Newton's Law of cooling: A piece of steel

wind

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I'm really having trouble with this stuff

A piece of steel will be forged into a turbine. The steel is held in a furnace at a temperature of 1250°c, and once it is taken from the furnace it can be worked until it reaches a temperature of 1100°c, after which time it will become too hard to forge successfully. Experiments have shown that the relative cooling rate for this grade of steel is about k=1.4% per minute. Suppose that the temperature of the forge shop is 30°c.

a)determine a formula for the temperature of the steel, t minutes after it is taken out of the furnace.

\(\displaystyle \L\ T-T_{s}=(T_{o}-T{s})e^{Kt}\)

\(\displaystyle \L\ T-30=(1250-30)e^{0.014t}\)

\(\displaystyle \L\ T=(1220)e^{0.014t}-30\)

b) How long can the workers forge the steel before it must be returned to the furnace for reheating?

T=1100
Ts= 30
To= 1250
K=0.014
t=?

\(\displaystyle \L\ 1100=(1220)e^{0.014t}-30\)

\(\displaystyle \L\ 1130=(1220)e^{0.014t}\)

\(\displaystyle \L\ log1130=(0.014t)log(1220)e\)

\(\displaystyle \L\ 26.05=t\)

pls help! Thanks
 
wind said:
I'm really having trouble with this stuff

A piece of steel will be forged into a turbine. The steel is held in a furnace at a temperature of 1250°c, and once it is taken from the furnace it can be worked until it reaches a temperature of 1100°c, after which time it will become too hard to forge successfully. Experiments have shown that the relative cooling rate for this grade of steel is about k=1.4% per minute. Suppose that the temperature of the forge shop is 30°c.

a)determine a formula for the temperature of the steel, t minutes after it is taken out of the furnace.

\(\displaystyle \L\ T-T_{s}=(T_{o}-T{s})e^{Kt}\)

\(\displaystyle \L\ T-30=(1250-30)e^{0.014t}\) {temperature is decreasing ... k = +.014 ???}

\(\displaystyle \L\ T=(1220)e^{0.014t}-30\) {- 30 ???}

equation should be T = 1220e<sup>-.014t</sup> + 30


b) How long can the workers forge the steel before it must be returned to the furnace for reheating?

T=1100
Ts= 30
To= 1250
K=0.014
t=?

\(\displaystyle \L\ 1100=(1220)e^{0.014t}-30\)

\(\displaystyle \L\ 1130=(1220)e^{0.014t}\)

\(\displaystyle \L\ log1130=(0.014t)log(1220)e\)

\(\displaystyle \L\ 26.05=t\)

pls help! Thanks
 
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