Evaluate — 4 marks
A student investigates energy changes in a system by dropping a rubber ball from a height of 2.0 m onto a hard floor. The ball bounces back to a height of 1.4 m on the first bounce. The student claims that energy is not conserved in this system because the ball does not return to its original height.
-
(a) Calculate the percentage of gravitational potential energy lost during the first bounce. (Assume g = 10 m/s²)
[1 mark]
-
(b) Evaluate the student's claim that energy is not conserved in this system. In your answer, explain what happens to the 'missing' energy and identify the type of energy transformation that occurs.
[2 marks]
-
(c) The student repeats the experiment with a tennis ball dropped from the same height. The tennis ball bounces to only 0.8 m. Suggest why the tennis ball loses a greater percentage of its energy than the rubber ball, and explain whether this observation supports or contradicts the principle of conservation of energy.
[1 mark]
Show mark scheme
- (a) Award 1 mark for correct calculation: (2.0 - 1.4) / 2.0 × 100 = 30% energy lost (or equivalent method showing 70% retained)
- (b) Award 1 mark for explaining that energy IS conserved but is transformed into other forms (thermal energy, sound energy, or elastic deformation). Award 1 mark for identifying that the 'missing' gravitational potential energy is converted to internal/heat energy and/or sound energy during the collision with the floor, rather than being lost from the system entirely.
- (c) Award 1 mark for: recognising that the tennis ball has a less elastic collision with the floor (or greater deformation/energy absorption) compared to the rubber ball, causing more energy to be converted to thermal energy and sound / AND correctly stating that this SUPPORTS conservation of energy because the energy is still conserved but transferred to different forms rather than disappearing.
Describe — 3 marks
A student is investigating energy changes by dropping a rubber ball from a height of 2 metres onto a hard floor. The ball bounces several times before coming to rest on the ground.
-
(a) Describe the energy change that occurs as the ball falls from the height of 2 metres towards the floor.
[1 mark]
-
(b) Describe what happens to the energy when the ball hits the floor and bounces back up.
[1 mark]
-
(c) Describe why the ball eventually comes to rest on the ground, in terms of energy changes in the system.
[1 mark]
Show mark scheme
- (a) Gravitational potential energy decreases/converts to kinetic energy as the ball falls (accept: PE converts to KE)
- (b) Kinetic energy is converted to elastic potential energy during compression / some energy is lost as heat and sound (accept: energy is dissipated or wasted during collision)
- (c) Energy is dissipated/lost as heat and sound with each bounce, so less kinetic energy is available for the next bounce until all energy is transferred to the surroundings (accept: friction causes energy loss / energy is transferred to thermal energy)
Calculate — 2 marks
A student uses an electric kettle to heat water for a hot drink. The kettle has a power rating of 2400 W and is switched on for 45 seconds. The student wants to understand the energy changes involved in heating the water.
-
(a) Calculate the total electrical energy supplied to the kettle. Give your answer in joules (J).
[1 mark]
-
(b) If 85% of the electrical energy is transferred to the water as thermal energy, calculate the thermal energy gained by the water.
[1 mark]
Show mark scheme
- (a) Award 1 mark for correct calculation: E = P × t = 2400 × 45 = 108,000 J (or 1.08 × 10⁵ J). Accept answers in kJ (108 kJ).
- (b) Award 1 mark for correct calculation: 0.85 × 108,000 = 91,800 J (or 9.18 × 10⁴ J). Accept 91.8 kJ. Must use their answer from part (a) if different.
Suggest — 2 marks
A student is investigating energy changes when a ball is dropped from different heights onto a concrete floor. The ball bounces to different heights each time it is dropped. The student notices that the ball becomes slightly warmer after being dropped multiple times.
-
(a) Suggest why the ball becomes warmer after being dropped multiple times.
[1 mark]
-
(b) Suggest why the ball does not bounce back to its original height after each drop.
[1 mark]
Show mark scheme
- (a) Energy is dissipated/converted to heat energy during the collision/impact with the floor
- (a) Friction/air resistance converts kinetic energy to heat
- (a) Internal energy of the ball increases due to energy transfer from mechanical energy
- (b) Some energy is lost/dissipated as heat energy during each bounce
- (b) Energy is transferred to the surroundings/floor as sound and heat
- (b) Not all gravitational potential energy is converted back to kinetic energy
- (b) Energy is dissipated due to friction/air resistance and inelastic collision
Define — 2 marks
A student is investigating energy changes in a system by dropping a ball from different heights onto a force plate connected to a motion sensor. The ball bounces to different heights depending on the initial drop height. The student needs to understand how energy is transferred and transformed during this process.
-
(a) Define what is meant by a 'closed system' in the context of the bouncing ball experiment.
[1 mark]
-
(b) Define the term 'energy dissipation' and identify one way energy is dissipated when the ball bounces.
[1 mark]
Show mark scheme
- (a) A system where no energy (or matter) enters or leaves the system from outside / energy is conserved within the system
- (b) Energy dissipation is the loss of energy from a system (as heat/sound/deformation) OR energy that is transferred to the surroundings in non-useful forms. Accept any one valid example: heat generated during impact, sound energy produced during bounce, energy used to deform the ball/force plate
GCSE Perfect