Question

8. Three children, each weighing 500 N, decide to make a raft by lashing together logs of diameter 0.50 m and length 1.50 m. How many logs will they need to keep them afloat in fresh water? Take the density of each log to be 800 kg/m3.

9. A garden hose with an internal diameter of 2 cm is connected to a nozzle that has an exit hole with a diameter 0.1 cm in diameter. If the water enters the hose at 1 m/s, at what speed does it exit the nozzle?

Answer #1

8.

Assuming the children and the logs are just afloat in water, then the logs will be fully submerged by the weight of the children.

Archimedes principle states that the upward buoyant force on a floating object is equal to the weight of the displaced water by the object.In this case, the force balance equations will be:

where n is the number of logs

**The children need minimum 3 logs to stay
afloat.**

9.

For a laminar flow of an incompressible fluid through a system, the Conservation of mass or Continuity equation gives:

As the inlet to the nozzle is a stream of water with circular cross-section and uniform velocity of 1 m/s, and outlet is also a uniform stream of water, assuming laminar flow throughout and thus applying continuity equation gives:

**The water exits the nozzle with a speed of 400
m/s.**

A raft is made of 11 logs lashed together. Each is 42 cm in
diameter and has a length of 6.0 m . Part A How many people can the
raft hold before they start getting their feet wet, assuming the
average person has a mass of 70 kg ? Do not neglect the weight of
the logs. Assume the specific gravity of wood is 0.60. Express your
answer using two significant figures.

A raft is made of 13 logs lashed together. Each is 45 cm in
diameter and has a length of 6.5 m .
1) How many people can the raft hold before they start getting
their feet wet, assuming the average person has a mass of 65 kg ?
Do not neglect the weight of the logs. Assume the specific gravity
of wood is 0.60.
Please Recheck work when finished! and be clear please/

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