A battery-powered motor stretches a rubber cord in a launching machine before the machine is used.
The motor starts at rest and is switched off when the cord is fully stretched, so the motor finishes at rest. The battery, motor, cord, casing and air inside the casing are treated as a closed system while the cord is stretched.
The only overall changes in energy stores are shown in the table.
| Component | Change in energy store |
|---|---|
| battery | chemical energy store decreases by 60 J |
| motor, casing and air | thermal energy stores increase by 15 J |
| stretched cord | unknown |
Calculate the increase in the energy store of the stretched cord.
Name the energy store of the stretched cord.
Explain how energy is transferred from the battery to the cord while the motor is stretching the cord.