Year 7 Science · Unit 3 · Lesson 6
Extension · Challenge Worksheet
Learning Goals
Extension · optional
Newton's Second Law sits in Stage 5 (SC5-WAM-01), beyond the Stage 4 Forces dot points this unit is built on. Nothing on this sheet is assessed as part of Year 7 Forces, so it is enrichment rather than required practice. Do it if you are interested, and skip it without losing any marks if you are not.
1. What is acceleration? Name the two quantities that decide how much acceleration a force produces.
2. Student A says: “More force always means an object goes faster.” Student B says: “More force means the object accelerates more, not that it is necessarily going faster.” Who is correct? Explain, and give an example that supports that student.
Scenario: electric versus petrol
An electric car (Car E) is compared with the petrol version of the same model (Car P). Car E’s motor gives twice the driving force of Car P’s engine, but its battery makes Car E 1.5 times as heavy. A journalist writes: “The electric car must accelerate twice as fast, because it has twice the force.”
3. Predict how Car E’s acceleration compares with Car P’s, and evaluate the journalist’s claim.
Find the error
Zac ranked four balls by acceleration
Ball A: light, pushed hard. Ball B: heavy, pushed gently. Ball C: light, pushed gently. Ball D: heavy, pushed hard. Zac ranks them A, C, D, B and writes: “Light balls always accelerate more than heavy balls, so both light balls come first.”
4. Which parts of Zac’s ranking are certain, and which are not? Explain the error in his reasoning.
Read the data
Scenario: pushing skaters
A skating coach pushes five skaters, one at a time, with a steady force for 1 second on smooth ice, where friction is tiny. A sensor records how much speed each skater gains.
| Skater | Mass (kg) | Pushing force (N) | Speed gained (m/s) |
|---|---|---|---|
| Ari | 30 | 60 | 2.0 |
| Bea | 40 | 60 | 1.5 |
| Cal | 50 | 60 | 1.2 |
| Dev | 60 | 60 | 1.0 |
| Eli | 60 | 120 | 2.0 |
Source: made-up results for this worksheet.
5. Use the results for Ari, Bea, Cal and Dev to describe how mass affects acceleration when the force is the same. Quote data.
6. Maya claims: “Heavier objects always accelerate less.” Use the results for Ari and Eli to evaluate her claim.
7. In a crash, a car stops very suddenly. An airbag makes the driver’s body take longer to slow down. Use Newton’s Second Law to explain why this reduces the force on the driver, and so reduces injury.
Question 1
A change in speed or direction (1). The force on the object and its mass (1). [2 marks]
Question 2
Student B (1). Force gives acceleration, a change in speed; how fast it ends up also depends on how long the force acts (1). Example: a braking car has a big force on it but is getting slower; or a dragster just off the line is slower than a cyclist who has been riding for minutes (1). [3 marks]
Question 3
Twice the force alone would double the acceleration (1); 1.5 times the mass alone would reduce it (1). Force has gone up more than mass, so Car E accelerates faster, but by about a third, not double (1). The journalist is partly right: the conclusion “faster” holds, but ignoring mass gives “twice”, which is wrong (1). [4 marks]
Find the error: Question 4
Certain: A is greatest (light mass, big force) and B is least (big mass, small force) (1). C against D cannot be decided without knowing how much bigger D’s mass and force are (1). Error: Zac compared mass only and ignored that the forces differ (1). [3 marks]
Read the data: Question 5
With the same 60 N, more mass gives less speed gained (1): Ari (30 kg) 2.0 m/s down to Dev (60 kg) 1.0 m/s; double the mass, half the acceleration (1). [2 marks]
Question 6
Eli has twice Ari’s mass, yet both gained 2.0 m/s (1), because Eli was pushed with twice the force (1). The claim is only true when the force is the same (1). [3 marks]
Question 7
Marking guide: 1 the driver must slow from the car’s speed to zero, and slowing is an acceleration; 1 the airbag spreads this over a longer time; 1 so the acceleration (rate of slowing) is smaller; 1 the driver’s mass stays the same; 1 for a fixed mass, a smaller acceleration needs a smaller force; 1 a smaller force on the body causes less injury. [6 marks]
Model answer: Slowing to a stop is an acceleration. The airbag makes it take longer, so the acceleration is smaller. The driver’s mass is unchanged, and for the same mass a smaller acceleration needs a smaller force, so there is less injury.
Common error: saying the airbag “absorbs the force” without linking time, acceleration and force.