1. 2. 3. 4. 5. 6. Define the system to only include the ball. State whether each of the following
quantities are positive, negative, or zero:
• the net work done on the system by external forces
• the change in the system’s kinetic energy
• the change in the system’s gravitational potential energy
Explain your reasoning.
Now define the system to include both the ball and Earth. State whether each of
the following quantities are positive, negative, or zero:
• the net work done on the system by external forces
• the change in the system’s kinetic energy
• the change in the system’s gravitational potential energy
Explain your reasoning.
Let m represent the ball’s mass, y represent the distance the ball falls, and v
represent the ball’s final speed. Define the system to only include the ball and
use the Work-Energy Theorem to find an expression for v in terms of the other
variables given and any constants that are necessary.
Repeat question A3 but define the system to include the ball and Earth.
Is your answer to question A4 the same as your answer to question A3? Explain
why or why not.
Two students are discussing their answers to question A5.
Chuck: Our answers must be the same. In one case, we are accounting for the
effects of the gravitational force by saying it does work on the system.
In the other case, we are accounting for this force by saying it causes a
change in the system’s potential energy. Either approach is valid and
we should get the same answer for both.
Logan: I disagree. When the system includes both the ball and Earth, the
gravitational force is doing work on the system and it is also causing a
change in the potential energy of the system. Since the definition of our
system changed, our expression for the speed also has to be different.
Do you agree or disagree with either or both of the students? Explain your
reasoning.