A clock with an iron pendulum keeps correct time at 20C.How much will it lose or gain per day if temperature changes to 40C?Coefficient of cubical expansion of iron =36×10-6C-1

We know that for an ideal pendulum time period ##T## is given by the expression##T=2pisqrt(L/g)##where ##L## is the length of the pendulum and ##g## due to gravity.Given that the pendulum keeps correct time at ##20^@C####:.T_”correct”=2pisqrt((L_”20″)/g)## …….(1)

Now we need to find time period at ##40^@C##

We know that with increase in temperature length ##L_0## of metallic rod expands and the expression is##L_T = L_0 ( 1 + α ΔT)## …….(2)where ##α## is the coefficient of linear expansion and ##DeltaT## is the change in temperature.

It can be shown that the coefficient of volumetric expansion##gamma~~3alpha##As such from (2) we get##L_40= L_20 ( 1 + (36xx10^-6)/3xx (40-20))####L_40=1.00024 L_20 ## …..(3)

We see that at ##40^@C## length of the pendulum is greater than at ##20^@C##. As the time period is directly proportional to the square root of length, increase in length implies increase in time period. Which amounts to clock loosing time.From (1)##T_40=2pisqrt((L_40)/g)##Using (3)##T_40=2pisqrt((1.00024 L_20 )/g)####=>T_40=sqrt1.00024 xxT_”correct”##To calculate loss per day we insert seconds in ##1## day (##24## hours) as the correct time and deduct seconds equal to one day. We get##DeltaTime=sqrt1.00024 xx86400-86400####=10.4s##, rounded to one decimal place.