To start a great night of doing physics homework, you sit down to pour yourself

Question

To start a great night of doing physics homework, you sit down to pour yourself a good cup of coffee. Your coffee mug has a mass 127 g and a specific heat of 1090 J/kg K. The mug starts out at room temperature (24.1 o C). Your coffee has an initial temperature of 80.7 oC and has the same specific heat as water (4186 J/kg K).

1)What is the final temperature of coffee and mug once they come to thermal equilibrium? ---69.01

2)Now lets assume that instead of the 127 g of coffee, you pour in 235 g of coffee. What is the final temperature of the coffee and mug? (Again, assume that you loose no heat to the outside.) ----75

3) Now lets say that along with the 235 g of coffee, you pour in 12.1 g of cream in your mug. The cream has an initial temperature of 5.4 oC and also has the same specific heat as water. What is the final temperature of the coffee, cream and mug? (Again, assume that you loose no heat to the outside.)

I need help on three!

Explanation / Answer

Part 3)

By conservation of energy

mc(delta T) = mc(delta T) + mc(delta T)

(.235)(4186)(80.7 - Tf) = (.127)(1090)(Tf - 24.1) + (.0121)(4186)(Tf - 5.4)

79385 - 983.7Tf = 138.4Tf - 3336.2 + 50.7Tf - 273.5

82995 = 1172.8Tf

Tf = 70.8o

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