PHYSICS HELP FORUM

Persons visit PHF to discuss physical reality as we know it. I visit to read what's happening and to add my thoughts to the confusion. A visit, now and then, causes a spark when some student asks the same physics question I asked some 50 some years ago. Into that dialog, I write and post "answers, truth, wisdom," as I see it. Thank you Physics Help Forum!

Dog Greets Owners

Problem statements of HS physics sometimes omit detail then request an "answer" when the only an "approximate answer" as a "minimum" or "maximum" is obtainable. Students are expected to approximate, hence answers are approximate. Once an answer is obtained, it is proper to ask if its magnitude is smaller (or larger) than the truth and why so. Consider the following physics text problem:

Statement:  "A dog runs back and forth between its two owners, who are walking toward one another. The dog starts running when the owners are 10 meters apart. If the dog runs with a speed of 3 m/s and the owners each walk with a speed of 1.3 m/s how far will the dog have traveled when the owners meet?"

Parachutist's Drag Force

At an instance in free fall, the acceleration of a parachutist becomes 6.8 m/s². The mass of the person is 90 kilograms. Calculate the frictional drag force of the surrounding air at that instance of the fall?

Note:  At the precise instance of leap, the acceleration is Earthward at 9.8m/s², the velocity downward is zero and so also is the resistance of surrounding air. Speed of fall through air increases, hence air resistance to that motion increases. Ultimately the speed of fall becomes "all the surrounding air" will permit... terminal velocity is attained.

Box Slides Between Springs

The sketch (left part) shows a box shoved against the strong spring (ss) compressing it 4 centimeters and restrained there by a "trigger" mechanism. When the trigger is pulled (our catalyst) the box is projected to slide across the assumed smooth plane, ultimately to encounter the weaker spring (ws) and compress it.

Calculate the greatest length of compression the weaker spring will experience.

When will the Teapot Whistle?

When you turn it on, a thermally insulated, electrically heated teapot pour contains 1 liter of water at @25°C. The metal of the teapot is equivalent to 200 cubic centimeters of water and the label of the teapot says it is rated at 1250 W. How long will you have to wait for the water to boil and for the pot to whistle?

Work to Move a Wall

A tank contains a shallow body of water. The tank bottom, left wall, and ends (not shown ~ sectional view) are fixed. The right wall can be pushed to the left and will remain vertical should that happen. Seals prevent all leakage of water. Dimensions of the device are given below.

Calculate the work required of the worker to push the movable wall a short distance to the left.