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Question to Marilyn: "If you drop a bullet from one hand and, at the same time, use the other hand (held at the same height) to shoot another bullet straight across an open field, which bullet will hit the ground first?"
 

Marilyn's answer: "... both will hit the ground about the same time. The sideways force and the downward force don't interfere with each other as they affect the flight of the bullet itself: One force takes the bullet across the field; the other brings it down to earth."

Physicist's comments: While indeed both bullets will hit the ground about the same time, there is no sideways force that "takes the bullet across the field". A horizontal force produced by expanding gasses acted on the bullet only while the bullet was inside a barrel, causing the bullet to accelerate horizontally. However, once the bullet leaves the barrel, there is no horizontal force anymore, and the horizontal component of bullet's velocity does not change (neglecting an effect of air resistance). The law of inertia (Galileo, Newton) states that any moving object has a tendency to keep on moving along a straight line with a constant speed just by the nature of things. A force is needed to change object's speed or direction of motion. It seems that Marilyn does not understand a concept of inertia, and holds onto ancient Aristotelian idea refuted by Galileo and Newton long time ago.

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Question to Marilyn: "Say that I could install a tube right through the middle of the Earth. If I jumped into one end of it, what would happen?"

Marilyn's answer: "... you would fall faster and faster until you arrived at the center of the Earth. (Although gravity would get weaker as you approach the center, your motion would help to keep you accelerating.)"
 

Physicist's comments: The wording "your motion would help to keep you accelerating" is incorrect. You would keep accelerating simply because gravity would be pulling you towards the center. It is perhaps interesting to note that as you passed the center of the Earth, your speed would be some 7.9 km/s (about 5 miles per second). Also, to eliminate a danger of bumping into a side of the tube, the tube would have to be installed between the North and South Poles.

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"... the turbulence was so violent that the plane suddenly dropped, sending passengers and flight attendants into overhead luggage compartments and the ceiling."

Physicist's comments: This is wrong. If the plane has simply dropped, the passengers would have dropped at the same rate too, therefore not moving with respect to the cabin. Try it yourself (this is one of my favorite lecture demonstrations): take off your shoe (it is a jumbo jet) and put some coins inside (passengers). Then simply drop the shoe and observe that coins are not left behind. Now, instead of dropping the shoe, throw it down, and observe that as you were pushing the shoe down, it separated from the coins. The shoe accelerated downward faster than g, while the unbuckled passengers found themselves in a free fall, accelerating downward only at g. So the AP should have reported that the turbulence has thrown the jumbo jet down, and since the jet accelerated downward faster than g, the ceiling of the cabin hit the unbuckled passengers who accelerated downward only at g. Buckle up!

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"Most astronomers now accept the idea that the universe began with a "big bang" about 15 billion years ago, when a superdense point exploded in the most gigantic bang imaginable. (...) In a closed universe, the expansion would continue until gravity from the mass of matter canceled the outward force and the motion reversed directions."
 

Physicist's comments: There are several problems with this simplified description of the Big Bang, but let's address only the issue of inertia here. If the Big Bang was a gigantic explosion, then the matter was set into outward motion during the explosion. After the explosion, there would not be anymore outward force, and  the mutual gravitational pull of the expanding matter would gradually slow the expansion. In a closed universe, it will eventually pull the matter back inward to the point of "big crunch".

Toss a rock up and observe how it slows down and then falls back. There is no upward force on the rock once it has left your hand. The only force acting on it is gravity.

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This old cartoon strip was send to me by Dr. R.R. Hake, who also provided the following verbal description: A Funky Winkerbean cartoon strip shows a stewed old codger driving a school bus. He says: "Fooey, I never have any trouble" (with kids). "If you go fast enough, they just stay pinned to their seats."

Physicist's comments: Dr. Hake (Am. J. Phys., Vol. 55, No. 10, October 1987,  page 882), provided Socratic explanation in a form of three questions:
    a) Is there a horizontal force on the kid when the bus moves with a constant velocity?
    b) Is the kid pinned to her seat?
    c) How might a bus driver pin a kid to her seat by control of the bus motion?

I hope you see that the answers are:
    a) no;
    b) no;
    c) put the pedal to the metal; as long as the speed increases, the kid stays pinned to her seat.

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Here is a dialogue between two characters, Lomax and Martin:

    "Two racing cars enter a curve at speed, a heavy car and a light car. Which one ends up on the outside track?"

    "The heavy one" said Martin.

    "Right."

Physicist's comments: Wrong. The weight of the car does not matter.

For each car to follow the same curve at the same speed, the same force per kilogram of mass is required. Hence a greater force is needed to make the heavier (actually - the more massive) car to follow the curve. But this force is provided by a (static) friction between the rubber of the tires and the roadway, and this frictional force is proportional to the weight of the car, so the mass of the car cancels out.

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“Assume”, Polton said, “that you have a road, with a sharp corner. Now assume that you have two automobiles, a sports car and a large truck. When the truck tries to go around the corner, it slips off the road; but the sports car manages it easily. Why? The sports car is lighter, and smaller, and faster; it is better suited to tight, sharp curves. On large, gentle curves, the automobiles will perform equally well, but on sharp curves, the sports car will do better.”

Physicist's comments: This is vaguely written paragraph confusing two different concepts. To see why, let us assume that both automobiles travel at the same speed, and the static coefficient of friction between the tires and roadway is same in each case.  As I explained above, the same force per kilogram of mass is required to make each car turn the same curve at the same speed. Hence a greater force is needed to make the heavier (actually - the more massive) car to follow the curve. But this force is provided by a (static) friction between the rubber of the tires and the roadway, and this frictional force is proportional to the weight of the car, so the mass of the car cancels out. Hence neither automobile will slip off the road more easily than other.

However, the truck is much more likely to tip over than the car. That's because the center of mass of the truck is so much higher above the road. Specifically, what matters here is the ratio of the center of mass height to the width of the base (a distance between left and right wheel). This ratio is much higher for the truck, while quite low for a sleek sports car which won't tip over so easily.

The truck may tip over before it starts slipping, but we don't have enough information to determine that.

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