# Physics A-level density exam Q help needed.Thanks.

A group of civil engineers are assessing whether or not to use solid concrete pillars or hollow
metal tubes to support a building. One such tube is shown below. The tube is placed on a
horizontal surface. The tube is made of metal of thickness t. The tube has height h and a mean
A heavy metal block of mass m is placed on top of the tube.
What is the approximate pressure p acting on the tube?
Original post by AJzillaPro
A group of civil engineers are assessing whether or not to use solid concrete pillars or hollow
metal tubes to support a building. One such tube is shown below. The tube is placed on a
horizontal surface. The tube is made of metal of thickness t. The tube has height h and a mean
A heavy metal block of mass m is placed on top of the tube.
What is the approximate pressure p acting on the tube?

hey, i can help with this question
Original post by AJzillaPro
A group of civil engineers are assessing whether or not to use solid concrete pillars or hollow
metal tubes to support a building. One such tube is shown below. The tube is placed on a
horizontal surface. The tube is made of metal of thickness t. The tube has height h and a mean
A heavy metal block of mass m is placed on top of the tube.
What is the approximate pressure p acting on the tube?

Are you referring to the following question?

(edited 2 months ago)
Original post by julian015
hey, i can help with this question

Thank you so much!
Original post by Eimmanuel
Are you referring to the following question?

Yes, that's the one.I apologise and I'll keep that in mind for future posts.Thanks.
Hi, did anyone work out an answer for this as I don't see why it's not B, please?
Original post by blond-insincerit
Hi, did anyone work out an answer for this as I don't see why it's not B, please?
Actually I just figured it out as the cone is hollow.
Original post by blond-insincerit
Actually I just figured it out as the cone is hollow.
did u get the answer? if yes, can u pls tell me how? been trying for a long time
Original post by j_07
did u get the answer? if yes, can u pls tell me how? been trying for a long time

You would be good if you described what you have tried.

Hint: What is the cross-sectional area of the hollow tube in contact with the horizontal surface?

Spoiler

right i got the correct answer anyways-im sure im correct. the cross sectional area if it was fully hollow as in we wouldnt account for thickness t would be piR^2.naturally the question states that the engineers are testing hollow metal columns. now the thickness is the decrease in the HOLLOW cross sectional area as in a smaller cocentric circle- (which is a circle which originates at the same centre but of smaller or larger radius), with radius of (r-t) occurs where its area would be pi(r^2-2rt+t^2) . now we are assuming that R is much greater than t so t on its own is negligible in any calculation, it is too small on its own to make any noticeable effect so we can say t^2=0. hence subtracting the original cross sectional area from the smaller adjusted area for thickness we get piR^2-piR^2-0--2Rt= 2piRt. now we know pressure=force/area of cross section in contact which means we obtain the following. force due to the rectangular weight is mg. ASSUMPTION: only pressure is due to the rectangular mass and the hollow tube is said to be massless or very negligible mass, say aluminium. only assuming this because the question hadnt stated otherwise. thus pressure=mg/2piRt as required