Chapter 6, Topic 1
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# Energy

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### What is energy?

Energy is the capacity to do work. SI unit of energy Joules (J)

#### Different forms of energy:

• Kinetic Energy
• Potential Energy (Gravitational, Chemical, Elastic)
• Electrical Energy
• Light
• Thermal Energy
• Nuclear Energy

### What is Principle of Conservation of Energy

The principle of conservation of energy states that energy cannot be created nor destroyed but can be converted from one form to another. The total energy in an isolated system is constant.

### Pendulum

• When a pendulum is held up to A, it gains GPE.
• When the raised pendulum is released, it swings towards the equilibrium position at B.
• The GPE is converted to KE.
• At B, GPE is at its minimum while KE is at its maximum.
• From B, the bob now swings upwards C. It now loses KE and gains GPE.

#### What is the ideal pendulum?

The bob will swing to the position with the same height as when it was released.

#### What is the non-ideal pendulum?

Energy lost due to friction and air resistance. Hence, Final KE at rest and Final GPE at final position are lower than ideal pendulum.

### Barbecue

• As charcoal burns, the chemical potential energy in charcoal is converted to thermal and light energy, the thermal energy is used to cook food.

### Classic Energy Questions

#### Describe the energy changes that take place when a ball is being thrown upwards by a hand. (Neglecting air resistance)

• As the ball just leaves the hand, it possesses the maximum amount of KE. Its GPE is zero due to the zero height from the hand.
• As it moves upwards, it loses speed. The KE is converted to GPE as the ball looses speed and gain height.
• At the highest point from the ground, the stone is momentarily at rest. The KE is zero while the GPE reaches a maximum. All the KE have been converted to GPE. The GPE at the top equals the KE as the ball just left the hand (at the bottom).
• As the object moves downwards, its speed increases. GPE is converted back to KE.
• Just before hitting the hand, the KE reaches maximum, and GPE is zero.
• According the Principal of Conservation of Energy, the sum of KE and GPE is always constant at any point in time. 