Bob's Bad Day

Bob went home with a red print on his face yesterday. He was involved in a fight in school. Someone slapped his face which caused the temperature of the affected area of his face to rise. How fast the slapping hand is?

Known:

• The mass of the slapping hand m_hand = 1.5 kg
• The mass of the affected tissue m_{affected tissue} = 0.2 kg
• The specific heat of the tissue c = 3.8 kJ/(kg-^oC)
• Temperature of face rise 1.5^oC
  

• Take the hand and the affected portion of the face as a system
• The energy equation:
  
  E_in - E_out = ΔE_system
                = (ΔU + ΔKE + ΔPE)_{affected tissue} +
                   (ΔU + ΔKE + ΔPE)_hand