# Questions d'entretiens - Product engineer

# 5 k

Questions d'entretien pour Product Engineer partagées par les candidats## Principales questions d'entretien

### There is a refrigerator in a perfectly insulated room. The door is then opened. Is it an open or closed system? After a long time has passed, has the temperature in the room increased or decreased?

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It is a closed system as electricity is adding energy into the room. After a long time passes, the temperature increases as there's energy going in, but not leaving. Moins

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The refrigerator door is open, but it is plugged in, thus providing energy to the system. However, there is no energy leaving the system. Moins

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It's a closed system. Since the efficiency of heat transfer is not 100%, will generate heat and eventually heat up the room. Moins

### Show me the back of an iPhone and asked me how it might have been manufactured.

8 réponses### If I have a solid rod and hollow rod with the same mass and I let them slide in a ramp. Which one reaches the bottom first and why?

6 réponses↳

Since they've clearly told sliding. Acceleration along the ramp is independent of the mass or the moment of Inertia. Hence, both reaches down at the same time. Moins

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Solid bar, smaller moment of inertia allows it to spin faster

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Solid reaches first

### If you and a large brick are in a boat floating in a pool, will the water level rise or fall when the brick is tossed into the water? What if the brick is a large piece of Styrofoam and thrown into the water?

6 réponses↳

Your answer indicated the necessity of this question.

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The water level will decrease ( if the brick has a density greater than water, which of course a conventional brick does). When in the boat, the brick will displace a mass of water equal to it's own. So if it has a mass of 1kg it will displace 1kg of water. When thrown overboard it will now displace an amount of water equal to it's volume. As it sinks, it's density must be grater than water and therefore it's 1kg mass will take up a smaller volume than 1kg of water. Therefore the water level will decrease. As a styrofoam brick has a lower density than water it will not sink when thrown overboard. When floating it will displace a mass of water equal to that of the brick and therefore the water level will remain the unchanged. Moins

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A powerful tool in looking at questions like these intuitively rather than by the raw physics is to go to the extreme. Say the brick were of normal brick size but were made of some ultra-dense material...so dense that it would push an aircraft carrier down in the water to the brink of sinking. This would obviously cause the carrier to displace many thousands of gallons more water than it had been displacing before the brick was added. When the brick is thrown overboard, the carrier would come shooting up out of the water, now not displacing those additional thousands of gallons of water The volume of water displaced by the brick once it's in the water is comparatively insignificant. So clearly the water level will fall. Moins

### You have 5kg of Aluminum and 5kg of Steel. You can make whatever cross section beam you want with these materials. If you have to make a cantilever beam 1m long with a load at the end. And you want to minimize the deflection. What material would you use?

6 réponses↳

Aluminum. While the previous post on Sep 5th is correct in stating that P, L are the same, since you are given a mass of both metals, you need to find you allowable "volume" of material, which you can estimate using 2700kg/m3 for Al and 7800kg/m3 for steel. MoI is given by (1/12) b h^3 if you assume b (base) is the same for both materials, you'll find that h for aluminum is greater than h of steel, and since h is cubed, it actually makes a big difference. In my test case, E of steel ~3x greater (200 GPa vs 70 GPa), but the moment of inertia of the Aluminum was ~3.75x than steel, meaning that deflection of aluminum would be less than steel! Moins

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Aluminum

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Why is it being assumed that the base be the same for both materials. Since the question allows to choose whatever cross section we are allowed, can we not just assume that we'll come up with a MoI that's comparable? Or we simply switch the b and h values for both materials. Given these assumptions, one could argue that the deflection is a function of E and thus, higher in aluminum and lower in steel. Moins

### The most unexpected question was to write a program that would take a 3x3 matrix and move around the values. [[1 2 3][4 5 6][7 8 9]] -> [[9 8 7][6 5 4][3 2 1]] like this.

5 réponses↳

Is it just kind of like temp=a[i];a[i]=a[n-i];a[n-i]=temp?

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Could you tell me how long it takes to receive the job offer?

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Anonymous II, I received an offer the week after the interview. Hope that helps.

### In the 2nd round they asked: 1. An egg has to be boiled which takes exactly 11 minutes to boil. There are only 2 sand watches available to you one of 9 minutes and the other of 5 minutes. Using only these sand watches boil the egg.

5 réponses↳

It takes 4 flips to calculate 11 minutes. You have tu turn both sand watches same time as anyone of them is been finished first. $ = 1 minute and @ = Flip. $$$$$@$$$$@$@$ Explanation: Once 5 minute over and flip both watches. Timer is 4 minute and 5 minute as 4 minute over flip the watch so till now 9 minutes over now watches running for 1 minute and 9 minute. Once 1 minute over flip watches again watches running for 1 Minute and 5 Minute. Once one minute watch is finished egg is cooked. Moins

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I ll start both watch After 5 mins I ll have 4 remaining in 9 min watch and 5 min watch will go down Then I ll start 5 min watch again When 9 min watch finishes it's time we will have 1 min remaining in 5 min watch Now I ll start 9 min watch again in 1 min I ll have 1 min sand down in 9 min watch and 5 min watch will have all its sand down Until now we are done with 10 mins with 5 min watch used 2 times Now we will use 1 min sand which is down in 9min watch to calculate 11 th min Moins

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start both hour glasses at 0 let 5 min hour glass be x and 9 min hour glass be y. after 5 min- x will have 0; y will have 4. turn and start;after 4 mins- x will have 1; y will have 0. Now start boiling the egg turn the glass and after 1 min x will have 0; y will 0; reset and start again. after 5 min- x will have 0; y will have 4. turn and start- after 4 min- x will have 1; y will have 0. turn and start after 1 min- total 1+5+4+1=11. done! Moins

### We have a cup of hot coffee and a small cold milk out of the fridge. The room temperature is in between these two. When should we add milk to coffee to get the coolest combination earliest ( at the beginning, in the middle, or at the end)?

5 réponses↳

At the beginning. They are not asking inserting at beginning or end.(Question is when and not where) If you add at start, it will have max. temperature difference and it will cool max. At end, all will be anyway at room temperature so will not have any difference. Moins

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Apple likes the answer: At the end. I was at the onsite interview. The engineering director asked me this question. He likes the answer "at the end". But, he said he wasn’t sure. He said he doesn’t know the answer to that question either. So, in other words they are asking you questions that they don’t know the answer either. Moins

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If you do the math, it should be in the middle. The system is a first order system so that it reaches to steady states takes 4*time constant. The time constant is Ah/mCp. Based on this, those two will reach steady states based on their own surface Area (For convection) and mass at different rates. Once one of them reach to the steady state, you should mix them. Moins

### If I need to make a cantilever beam 1mm thick and I only have 0.5mm sheet stock. How can I make a beam that has the same properties as the 1mm.

5 réponses↳

same length but, 8 times width for inertia

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You cut 1mm width strips and use the width as the thickness.

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The width needs to be 4X greater on the .5mm thick beam to match the MoI

### Questions that popped out as I can remember. 1) A cube (1-1-1m) of ice in a room (50C) sitting on a wooden table. the ice is 1m away from the walls around it's 4 sides, except for 1 side is 30cm away. You're given 2 insulating blankets (1m by 1m) that can be used to cover the ice block. The goal is to keep the ice in solid form as long as possible. Where would you put the blankets? 2) 2 rectangular beams, one is Aluminum a by 2b cross-section one is Steel a by b cross-section. If you were to climb a mountain and find these two sticking out on a spot where you need to rest, which one would you latch on to? 3) Interviewee shows a plastic part and asks: How do you think I built this part? What other ways can you think of this part could have been done?

5 réponses↳

This is a beam bending question, the max deflection will be at the end of the bend, which, if you assume it is simple beam bending, will be y=FL^3/EI The Young's Modulus of Al is around 70 GPA while SST's is around 200 GPA. Now we need the inertia I, if, Ial is bigger than 20/7 of Isst, then Yal will be smaller than Ysst, so you would like to choose Al because it has a smaller deflection. If it is the other way around, then you will need to choose SST because the deflection is smaller for SST. So you will need to clarify 2b is the high side or the width side. Moins

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This is a beam bending question, the max deflection will be at the end of the bend, which, if you assume it is simple beam bending, will be y=FL^3/EI The Young's Modulus of Al is around 70 GPA while SST's is around 200 GPA. Now we need the inertia I, if, Ial is bigger than 20/7 of Isst, then Yal will be smaller than Ysst, so you would like to choose Al because it has a smaller deflection. If it is the other way around, then you will need to choose SST because the deflection is smaller for SST. So you will need to clarify 2b is the high side or the width side. Moins

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1) there are many answers to this.. but the key is asking many questions to learn more about the problem and your capabilities. depending on how you go about the questions expect follow up questions on your decisions. I put the 1 blanket between ice and wood, 1 blanket on the surface close to the wall. Although there is no air circulation initially, as time passes heat exchange between ice and air start movement and creates convection (downward draft), higher velocity on the 30cm side and more heat exchange. 2) Remember beam theory, UTS is the key to answer this question. 3) Try to go over all the Apple components you can find and make a few practices before you go. I actually looked at ifixit.com to learn more about the Apple products. Think how it could have been manufactured. Moins