- An object, such as the person on the sled in Example 10.13, is sliding down an
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Question
- An object, such as the person on the sled in Example 10.13, is sliding down an incline. However, friction is just right so that the object moves at constant speed. For the system energy to be conserved what must we include in the system?
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As the object in the previous question moves down the hill it (match each type of energy to the correct change).
a-gains 1- thermal energy
b- loses 2- kinetic energy
c-neither gains nor loses 3-gravitational potential energy
A sledder, starting from rest, slides down a 10-m-high hill. At the bottom of the hill is a long horizontal patch of rough snow. The hill is nearly frictionless, but the coefficient of friction between the sled and the rough snow at the bottom is mu %= 0.30. How far will the sled slide along the rough patch? PREPARE In order to be isolated, the system must include the sled, the earth, and the rough snow. As Table 10.2 shows, this makes the friction force an internal force so that no work is done on the system. We can use conservation of energy, but we w ill need to include thermal energy. A visual overview of the problem is show n in FIGURE 10.24. FIGURE 10.24 Visual overview of a sledder sliding downhill. SOLVE At the top of the hill the sled has only gravitational potential energy (Ug)i = mgyi. It has no kinetic or potential energy after supping at the bottom of the hill, so Kt = (Ug)t = 0. However. friction in the rough patch causes an increase in thermal energy. Thus our conservation of energy equation Kt + (Ug) + Delta Eth = K1 + (Ug)i is The change in thermal energy is normal force n balances the sled's weight w as it crosses the rough patch, so n = w = mg. Thus from which we find ASSESS It seems reasonable that the sledder would slide a distance that is greater than the height of the hill he started downExplanation / Answer
thermal energy due to friction must be included during energy conservation.
gain in thermal energy
lose in gravitaional potential energy
and kinetic energy is constant
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