Year 9 Science · Unit 3 · Lesson 5
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Learning Goals
1. Name two forms of stored energy from the lesson, and one form of energy that travels as a wave.
Real-world context: lightning in the Blue Mountains
During a summer storm, a lightning bolt strikes a gum tree in the Blue Mountains. The bolt discharges in a fraction of a second, splitting the tree and setting fire to its dry bark. Flames then spread through the surrounding bush.
2. When the bolt strikes the tree, it releases light, sound and thermal energy. What form of energy did the lightning bolt have before it struck, and where did that energy come from?
Cause and effect: a hydrogen car
Complete the chain. In each empty box, name the energy change that happens at that step.
3. A 0.16 kg cricket ball is bowled at 35 m/s. (a) Calculate its kinetic energy. (b) A junior bowler bowls the same ball at half the speed. Calculate its kinetic energy now, and state how many times smaller it is.
(a) J
(b) J
(b) times smaller
Investigation: a swinging pendulum
A class hung a 0.20 kg bob on a long string and released it from three heights above its lowest point. A light gate at the lowest point measured the bob’s speed. Use g = 10 m/s². The first row is done for you.
| Release height (m) | GPE at release (J) | Speed at the lowest point (m/s) | Kinetic energy at the lowest point (J) | Kinetic energy ÷ GPE (%) |
|---|---|---|---|---|
| 0.20 | 0.40 | 1.9 | 0.36 | 90 |
| 0.45 | 2.8 | |||
| 0.80 | 3.8 |
Source: sample class results. Kinetic energy is rounded to 2 decimal places.
4. Complete Table 1.
5. In every trial, the kinetic energy at the lowest point is a little less than the GPE at release. Explain where the rest of the energy went. Then predict the speed at the lowest point for a release height of 1.25 m, if there were no air resistance or friction.
Predicted speed m/s
6. Once the gum tree catches fire, chemical energy stored in the wood is released. Describe the complete chain of energy changes, from the lightning bolt to the spreading fire. Name at least five forms of energy and the object or substance that has each one, and explain whether energy is conserved.