Apple Interview Question: You put a glass of water on a... | Glassdoor

## Interview Question

Mechanical Engineer Interview(Student Candidate) Cupertino, CA

# 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.

2

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).

Anonymous on Jan 16, 2010
1

-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.

Anonymous on Nov 6, 2011
21

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.

Previous provided answers are NG. Here is correct answer on Dec 8, 2014
3

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 on Nov 16, 2015
1

What's a record turntable?

Kubla on Nov 19, 2015