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Cambridge IGCSE Physics · 0625

Chapter 1: Motion, forces and energy — Part 9

Topic 1.7.1 · Energy

Energy stores

Energy may be stored as: kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic, and internal (thermal).

Energy is transferred between stores by: forces (mechanical work), electric currents (electrical work), heating, and waves (electromagnetic, sound and other waves).

Conservation of energy

Energy cannot be created or destroyed. The total energy of a closed system is constant. Apply this to simple examples and to simple flow diagrams (arrows between stores).

Simple energy flow diagram for a falling object: gravitational potential store decreasing, kinetic store increasing, with a small thermal store from air resistance.
Diagram 1: Simple energy flow diagram for a falling object: gravitational potential store decreasing, kinetic store increasing, with a small thermal store from air resistance.

Kinetic and gravitational potential energy

Ek = ½ mv2

Change in gravitational potential energy ΔEp = mgΔh

Apply conservation to multi-stage processes and interpret Sankey diagrams (arrow width ∝ energy; wasted energy shown branching off).

A 0.40 kg ball moves at 5.0 m/s. Calculate its kinetic energy.

Worked example: kinetic energy is one half times 0.40 times 25 equals 5.0 joules
Ek = ½ mv2 = 5.0 J.

A 2.0 kg box is lifted 3.0 m at constant speed. Take g = 9.8 N/kg. Calculate the gain in gravitational potential energy.

Worked example: change in gravitational potential energy is 59 joules
ΔEp = mgΔh = 59 J (2 s.f.).

A 0.50 kg stone is dropped from rest through 5.0 m. Ignore air resistance. Take g = 9.8 N/kg. Calculate its speed just before it hits the ground.

Worked example: loss of gravitational potential energy becomes kinetic energy, speed 9.9 metres per second
mgΔh = ½ mv2 gives v = 9.9 m/s.
Sankey diagram with a wide input arrow splitting into a useful output arrow and thinner wasted-energy arrows, widths proportional to energy.
Diagram 2: Sankey diagram with a wide input arrow splitting into a useful output arrow and thinner wasted-energy arrows, widths proportional to energy.

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