Answer to Question #224171 in Physics for yang

Question

1. As an oil well is drilled, each new section of drill pipe supports its own weight and the weight of the pipe and the drill bit beneath it. Calculate the stretch in a new 8.00-m-long steel pipe that supports a 200000-g drill bit and a 5.00-km length of pipe with a linear mass density of 20.0 kg/m. Treat the pipe as a solid cylinder with a 8.00-cm diameter.
2. A large uniform cylindrical steel rod of density ρ = 8.8 g/cm3 is 4.0 m long and has a diameter of 5.0 cm. The rod is fastened to a concrete floor with its long axis vertical. What is the normal stress in the rod at the cross section located at (a) 2.0 m from its lower end? (b) 2.5m from the lower end?
3. Two children push on opposite sides of a door during play. Both pushes horizontally and perpendicular to the door. One child push with a force of 20.5 N at a distance of 0.600 m from the hinges and the second child pushes at a distance of 0.650 m. What force must the second child exert to keep the door from moving? Assume friction is negligible.

Answer 1

1)

$$m=(8+5000)20+200=100360\ kg\\F=(100360)(9.8)=983528\ N \\ \Delta L=4\frac{983528}{1.6\cdot10^{10}(\pi (0.08)^2)}8=9.78\ cm$$

2)

$$p=(8800)(9.8)(4)=3.4\cdot10^5\ Pa$$

3)

$$F=\frac{0.6}{0.65}20.5=18.9\ N$$