Conservation of energy: why a ball thrown upwards slows down, mark by mark

THE QUESTION

3 marks, and each one is for a different sentence

The trap here is in the question itself. It asks for an explanation using energy, and the most natural answer uses forces. That answer is true and scores nothing.

WHAT EARNS THE MARKS

The sentence each mark is for

1. Mark 1 of 3As the ball rises, its kinetic energy is converted to gravitational potential energy.The first mark is for naming both forms. Whether your school teaches energy as conversions or as stores and transfers, the scheme wants kinetic and gravitational potential, with the direction of the change.
2. Mark 2 of 3The total energy of the ball stays constant, because energy is conserved.The question names the principle, so the answer has to use it. This is the mark students skip because it feels too obvious to write down.
3. Mark 3 of 3Since its kinetic energy decreases, the speed of the ball decreases.The question asked why the ball slows down. The last mark is for getting back to speed: kinetic energy depends on speed, so less of one means less of the other.

WHAT LOSES THEM

Answers that sound right and score less

• A student writesGravity pulls the ball down, so it slows.It is correct physics and it answers a different question. Asked to explain with energy, a forces answer earns no marks here.
• A student writesThe ball loses energy as it goes up.Energy is not lost. With air resistance negligible the total stays the same, and saying otherwise contradicts the principle the question asked you to use.
• A student writesKinetic energy changes to potential energy.It may earn the first mark, but it stops there. Nothing says the total is constant and nothing links the change back to speed.
• A student writesAt the top the ball has no energy.At the top it has no kinetic energy. All of it is now gravitational potential energy.

THE REST OF THE CHAPTER

The same pattern on other energy questions

Most explain questions in this chapter want the same three moves: name the forms, state that the total is conserved, and return to what the question asked about.

• An object fallingGravitational potential energy is converted to kinetic energy, the total stays constant, so the speed increases as it falls.
• A pendulum swingingEnergy passes between gravitational potential and kinetic, with the most kinetic energy at the lowest point.
• When air resistance is not negligibleSome energy is transferred to the surroundings as thermal energy, so the object ends with less kinetic energy than it started with. The total is still conserved.