Every day math

Question

In a certain school exams are taken outside the hall. One of the students drops her eraser and it takes 0.77 seconds for the eraser to hit the ground (from the desk to the ground). If the shadow of the desk is 2.92 m and the shadow of the hall is 8× the height of the desk under the same conditions. Calculate the the height of the hall

Answer 1

Let the height of the desk be h m. Time of fall of the eraser from the top of the desk being t=0,77s we can write by considering the motion under gravity

h=0xt+1/2gt² 

h=1/2x9.80x0.77²=1.45 m

If the height of the school building be H m and angle of elevation of sun be A then

h/H=H/S=tanA , where s and S are length of shadows of desks and hall.

given s =2.92 m. S =8h=8*1.45=7.6 m

now h/H=s/S=2.92/7.6

So H = 7.6/2.92*h = 7.6/2.92*1.45 m=3.77 m

Comments

The approach is very correct but the calculation and final ans is wrong and this started as a result of wrong calculation of height of desk h= ½gt² = 2.91m

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