Right!

• The "total" potential energy is the spring potential energy only (since the Earth is not in the system, gravitational potential energy is not included). As the spring stretches, U_spring goes down.
• The kinetic energy does not change overall, since the toy starts from rest and ends momentarily at rest at the top of its flight.
• The normal force from the table does no work (the point of application does not move). Gravity, an external force given our choice of system, does do work, and it is negative (since the object is moving upwards and gravity points downward).
• Lastly, for a closed system (the toy), the work done by the external force must equal the change in the system's energy.

Now answer this: if we chose the system to be the toy and the earth, what would the work-energy diagram look like? Choose from the pictures shown below.

Click here to choose diagram (a).

Click here to choose diagram (b).

Go back if you want to look at the original question.