Q: A 70 kg swimmer swims 1.0 km at a steady speed of 1.4 m/s in water with a drag force of 35 N. How much thermal energy is generated in the water?

$$AE_{th} = (35\, \text{N})(1000\, \text{m}) = 3.5 \times 10^4$\, \text{J}$$

Question 1

Which of the following best describes thermal energy in a substance?

Accepted Answer

The sum of the kinetic energy of atoms/molecules and the elastic potential energy stored in molecular bonds.

Question 2

A 70 kg swimmer swims 1.0 km at a steady speed of 1.4 m/s in water with a drag force of 35 N. How much thermal energy is generated in the water?

Accepted Answer

$$AE_{th} = (35\, 	ext{N})(1000\, 	ext{m}) = 3.5 	imes 10^4$\, 	ext{J}$$

Question 3

Which equation correctly expresses the work-energy principle including thermal energy?

Accepted Answer

$$\Delta K + \Delta U + \Delta E_{th} = W$$

Question 4

A spring-loaded toy gun launches a 10 g ball using a spring with $$k = 10\, 	ext{N/m}$$ compressed by 10 cm. What is the ball's speed as it leaves the barrel (neglecting friction and gravity)?

Accepted Answer

$$v = \sqrt{\frac{kx^2$}{m}} = \sqrt{\frac{(10)(0.10)^2$}{0.01}} = 3.2\, 	ext{m/s}$$

Question 5

If the spring in the toy gun above is compressed twice as far, what happens to the ball's launch speed?

Accepted Answer

It increases by a factor of $$\sqrt{4} = 2.$$

Question 6

Quinn slides down a 5.0 m long slide at $$30^\circ$$ and leaves at 2.0 m/s. If his mass is 24 kg, how much thermal energy is generated due to friction?

Accepted Answer

$$AE_{th} = mgh - \frac{1}{2}mv^2$ = (24)(9.8)(2.5) - \frac{1}{2}(24)(2.0)^2$ = 588 - 48 = 540\, 	ext{J}$$

Question 7

In an energy diagram for a spring, what does the minimum of the potential energy curve represent?

Accepted Answer

The equilibrium position where the object is at rest.

Question 8

Which statement about molecular bonds and chemical energy is correct?

Accepted Answer

Molecular bonds store electric potential energy due to interactions between electrons and nuclei.

Question 9

A photon with energy $$2.5 	imes 10$$^{-18}$$\, 	ext{J} is $$absorbed by an $$O_2$$ molecule. If the bond energy is $$1.2 	imes 10$$^{-18}$$\, 	ext{J}$$, what happens?

Accepted Answer

The molecule breaks apart and the atoms gain kinetic energy equal to $$2.5 	imes 10$$^{-18}$$ - 1.2 	imes 10$$^{-18}$$ = 1.3 	imes 10$$^{-18}$$\, 	ext{J}.$$

Question 10

Which of the following best describes the role of activation energy in chemical reactions?

Accepted Answer

It is the energy required to break the bonds of reactants.

Question 11

In a perfectly inelastic collision, which of the following is true?

Accepted Answer

Momentum is conserved, but mechanical energy is not.

Question 12

Two railroad cars (20,000 kg and 40,000 kg) collide and couple together. If the initial kinetic energy is $$4.7 	imes 10^4$\, 	ext{J}$$ and the final is $$1.9 	imes 10^4$\, 	ext{J}$$, how much energy is transformed into thermal energy?

Accepted Answer

$$AE_{th} = 4.5 	imes 10^4$\, 	ext{J}$$

Question 13

In a head-on perfectly elastic collision between two identical air hockey pucks, one moving at 2.3 m/s and the other at rest, what are their final velocities?

Accepted Answer

The moving puck stops, and the stationary puck moves at 2.3 m/s.

Question 14

Which formula gives the output power of a force $$F$$ acting on an object moving at velocity $$v$$?

Accepted Answer

$$P = Fv$$

Question 15

A student runs up a flight of stairs in 3 seconds, while another takes 10 seconds. Both do the same amount of work. Which student has the greater power output?

Accepted Answer

The student who takes 3 seconds.

Question 16

Which of the following is true about energy diagrams for oscillating systems?

Accepted Answer

The object cannot be at a position where the potential energy curve is above the total energy line.

Question 17

Which of the following statements about the conservation of energy in an isolated system is correct?

Accepted Answer

$$\Delta K + \Delta U + \Delta E_{th} = 0$$

Question 18

Which of the following best describes the effect of a catalyst in a chemical reaction?

Accepted Answer

It lowers the activation energy, increasing the reaction rate.

Physics for Life Sciences I – Lecture 17 (Sections 10.5–10.9): Energy, Thermal Energy, Conservation, Molecular Bonds, Collisions, and Power

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