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Challenge Questions

Stop in and exercise your brain. Talk about this month's Challenge from Specs & Techs or similar puzzles.

So do you have a Challenge Question that could stump the community? Then submit the question with the "correct" answer and we'll post it. If it's really good, we may even roll it up to Specs & Techs. You'll be famous!

Answers to Challenge Questions appear by the last Tuesday of the month.

The Merlin Rocket Engine: Newsletter Challenge (October 2014)

Posted October 01, 2014 12:00 AM
Pathfinder Tags: challenge question

This month's Challenge Question: Specs & Techs from IHS Engineering360:

The SpaceX Falcon 9 v1.1 uses nine Merlin 1D rocket engines in its first stage. The thrust of a Merlin 1D engine in a vacuum is 720 kN, about 10% more than its 650 kN thrust at sea level. Why does the thrust of the Merlin 1D increase with increasing altitude?

The answer to this challenge will be posted later this month, right here on CR4.

45 comments; last comment on 10/17/2014
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Balloon Ride: Newsletter Challenge (September 2014)

Posted September 01, 2014 9:00 AM
Pathfinder Tags: challenge question

This month's Challenge Question: Specs & Techs from GlobalSpec:

You are inside a car with the windows closed, and you are holding a helium balloon by a string. You are cruising at 30 mph. Now you make a right turn. What happens to the balloon? Does it move to the left? Or to the right? Or does it stay in the same position? Find the right answer and explain the reason.

And the answer is:

The air inside the car tends to continue in the same straight-line direction (Newton's laws), so when turning the pressure on the outside radius of the turn will be a bit higher. Therefore the balloon will be pushed to the right (to the inside radius of the turn).

40 comments; last comment on 09/23/2014
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The Sprinkler Mystery: Newsletter Challenge (August 2014)

Posted August 01, 2014 12:00 AM
Pathfinder Tags: challenge question

This month's Challenge Question: Specs & Techs from GlobalSpec:

An old home's new sprinklers are scheduled to start up at 9 PM but do not. A tech manually turns on the sprinklers from the controller located in the basement boiler room. He hears from upstairs that they are working fine so he closes up the controller, goes upstairs and sees that the sprinklers have stopped working again. What's going on?

And the answer is:

Based on a true story from someone here at IHS, the Sprinkler Mystery gave us all a good laugh. It turns out the answer to the mystery was that the sprinkler controller was plugged into a switched outlet. When the sprinkler system tech went into the basement, he turned on the light, which also turned on the outlet for the controller. Thus the sprinkler started working. When the tech left the basement, he turned out the light, which turned off the outlet for the controller. The sprinkler stopped working. The solution? A little rewiring of the old house! Glad to report the sprinkler system now works great!

144 comments; last comment on 09/04/2014
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Space Trail: Newsletter Challenge (July 2014)

Posted June 30, 2014 12:00 AM
Pathfinder Tags: challenge question

This month's Challenge Question: Specs & Techs from GlobalSpec:

The path that a spacecraft follows when traveling to the moon is a distorted figure eight, instead of an ellipse surrounding the earth and the moon. Why is this?

And the answer is:

The reason is energy-saving. The figure eight path allows the spacecraft to remain almost equidistant from the centers of the earth and the moon. At this path the gravitational pull of the earth and of the moon attract the ship in opposite directions, so the net force exerted on the craft is minimized, and it is certainly smaller than the force exerted on an elliptical path.

21 comments; last comment on 08/10/2014
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Rolling, Rolling, Rolling...: Newsletter Challenge (June 2014)

Posted June 01, 2014 12:00 AM
Pathfinder Tags: challenge questions

This month's Challenge Question: Specs & Techs from GlobalSpec:

Two cylinders of equal length and radius are set at the same height on a ramp and allowed to roll to the bottom. The first is a solid aluminum cylinder. The second is a hollow lead cylinder with an inner radius slightly more than 3/4 of its outer radius. Assuming the frictional effects are negligible, which cylinder reaches the bottom of the ramp first?

And the answer is:

The solid aluminum cylinder will reach the bottom first.

The key to this problem is that the cylinders are rolling down the ramp. In this case the masses of the cylinders are unimportant, for the same reason that they would be unimportant if the cylinders were just dropped from the same height (neglecting air friction). What matters in this problem is the difference in the moment of inertia of a solid cylinder as compared to a hollow cylinder (cylindrical shell). This difference leads to different accelerations for the respective cylinder's center of mass.

Center of Mass Acceleration of the solid cylinder:

Where g is the acceleration due to gravity (9.8 m/s^2) and θ is the angle of the ramp.

Center of Mass Acceleration of the hollow cylinder:

Where g is the acceleration due to gravity (9.8 m/s^2) and θ is the angle of the ramp.

Since the acceleration of the center of mass of the solid cylinder is greater than the acceleration of the center of mass of the hollow cylinder, the solid cylinder reaches the bottom of the ramp first.

138 comments; last comment on 07/06/2014
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Supercooled Water: Newsletter Challenge (May 2014)

Posted May 01, 2014 12:00 AM
Pathfinder Tags: challenge questions

This month's Challenge Question: Specs & Techs from GlobalSpec:

We know that water freezes at 0º C in standard conditions. However, we also know that water can stay in its liquid state well below 0º C in the same conditions; this is called supercooled water. What causes this to happen?

And the answer is:

In order to form ice crystals water needs a nucleating agent. This can be any type of small material (like dust or dissolved air) which causes the deposition of water molecules, creating the crystal. The reason has to do with the minimum energy required to start the creation of the first piece of ice with a minimum given radius so it can grow to larger radius. If the initial radius is smaller than the minimum critical radius needed, ice growth will require lots of energy, making the growth too difficult or impossible. If the initial ice forms on a nucleating agent, growth will be easier because the initial radius may already be larger than the critical radius.

Water without nucleating agents can still form ice if the water molecules meet in certain orientations, but the probability of encountering this molecular orientation increases if the water is below the freezing point. Below the freezing point, the water molecules become less mobile and produce ice crystals more easily.

37 comments; last comment on 06/12/2014
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