Momentum and Heat

  • Thread starter tamir102
  • Start date
  • #1
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Homework Statement


A 30 kg boy on a 5 kg sled takes a running start from the top of a slippery hill, moving at 4.00
m/s. The hill is 60 m long at an angle of 11.5 degrees above the horizontal. The boy and sled
reach the bottom of the hill moving at 10.0 m/s. (a) How much mechanical energy has been
transformed into thermal energy? (b) If the steel runners of the sled have a mass of 1.500 kg and
half of the heat generated remains in the runners, by how much does the temperature of the steel
runners rise during the ride downhill? Specific heat for steel = 452 J/(kg·°C). [ANS: 1.94°C]


Homework Equations



Wnc=delta KE+ delta PE
delta T = Q/nc


The Attempt at a Solution





Wnc= delta KE + delta PE
=.5(35)(6^2)+35(9.8)(30)
= 10920 J

delta T = Q/nc
= .5(10920)/(1.5*452)
= 24.16 C


the answer as shown is 1.94 C so i dont know where i am going wrong... i am also solving part b because part a is theory.

Thank you
 

Answers and Replies

  • #2
PeterO
Homework Helper
2,431
54

Homework Statement


A 30 kg boy on a 5 kg sled takes a running start from the top of a slippery hill, moving at 4.00
m/s. The hill is 60 m long at an angle of 11.5 degrees above the horizontal. The boy and sled
reach the bottom of the hill moving at 10.0 m/s. (a) How much mechanical energy has been
transformed into thermal energy? (b) If the steel runners of the sled have a mass of 1.500 kg and
half of the heat generated remains in the runners, by how much does the temperature of the steel
runners rise during the ride downhill? Specific heat for steel = 452 J/(kg·°C). [ANS: 1.94°C]


Homework Equations



Wnc=delta KE+ delta PE
delta T = Q/nc


The Attempt at a Solution





Wnc= delta KE + delta PE
=.5(35)(6^2)+35(9.8)(30)
= 10920 J

delta T = Q/nc
= .5(10920)/(1.5*452)
= 24.16 C


the answer as shown is 1.94 C so i dont know where i am going wrong... i am also solving part b because part a is theory.

Thank you


You can only do part B correctly if you have part A done correctly.
That energy figure is way too high.

For a start .5(35)(10^2) - .5(35)(4^2) is not equal .5(35)(6^2)

That is like saying (a - b)2 = a2 - b2 which it certainly is not

Also the change in height is way less than 30m. The slope is only 11.5o. The change in height is [co-incidentally] alarmingly close to 11.5 - I thought I had made a calculator error.
 
  • #3
8
0
you used sine law to find the height correct ???

and part a was just a theoretical answer of explanation not writing.
 
  • #4
PeterO
Homework Helper
2,431
54
you used sine law to find the height correct ???

and part a was just a theoretical answer of explanation not writing.

if you call sin(11.5) = h/60 the sine law, then yes.
 

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