For the situation: "A moving crate slides to a halt on a rough surface"
Does temperature increase, decrease, or not change? Are work and heat positive, negative, or zero? Does thermal energy increase, decrease, or not change?
My answer key says temperature increases, work is positive, heat is zero, and change in thermal energy is positive. I don't get why! Can I please get an explanation? Thank you.
To understand why the temperature increases, work is positive, heat is zero, and the change in thermal energy is positive in the given situation, let's break down the scenario and the concepts involved.
1. A moving crate slides to a halt on a rough surface: When the crate slides on a rough surface, it experiences friction, which acts against its motion. Friction is a force that opposes relative motion between two surfaces. In this case, the friction force acts in the opposite direction to the crate's motion and eventually brings it to a stop.
2. Temperature increase: As the crate slides to a halt, the friction between the crate and the rough surface converts the kinetic energy (energy of motion) of the crate into other forms of energy. One of these forms is thermal energy, which is related to the kinetic energy of the molecules in the system. The conversion of kinetic energy to thermal energy leads to an overall increase in the temperature of the crate and the surface it is moving on.
3. Work is positive: Work is done when a force acts upon an object to cause it to move. In this situation, the friction force acts against the crate's motion. Since the crate initially has kinetic energy due to its motion, the friction force does negative work on the crate. However, the work-energy principle states that the total work done on an object is equal to the change in its kinetic energy. As the crate slows down and comes to a halt, its kinetic energy decreases, and therefore, the negative work done by friction is positive.
4. Heat is zero: Heat is a form of energy transfer that occurs due to a temperature difference. In this specific situation, since the crate is sliding to a halt, there is no heat transfer between the crate and its surroundings. Therefore, the heat is zero.
5. Change in thermal energy is positive: As mentioned earlier, the conversion of the crate's kinetic energy into other forms of energy, like thermal energy, leads to an increase in temperature. This increase corresponds to a positive change in thermal energy.
In summary, when a moving crate slides to a halt on a rough surface, the temperature increases because the kinetic energy is converted into thermal energy. The work done is positive because it opposes the crate's motion but still results in a decrease in kinetic energy. The heat transfer is zero because there is no temperature difference, and the change in thermal energy is positive as the temperature of the crate increases.