
asked: A little sled weighs one pound. It is set in motion over frictionless ice by a toy rocket motor. After the rocket fuel is expended, the sled is coasting over the ice at one foot per second. How much force did the rocket exert on the sled to make it go?
a)1 pound
b)4 pounds
c)16 pounds
d)32 pounds
e)There is no way to tell from the information given.
Choose one answer only please. A correct answer accompanied by a correct explanation trumps a correct answer alone (so first to answer doesn't exclude others from being selected for "Best Answer."
I already know the answer (I think!) to the question. I offer it for the fun and educational value. I love to share physics puzzles that at once make a person think while simultaneously teaching basic physics concepts in a different and creative manner. My questions are meant for individuals of all levels of expertise. If you don't like a question (for whatever reason), just move on to another that suits you better.
General Advice: Equations are useful abbreviations of relationships and are indispensable to the physcist. But often they are misused -- when they become substitutes for understanding. Never put your efforts toward memorizing equations until you understand the concepts the symbols represent. Only after a conceptual understanding are the equations truly meaningful.
There are some terrific answers so far! Don't worry. I never ask a question that requires more than a basic knowledge of algebra. The concept is the important thing.
Alexander: You're right. It's an honor to have someone who asks such excellent questions of their own answer one of my questions!
Linlyons: It's always an educational experience. You have a talent for taking a question and probing deeper. Thank you!
Vincent G: Thank you. Great explanation as well as a correct answer!!!
BB: Strange but poetically creative! And correct!
Aaron: Excellent work! I'm sorry I can only choose one best answer. Please try again. I very much appreciate your answer. It helped not only myself but others who are learning what you know.
Now for my "official" explanation...
The correct answer is E. That's right, you can't tell you say! The rocket motor might have provided a small force for a lng time or a big force for a short time, but from the information givein you can't tell which. This question is very much like asking how long is a rectangle which contains 12 square inches. It might be one long and twelve high or two long and six high or three long and four high. In the case of the sled, its moomentum is like the area of the rectangle and the force and time during which the force acts are like the sides of the rectangle which multiply together to make the area.
Area = Height x Length
Change in Momentum = Force x Time
Thank you for your insightful answers and sharing your knowledge with others so that they can learn.
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