Year 7 Science · Unit 3 · Lesson 11
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Learning Goals
Read the graph
The bar graph below shows the relative kinetic energy of four sports balls when each is thrown at exactly the same speed (30 m/s). Use the graph to answer the questions.
Data: approximate standard ball masses (Cricket Australia, International Tennis Federation, R&A)
(a) Describe the trend shown in the graph. What is the relationship between ball mass and kinetic energy when all balls travel at the same speed?
(b) A cricket ball has approximately 2.7 times the mass of a tennis ball. By how many times is the cricket ball's KE greater than the tennis ball's at the same speed? Does this match what the graph shows?
(c) If the cricket ball's speed were doubled to 60 m/s while all other balls stayed at 30 m/s, predict how the cricket ball's KE bar would change. Explain your reasoning.
Scenario
Transport for NSW data shows that a car travelling at 100 km/h has approximately four times the kinetic energy of the same car travelling at 50 km/h — because KE depends on speed squared. The car's brakes must absorb all of that kinetic energy to bring it to a stop.
(a) Predict: If a car's speed doubles from 50 km/h to 100 km/h, predict what happens to its stopping distance — does it double, or change by a different amount?
(b) Justify: Why does stopping distance increase by more than the speed increase? Use the idea of kinetic energy in your explanation.
(c) Using this reasoning, explain why a reduction in speed limit from 60 km/h to 50 km/h in Australian school zones (introduced to improve pedestrian safety) is more effective than it might seem.
1. A truck and a small car are both travelling at 80 km/h. The truck has 5 times the mass of the car. How many times greater is the truck's kinetic energy? Explain how you worked this out.
2. The Snowy Mountains Hydroelectric Scheme converts the kinetic energy of flowing water into electricity. If the flow rate of the water (which affects its speed) were doubled, predict how the electrical output would change. Justify your answer.
Wrap Up
In one sentence, what was the main idea of this lesson?