Problems on internal energy with solutions

Physics problem #1.
Scissors and a pencil lying on the table have the same temperature. Why do the scissors feel colder than the pencil?
Problem Solution:
A human hand under normal conditions has a higher temperature than the scissors and pencil. Therefore, when the hand comes in contact with these objects the heat exchange starts: the hand loses some heat, and the pencil and the scissors get it. But the thermal conductivity of wood, of which the pencil is made, is much less than the thermal conductivity of metal scissors. Therefore, the heat transfer between the hand and the pencil will be much slower than between the hand and the scissors. Faster heat transfer in the latter case causes the feeling of cold, which occurs when we touch the scissors.
Physics problem 2.
By how much did the internal energy of a gas change, if the gas, having received the amount of heat of 8 MJ, made a work of 6 MJ?
Solution of the problem:

на сколько изменилась внутрення энергия газа
Physics problem #3.
An iron boiler with a mass of 10 kg is filled with water with a mass of 20 kg. How much heat must be transferred to the boiler with water to change their temperature from 10 to 100 ° C?
Solution of the problem
В железный котел массой 10 кг
A bucket has a mass of 5 kg of water and its temperature is 9 ° C. How much boiling water must be added to the bucket to make it 30 °C? Neglect the heat loss.
В ведро налита вода массой 5 кг

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