about friction?  Maybe there’s some horizontal friction between the shuttle and the ground, and a Toyota Tundra is forceful enough to “overcome” that friction whereas a mini-Cooper is not.  This is a valid point, but the writers of the commercial were definitely not thinking of this.  How do I know they were not thinking of this?  Well, because they say (as if it is important), “that bad boy weighed 292,000 pounds.”  If that’s all the information we are to be given, then we can’t conclude anything about the merits of their truck: if friction is zero, then the feat is less than impressive.  If instead the coefficient of friction is tremendous, and the normal force between the shuttle and the ground is truly 292,000 pounds, then the feat is amazing, in particular because I would wonder why the truck doesn’t subsequently pull itself back towards the shuttle à la Newton’s 3rd law.  But they don’t mention friction, and therefore they don’t get to play that card.  Occam’s razor suggests that the copy writers just don’t know squat about physics.

Anyway, I have nothing to say about the merits of the Toyota Tundra.  Maybe it’s a good truck, maybe it’s not.  But as for Toyota Truck commercials…please turn the channel.  You’d do better to watch a roadrunner cartoon.  The physics isn’t any better, but at least it’s entertaining.

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