Final answer:
The javelin's kinetic energy at its highest point is 200 J, calculated by subtracting the increase in gravitational potential energy (70 J) from the total work done to propel the javelin (270 J).
Step-by-step explanation:
The question asks us to calculate the kinetic energy of a javelin at its highest point after a throw, given that 270 J of work have been done to propel it and that its gravitational potential energy increased by 70 J. According to the work-energy theorem, the net work done on an object is equal to the change in its kinetic energy. In this case, the work done by Christina is transformed into kinetic energy and gravitational potential energy. At the highest point, the javelin's potential energy has increased by 70 J, which means the rest of the work done on it (270 J - 70 J) must have gone into its kinetic energy, giving us 200 J of kinetic energy at the highest point.