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U world questions and explanations 2024
A 100-g block of iron at 31°C is submerged in 50 g of water at 25°C in a thermally
insulated container. If the temperature of the system reaches equilibrium at 26°C,
what is the specific heat of iron? (Note: The specific heat of water is 4.2 J/(g⋅°C).)
A.0.42 J/(g⋅°C)
The specific heat of a substance is an intrinsic property (does not depend on mass) and is
the amount of heat required to increase a unit mass (1 g or 1 kg) by 1°C or 1 K. The total
amount of heat gained or lost by an object is the product of its mass m, its specific heat c,
and the change in temperature ΔT, as given by the calorimetry equation:
q=mc∆T
Because the block of iron is hotter than the water, heat flows from the iron to the water,
and the temperature of the water increases from 25°C to 26°C (ΔT = 1°C). The heat gained
by water is calculated using its given mass (50 g), specific heat (4.2Jg⋅°C) , and ΔT:
qwater=(50 g)(4.2Jg⋅°C)(1°C)=210 J
Because the container is thermally insulated, the heat gained by the water is equal to the
heat lost by the iron block, qiron = −210 J. According to the question, the 100-g iron block
decreases in temperature from 31°C to 26°C (ΔT = −5°C). The specific heat of iron is
calculated by rearranging the calorimetry equation and using these values:
According to the passage, the arm is modeled as a lever with the pivot point (fulcrum) at
the elbow. Mechanical advantage is the ratio of the output force Fo to the input force Fi:
mechanical advantage=FoFi
Work W is the amount of energy transferred by a force F over a distance d: W = Fd. A simple
machine can amplify an input force through the conservation of energy; eg, a lever can
transfer small forces over long distances into large forces over short distances. The work
done by the output force must equal the work done by the input force:
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