Question:

You work for a balloon delivery service and you are delivering a single, helium-filled balloon in your car. To prevent the balloon from bouncing around on the ceiling while you are driving you have tied a string with a weight on it to the balloon. The weight is resting on the floor and the balloon is floating just below the ceiling. When you accelerate, does the balloon stay where it is, move backward, or move forward? What does it do when you make a turn? Assume all the windows are closed and the vents are turned off so there is no air flow inside the car to affect the balloon.

Answer:

As the car accelerates forward, inertia forces the air inside the car backward compressing the air behind the balloon and pushing it forward. This is because the air is heavier than the helium-filled balloon, and thus acts as all heavy things do in a car that's accelerating: move backwards. Similarly, when rounding a curve, the balloon moves into the curve.

Question:

A new street is built with one hundred new houses, numbered 1 to 100. How many number 9's are required to number all the houses?

Answer:

Not 10 (people forget the 90), not 11 (people forget the 90's): The answer is 20: 9, 19, 29, 39, 49, 59, 69, 79, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 (99's 1st digit), 99 (99's 2nd digit).

Question:

An open straight-sided tank contains water, which drains through a small hole drilled in its base. If a short tube with the same bore as the hole is attached vertically downwards to the hole through which the water drains, why will the drainage rate be quicker?

Answer:

Although the tube offers frictional resistance to flow reducing the rate of drainage, for the short tube the rate is more heavily dependant on the increased head, which includes both the water level in the tank and length of tube. A long piece of tube would ultimately produce more resistance to flow than is provided by the increased head.

Question:

When a ball is placed in a vertical air stream why does it stay in the air stream?

Answer:

The ball is supported by the column of air delivered from the pipe end by the blower.

The velocity profile of the air delivered from the pipe shows the highest velocity in the centre of the pipe, with the velocity reducing slightly towards the circumference of the pipe due to the frictional effect of the pipe wall.

The curvature of the ball splits the air column to pan around the ball equally as the ball in centred on the air column, the pressure that the air exerts on the surface of the ball is in equilibrium on all sides as the air velocity past the ball is common at all points at the circumference of a plane through the centre of the ball perpendicular to the direction of air flow from the pipe.

If the ball should move laterally in this perpendicular plane, away from the centre of the column of air, then air of a higher velocity from the centre of the column of air is then passing the curvature of the ball surface at that point then at the side towards which the ball moved. The resultant imbalance of pressure forces acting on the ball pushes the ball back to the centre of the air column, the pressure exerted upon the surface is a function of the air velocity past the surface at any specified point.

This is the same principle that allows an aircraft wing to generate lift. The curved 'aerofoil' shape of the top surface of the wing forces the airflow to rise over the curves surface, and must therefore travel further to the wing trailing edge than the air passing under the flat lower surface. The air over the top surface must therefore travel faster than the air beneath, the faster air exerts a lower pressure on the top surface of the wing and the resultant imbalance between the top and bottom surfaces provides the lift that allows the wing to fly (or at lease a good proportion of the lift, other forces act also). 

Question:

Why does leaving a teaspoon in the neck of a half empty champagne bottle overnight stop the drink going flat?

Answer:

The spoon cuts the opening of the bottle neck into two unequal sections. This has the effect of starting an extremely gentle flow of air into one half and out of the other, which acts as a very weak but nonetheless effective air stopper across the neck of the bottle and keeps the air in contact with the champagne still, assisted by the fact that the liquid and the air directly above it are cold.

Question:

A child's toy consists of a plastic pipe shaped like a smoker's pipe and ball. By careful blowing, the ball can be made to elevate in the flow of air over the bowl of the pipe. Why is this a stable position?

Answer:

From the conservation of energy, a localised region of low pressure, within which the ball remains, accompanies the region of moving air from the pipe.