Question - Answer
What transparent object, found in many households, becomes less transparent when wiped with a clean cloth?
Wet ground glass. The clean cloth must be dry. The commonest example in a kitchen might be a Pyrex baking dish that has become worn or scratched on the bottom so as to be merely translucent, not transparent. When wet it is clear.
Posted: December 10, 2012, 9:53 am
A child sits in the back seat of the family car, holding a helium balloon on a string. All the windows are closed. As the car accelerates forward, does the balloon tilt forward, tilt backwards, or remain in the same place?
As the car accelerates forward, the balloon on its string tilts forward, too. Inertial forces push backwards in the car, pressing the people against their seats (an effect with which we are all familiar), but also compressing the air at the back of the closed car. This increased air pressure at the rear pushes the balloon forward. For similar reasons, as the car rounds a curve, the balloon tilts into the curve.
Posted: December 10, 2012, 9:47 am
A piece of solid iron in the shape of a doughnut is heated over a fire. As the iron expands, does the hole become larger or smaller or does it remain the same size?
As the doughnut expands, it keeps its same proportions; so the hole also gets bigger. This same principle is at work when an optician removes a lens from a pair of glasses by heating the frame, The next time you can't open the metal lid on a stubborn jar, heat the lid under hot water. The lid, inner circumference and all, will expand, making it easier to loosen.
Posted: December 10, 2012, 9:44 am
Study the drawing carefully. Can the man lift both himself and the block off the ground?
Although it looks as if the man were trying to lift himself up by his own bootstraps, he really isn't. True, for every pound of force that he pulls up on the rope, he also pushes down an equal force on the block, but if he is strong enough to lift his own weight plus the weight of the block, he will rise from the ground. (Tests have shown that a 190-pound man can lift both himself and a 110-pound block in this way.)
Posted: December 10, 2012, 9:40 am
The captain of a submarine tries at all costs to avoid letting his sub come to rest on a clay or sandy ocean bottom. He knows that if this happens, it can be fatal. Why?
Water pressure pushes perpendicular to a submarine's hull at every point, at the bottom as well as at the top and sides. When a sub settles on a clay or sandy bottom, the water layer may be squeezed out from beneath the hull, robbing the sub of much of its upward buoyant force. In effect, the downward forces can then "glue" the sub to the bottom.
Posted: December 10, 2012, 5:48 am