Heat transfer question

Vasquez

Vasquez

Vasquez

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I'm stuck on this question, I think I'm just being a noob and missing something simple

2. Estimate the rise in temperature of the brake discs of a car of mass 1200 kg travelling at 30 ms-1 which is brought to rest by the application of the brakes alone. The brake discs have a total mass of 5 kg. Assume there is no heat loss and that the material of the four brake discs has a total specific heat capacity of 800 Jkg-1K-1.

(Kinetic energy of a body of mass m and velocity v is 1/2mv2)
Click to expand...

I know Q=mc(t2-t1)

so energy from the car is 540000 Joules

540000=5x800(t2-t1)

540000
5x800 =(t2-t1)

= a change of 135K

But the answer says 66JK-1

am I missing something?
 
Is that even the right answer to the question? They've asked for a change in temperature, which should be measured in K, then given you an answer in JK-1.

Weird...

Your maths looks OK though.
 
AS physics? :)

I remeber doing this last year. And I came across a similar question, AFAIK youve done everything right. As vonhelmet said its weird that theyve given you an answer in JK-1 when they want a value of temperature.
 
Vasquez said:
I'm stuck on this question, I think I'm just being a noob and missing something simple



I know Q=mc(t2-t1)

so energy from the car is 540000 Joules

540000=5x800(t2-t1)

540000
5x800 =(t2-t1)

= a change of 135K

But the answer says 66JK-1

am I missing something?
Click to expand...

your answer looks right. their answer doesn't make sense.
 
Somebody named Vasquez asking about heat transfer. "Primary Heat Exchange" springs to mind, just don't slot in those extra M56 clips you have with you!
 
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