## Pregunta de entrevista

Entrevista de Mechanical Engineer

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# You put a glass of water on a record turntable and begin slowly increasing the speed. What happens first - does the glass slide off, tip over, or does the water splash out.

Respuesta

## Respuestas de entrevistas

7 respuestas

27

I was asked this question in separate interviews by Apple, then later by Lab126 (Amazon). I got the answer correct, and they told me so. This question evaluates candidates intuitive sense (engineering judgment) about how things behave when subject to multiple forces, including visualizing free body diagrams and understanding how multiple forces can be resolved into resultant forces (and their angles) to predict how a body moves. The conceptual answer is: this problem can be understood by considering a sufficiently small fluid element at the surface of the water in the glass. That tiny sub-volume of water experiences two forces: 1) gravitational force pulling down on that small mass of water; 2) the force of centripetal acceleration acting upon that small mass of water. When viewed in the (rotating) reference frame of that tiny volume of water under study, the centripetal force is a side-acting force and the gravitational is downward acting. The surface of the water will change to a shape that is NORMAL to the resultant vector of those two forces. Holding rotational freq constant, the water angle will get steeper as its radial location on the record player increases, because the centripetal force is proportional to radius multiplied by the SQUARE of the angular frequency. It can be shown that the water will take on the shape of a sub-portion of the surface of a PARABOLA centered with its minima at the center axis of the record player. As the angular frequency increases, the glass will either slide, or it will tip over, and which of these depends on the aspect ratio of the glass (position of COG) and the friction between the glass and the record player. Some water could splash out prior to either of those events, if the glass was nearly filled with water before the water angle started to change when the turntable started spinning Downward acting gravitational force: F = m X g Side acting centripetal force: F= (omega^2) X r, where omega is angular freq in rad/s If THETA = the tangential angle of the water relative to the plane of the record player, then: TAN (Theta) = (omega^2) r / g Interviewers often give major credit if candidate can at least get CONCEPTUAL answer correct.

6

Depends on several factors including where the glass is placed (the outer edge will experience stronger forces) and how much water is in the glass (I think).

Anónimo en

4

The above description is largely correct except this "It can be shown that the water will take on the shape of a sub-portion of the surface of a PARABOLA centered with its minima at the center axis of the record player." This is true only if the glass is placed at the center of the record player..in all other cases the shape will not be the same due to the effect of centripetal force acting on just direction (as opposed to uniform/all direction in the case of rotating glass at the center)

RajC en

1

Answer depends on: -height of water in the glass -friction between glass and table -how close to the center the glass is placed Knowing these factors, you could make a more educated guess.

Anónimo en

0

The post above is incorrect in saying that centripetal force is a force acting on the water. Centripetal force is a net force caused by centripetal acceleration and a tension, friction, or normal force opposing the motion. Just like you wouldn't put "ma" on a free body diagram, you wouldn't put mv^2/r on a free body diagram. The force acting on the water is an equal and opposite frictional force between the table and the glass. Hence, we have mg acting down and an equal but opposite force acting to the right on the water due to friction, not centripetal force. The resultant vector is indeed angled to the bottom right.

Hydr0gen en

0

If the glass is on the center, it should create a whirlpool effect and eventually splash out as the depths of the whirlpool increases. But of course turntables don’t go very fast.

Anónimo en

2

What's a record turntable?

Kubla en