U7 Work Power and Energy Quest

What are the only forces acting on a pendulum in motion, ignoring air resistance?

• Friction and tension

• Gravity and tension

• Gravity and friction

• Tension and air resistance

Answer explanation

In a pendulum's motion, the only forces acting are gravity, which pulls it downward, and tension, which is the force exerted by the string or rod holding the pendulum. Friction and air resistance are ignored in this scenario.

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NGSS.MS-PS2-4

What happens to the pendulum bob when it is displaced from equilibrium?

• It stops moving

• It accelerates towards equilibrium

• It moves faster

• It remains stationary

Answer explanation

When the pendulum bob is displaced from equilibrium, it experiences a restoring force due to gravity, causing it to accelerate back towards the equilibrium position. Thus, it accelerates towards equilibrium.

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NGSS.MS-PS2-2

Which statement BEST describes if the pendulum investigation supports the law of conservation of energy?

• The investigation supports the law of conservation of energy because the combined total of the kinetic and potential energies of the pendulum is always equal to the total mechanical energy of the system.

• The investigation supports the law of conservation of energy because the combined total of the kinetic and potential energies of the pendulum is greater than the total mechanical energy of the system.

• The investigation does not support the law of conservation of energy because the combined total of the kinetic and potential energies of the pendulum is always equal to the total mechanical energy of the system.

• The investigation does not support the law of conservation of energy because the combined total of the kinetic and potential energies of the pendulum is lesser than the total mechanical energy of the system.

Answer explanation

The correct choice states that the total of kinetic and potential energies equals the total mechanical energy, supporting the law of conservation of energy, as energy is conserved in the pendulum system.

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Select THREE statements that correctly describe the energy of the pendulum.

• The gravitational potential energy of the pendulum is equal at Position 1 and Position 5.

• Moving from Position 5 to Position 3, the pendulum is experiencing a decrease in kinetic energy and an increase in potential energy.

• Moving from Position 1 to Position 3, the pendulum is experiencing a decrease in potential energy and an increase in kinetic energy.

• The pendulum is experiencing an equal amount of kinetic energy and potential energy at Position 4.

• The pendulum is experiencing greater kinetic energy at Position 2 than Position 4.

Answer explanation

At Positions 1 and 5, the pendulum has the same height, thus equal gravitational potential energy. Moving from Position 1 to 3, potential energy decreases while kinetic energy increases. At Position 4, kinetic and potential energy are equal.

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What happens to the pendulum bob when it is displaced to the left?

• It moves to a higher position

• It moves to a positive position

• It moves to a negative position

• It stops moving

Answer explanation

When the pendulum bob is displaced to the left, it moves away from its equilibrium position, which is typically considered the positive direction. Thus, moving left corresponds to a negative position.

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What is the relationship between the pendulum bob's distance from equilibrium and its speed?

• The farther it moves, the faster it goes

• The closer it is, the slower it moves

• The farther it moves, the slower it goes

• Distance does not affect speed

Answer explanation

The correct answer is 'The farther it moves, the slower it goes' because as the pendulum bob moves away from its equilibrium position, it loses speed due to gravitational potential energy converting to kinetic energy, slowing it down.

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What is the total mechanical energy (TME) of a system composed of?

• Kinetic energy only

• Potential energy only

• Potential energy and kinetic energy

• Thermal energy and kinetic energy

Answer explanation

The total mechanical energy (TME) of a system is the sum of its kinetic energy and potential energy. Therefore, the correct choice is 'Potential energy and kinetic energy', as both are essential components of TME.

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NGSS.MS-PS3-2

Which statement BEST describes if the second pendulum investigation follows the law of conservation of energy?

• The second investigation does not follow the law; energy cannot leave and must remain within a system.

• The second investigation follows the law; adding air resistance adds additional gravitational force to a system.

• The second investigation follows the law; a change in energy of a system equals the energy transferred out of the system.

• The second investigation does not follow the law; different energies within a system cannot equal the total energy of the system.

Answer explanation

The correct choice states that the second investigation follows the law of conservation of energy, as it recognizes that a change in energy within a system corresponds to the energy transferred out, aligning with the principle.

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What happens to the energy of the system in the second investigation?

• Energy moves from the air particles to the pendulum, increasing the total mechanical energy within the system.

• Less potential energy is created from the air particles, which causes less kinetic energy within the system to be created.

• Some of the kinetic energy is transferred to the surrounding air particles outside the system, and some of it leaves as heat.

• The kinetic energy is destroyed as it encounters the air particles, which causes the potential energy to decrease in the system.

Answer explanation

In the second investigation, some kinetic energy is transferred to surrounding air particles and some energy is lost as heat, indicating energy is not conserved within the system.

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The equations for work and kinetic energy are shown. W = Fd, where F is Force and d is distance, and KE = 1/2 mv^2, where m is mass and v is velocity. The work-energy theorem shows the relationship between work and energy using the following equation: Work (W) = change in kinetic energy (ΔKE) A 70-kg person leisurely rides their bicycle at a speed of 4 m/s when they realize they will be late for school. How much work is done to increase their speed to 6 m/s?

• 79 J

• 700 J

• 2,152 J

• 3,500 J

Answer explanation

To find the work done, calculate the change in kinetic energy: ΔKE = KE_final - KE_initial. KE_initial = 1/2 * 70 kg * (4 m/s)^2 = 560 J; KE_final = 1/2 * 70 kg * (6 m/s)^2 = 1260 J. Thus, ΔKE = 1260 J - 560 J = 700 J. The correct answer is 2,152 J.

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