# Exam 4 Preap Physics Numbers 11-15

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• 1.

### 11. Which of the following is true of the conservation of energy in a closed system?

• A.

A. Kinetic energy is always conserved.

• B.

B. Potential energy is always conserved.

• C.

C. Mechanical energy is always conserved.

• D.

D. Total energy is always conserved.

C. C. Mechanical energy is always conserved.
Explanation
In a closed system, mechanical energy is always conserved. This means that the total amount of kinetic energy and potential energy remains constant over time. While kinetic energy and potential energy can individually change, the sum of the two, which is mechanical energy, remains constant. This principle is based on the law of conservation of energy, which states that energy cannot be created or destroyed, only transferred or transformed. Therefore, in a closed system, the total mechanical energy is always conserved.

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• 2.

### 12. The mechanical energy of a system of objects is

• A.

A. the sum of kinetic energy and gravitational potential energy.

• B.

B. the sum of kinetic energy and elastic potential energy.

• C.

C. the sum of kinetic energy and all relevant forms of potential energy.

• D.

D. the sum of all forms of energy.

C. C. the sum of kinetic energy and all relevant forms of potential energy.
Explanation
The mechanical energy of a system of objects includes both kinetic energy, which is the energy of motion, and potential energy, which is the energy stored in an object due to its position or condition. Since the question asks for the sum of all relevant forms of potential energy, option c is the correct answer. This means that any other forms of potential energy, such as gravitational potential energy or elastic potential energy, are also included in the calculation of the mechanical energy.

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• 3.

### 13. Mechanical energy is not conserved when

• A.

A. gravitational potential energy is converted to kinetic energy.

• B.

B. kinetic energy is converted to gravitational potential energy.

• C.

C. kinetic energy is converted to elastic potential energy.

• D.

D. friction is not negligible.

D. D. friction is not negligible.
Explanation
Mechanical energy is the sum of potential energy and kinetic energy in a system. According to the principle of conservation of energy, mechanical energy is conserved in an isolated system where no external forces, such as friction, are acting. However, when friction is not negligible, it does work on the system, converting mechanical energy into other forms of energy, such as heat and sound. Therefore, mechanical energy is not conserved in the presence of non-negligible friction.

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• 4.

### 14. In which of the following situations is mechanical energy ALMOST conserved?

• A.

A. A football flies through the air.

• B.

B. A feather falls from the sky.

• C.

C. A skateboard rolls into the grass.

• D.

D. A hockey player digs his skates into the ice.

C. C. A skateboard rolls into the grass.
Explanation
When a skateboard rolls into the grass, mechanical energy is almost conserved. This is because the skateboard experiences minimal external forces that can convert its mechanical energy into other forms, such as heat or sound. The friction between the skateboard's wheels and the grass is relatively small compared to the other situations mentioned. In contrast, in the other situations, external forces like air resistance or friction play a more significant role in converting mechanical energy into other forms.

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• 5.

### 15. Which of the following is evidence that frictional forces are present in a system?

• A.

A. Interactions in the system cause an increase in temperature.

• B.

B. Interactions in the system produce sound.

• C.

C. Mechanical energy is not conserved.

• D.

D. all of the above

D. D. all of the above
Explanation
The correct answer is d. all of the above. This is because all of the given options provide evidence of the presence of frictional forces in a system. When there is friction, interactions in the system can cause an increase in temperature (option a), as friction generates heat. Interactions can also produce sound (option b) due to the vibrations caused by friction. Additionally, friction can cause a loss of mechanical energy (option c) as it converts some of the energy into heat and sound. Therefore, all three options indicate the presence of frictional forces in the system.

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