# A collection of solutions from elementary dynamics

1. Dennis Chapman did not eat for a few days and went to a restaurant which says buffet all you can eat just $4.95. Dennis was so happy that he started eating and eating till the owner of the restaurant asked Dennis "May I help you sir?". Dennis said yes, just go away and let me eat. After eating Dennis weight from 185 lb became 200 lb. What is Dennis mass after eating?

2. If Dennis weights 100 kilograms on earth and we sent him to the moon. What would his mass on the moon be?

3. What are the dimensions of a square acre of ground if 1 square mile = 640 acres? Give the answer in feet x feet.

4. If a boat can move at 17 mph. in still water, how many minutes will it take for it to travel 13 miles upstream in a river flowing at 8.0 mph?

5. Consider a boat that can travel with speed V in still water. Give a condition that the elapsed time for that trip will be least?

6. Janice throws a stone horizontally from the roof edge of a 25 meter high dormitory. It hits the ground at a point 30 m from the building. What was the initial velocity?

7. An Olympic athlete throws a javelin at four different angles above the horizontal, each with the same speed: 30º, 40º, 60º, and 80º. Which two throws cause the javelin to land the same distance away?

8. A 32 lb fish is weighted with two spring scales, each of negligible weight. What will be the readings on the scales? The two scales are connected on the same straight line.

9. If a spring operated gun can shoot a pellet to a height of 300 feet on earth, how high could the pellet rise if fired on the moon?

10. If we push Dennis from the top of the Empire State Building, through the air (air friction is present), to the ground below. How does his kinetic energy (K) just before striking the ground compare to his potential energy (U) at the top of the building?

11. A girl throws a rock horizontally, with a velocity of 20 m/s, from a bridge. It falls 40 m to the water below. How far does the rock travel horizontally before striking the water?

12. A 40 N stone is dropped from a height of 20 m, and strikes the ground with a velocity of 20 m/s. What average force of air friction acts on it as it falls?

13. Concrete is sold by the cubic yard. What is the mass, in kilograms, of one cubic yard of concrete that is three times as dense as water?

14. A piece of iron sinks to the bottom of a lake where the pressure is 20 atm. What happens to the density of that piece of iron?

15. The weight that will cause a wire of diameter d to stretch a given distance, for a fixed length of wire, is proportional to what?

16. If the oxygen molecules are 16 times more massive than hydrogen molecules. At a given temperature, what is the average molecular kinetic energy of oxygen, compared to hydrogen?

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#### Solution Summary

The solution describes the answers for different problems from various fields submitted by a student.