Physical Science / Unit 8: Work, Energy and Machines
8.5 Forms of Energy
Notes
Energy is the ability to do work, measured in joules.
- Kinetic energy (energy of motion): $\displaystyle KE = \frac{1}{2}mv^2$. Example: 2 kg at 3 m/s has $\displaystyle \frac{1}{2}(2)(9) = 9$ J.
- Gravitational potential energy: $PE = mgh$. Example: 5 kg at 2 m high is $5 \times 9.8 \times 2 = 98$ J.
- Other forms: thermal, chemical, elastic, electrical, nuclear, and light (radiant) energy.
- Doubling the speed of an object multiplies its kinetic energy by 4.
Practice
What is the kinetic energy of a 4 kg ball moving at 5 m/s?
$\displaystyle \frac{1}{2}(4)(25) = 50$ J.
What is the gravitational potential energy of a 10 kg object held 3 m above the ground? Use $g = 10 \text{ m/s}^2$.
$PE = mgh = 10 \times 10 \times 3 = 300$ J.
If a car's speed doubles, its kinetic energy:
Kinetic energy depends on $v^2$, so $2^2 = 4$.
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