IGCSE Physics 0625: Energy Stores and Conservation of Energy on a Slide

A child sits at the top of a straight playground slide and pushes off, starting from rest.

(a) State the energy store that decreases and the energy store that increases as the child slides down. [2]

(b) The slide is not perfectly smooth. As the child slides down, some energy is transferred to the internal (thermal) energy store of the slide and the child's clothing. State the process by which this energy transfer takes place. [1]

(c) At the top of the slide, the child has $300\text{ J}$ stored in the gravitational potential energy store. By the time the child reaches the bottom, $210\text{ J}$ has been transferred to the kinetic energy store. Using the principle of conservation of energy, calculate the energy transferred to the internal (thermal) energy store. [2]

Part (a): Energy stores involved

As the child slides down, height above the ground decreases and speed increases.

The gravitational potential energy store decreases (the child loses height).
The kinetic energy store increases (the child speeds up).

Part (b): How the thermal energy transfer happens

Friction acts between the child and the surface of the slide. This friction force does mechanical work, and the process transfers energy from the child’s motion into the internal (thermal) energy store of the slide and the child’s clothing (they get warmer).

Part (c): Applying conservation of energy

By the principle of conservation of energy, the gravitational potential energy lost must be accounted for by the kinetic energy gained plus the energy transferred elsewhere:

$E_{p,\text{lost}} = E_{k,\text{gained}} + E_{\text{internal (thermal)}}$

$300\text{ J} = 210\text{ J} + E_{\text{internal (thermal)}}$

Rearranging:

$E_{\text{internal (thermal)}} = 300\text{ J} - 210\text{ J} = 90\text{ J}$

Final answers

(a) Gravitational potential energy store decreases; kinetic energy store increases
(b) Mechanical work done against friction transfers the energy to the internal (thermal) energy store
(c) Energy transferred to the internal (thermal) energy store $=$ $90\text{ J}$