Newton's Second Law — Qualitative
In 2020, JAXA's Hayabusa2 capsule re-entered Earth's atmosphere at 12 km/s — a speed only possible because its tiny 300 N thruster had been pushing a spacecraft that got lighter as it burned fuel for 6 years.
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Q1 · Push a shopping trolley gently vs as hard as you can. What's different about how it moves?
Q2 · Would you rather push a shopping trolley full of bricks or one full of feathers? Why?
● Know
- That applying more force produces more acceleration
- That more mass produces less acceleration for the same force
- The names: Newton's Second Law, force, mass, acceleration
● Understand
- Why heavier objects accelerate less when pushed with the same force
- How force and acceleration are proportional (same direction)
- Why we describe this conceptually before using the formula
● Can do
- Predict which object accelerates more from a given push
- Rank scenarios by acceleration (qualitative)
- Apply the concept to Australian sporting and vehicle examples
Push a friend on a skateboard gently. Now push twice as hard. They accelerate twice as fast. That's Newton's Second Law in action — more force means more acceleration.
There are two sides to this law:
- Relationship 1 — Force and acceleration: If you double the force on an object (keeping its mass the same), you double its acceleration. Push three times as hard → accelerate three times as fast. Force and acceleration are proportional when mass is fixed.
- Relationship 2 — Mass and acceleration: If you double the mass of an object (keeping the force the same), you get half the acceleration. More mass → harder to accelerate → less acceleration.
Both relationships work at the same time. A very light object pushed hard will accelerate a lot. A very heavy object pushed lightly will barely accelerate at all.
Key point for Year 7: You do NOT need to calculate anything. The conceptual rule — more force or less mass → more acceleration — is the goal for now. In Year 9 you will use the formula F = ma to calculate exactly how much.
Newton's Second Law shows up everywhere you look in sport and transport:
- Tennis vs shot put: A tennis player can accelerate a tennis ball (57 g) dramatically with one swing. The same swing would barely move a shot put (7.26 kg) — the huge mass difference means far less acceleration for the same force.
- Motorcycles vs semi-trailers: A motorcycle and a loaded semi-trailer may have similar engine power at low speed, but their masses are vastly different. The motorcycle (200 kg) accelerates off the mark; the semi-trailer (50,000 kg) takes a long time to build speed.
- Bathurst racing (Mount Panorama): Teams work incredibly hard to reduce car mass by grams — every kilogram removed means slightly more acceleration from the same engine force. Lighter cars accelerate faster from corners and achieve higher top speeds.
- AFL kick: A footballer applies roughly the same kicking force every time. Kick a standard football → it flies fast. Kick a waterlogged heavy football in the rain → much less acceleration, shorter kick.
The pattern is always the same: less mass → more acceleration from the same force.
You might wonder: why aren't we using F = ma right now? Here's the thinking:
- Patterns first, formula second. In science, understanding the relationship (more force → more acceleration) before plugging numbers in means the maths will actually make sense when you get there in Year 9.
- Rockets (a great example): A rocket burns fuel, which provides a large thrust force upward. As it burns fuel, the rocket gets lighter. Same thrust, less mass → more acceleration. That's why rockets accelerate faster as the launch progresses.
- Car safety: In a crash, a car decelerates suddenly. Airbags work by increasing the time over which your body slows down — this reduces your body's acceleration, which reduces the force on you. Newton's Second Law, saving lives.
- Preview: Force = mass × acceleration. When you study this in Year 9, every number you calculate will connect back to today's idea: force, mass and acceleration are linked.
Wrong: "Heavier objects always accelerate less." This is only true if the force is the same. Give the heavier object a proportionally larger force and it can accelerate just as much.
Right: For the same force, more mass → less acceleration. But if force also increases, acceleration can stay the same or increase.
Wrong: "More force always means faster in the end." Force produces acceleration, not speed. An object can have huge acceleration from rest and still be slower than one that's been accelerating for longer.
Right: Force → acceleration (rate of change of speed). Speed depends on how long that acceleration has been acting.
Wrong: "Newton's Second Law needs a formula to be useful." The conceptual version — more force, less mass → more acceleration — is already powerful enough to explain rockets, crashes, sport and engineering.
Right: The qualitative pattern is real science. The formula F = ma is how you quantify it — it comes later.
Rank these four scenarios from highest to lowest acceleration. Assume each push is the same unless stated. Explain your reasoning.
- A: Small ball pushed hard
- B: Large ball pushed gently
- C: Small ball pushed gently
- D: Large ball pushed hard
Show model answer
Model ranking: A → D → C → B
A: small mass + large force = biggest acceleration. D: large mass + large force — the large force compensates, still good acceleration. C: small mass + small force — the small force limits it. B: large mass + small force = smallest acceleration. Note: exact order of D and C depends on how much larger/smaller the ball masses and forces are — the key is the principle.
A cyclist on a light 8 kg bike and a cyclist on a heavy 16 kg bike both push with the same 80 N force. Before you see the answer: which cyclist accelerates more? What happens to each cyclist's acceleration if they both double their force?
How close was your prediction?
The hook at the start of this lesson described a rocket getting faster and faster as it burns fuel — not because it produces more thrust, but because it gets lighter. Can you now explain why?
Use Newton's Second Law to explain the rocket's increasing acceleration, and then connect it to why you'd rather push a trolley of feathers than a trolley of bricks. Use the words force, mass and acceleration at least once each.
Q1. A footballer kicks a soccer ball and a medicine ball with the same force. Explain which ball accelerates more and why, using Newton's Second Law. (2 marks)
Q2. A car engine produces a fixed force. What happens to the car's acceleration as it picks up a full load of passengers? Explain qualitatively. (3 marks)
Q3. Compare the acceleration of a racing car and a fully loaded truck, assuming both engines produce the same force. Explain your reasoning in terms of Newton's Second Law. (4 marks)
Answers
▾MCQ 1
C — Force and acceleration are proportional. Double the force (same mass) → double the acceleration.
MCQ 2
C — The 8 kg bike has half the mass of the 16 kg bike. Same force on half the mass → twice the acceleration.
MCQ 3
B — A bowling ball has far greater mass than a tennis ball, golf ball or ping-pong ball. For the same applied force, greater mass → less acceleration.
MCQ 4
C — As the rocket burns fuel, its mass decreases. The same engine thrust now acts on a lighter rocket → greater acceleration. This is Newton's Second Law: acceleration = force ÷ mass.
MCQ 5
D — Newton's Second Law: acceleration depends on BOTH the applied force AND the object's mass. Changing either will change the acceleration.
Short Answer 1
Model answer: The soccer ball accelerates more (1 mark). The soccer ball has much less mass than the medicine ball (1 mark). According to Newton's Second Law, for the same force, less mass produces more acceleration.
Short Answer 2
Model answer: When passengers are added, the total mass of the car increases (1 mark). The engine force remains the same (1 mark). According to Newton's Second Law, more mass with the same force means less acceleration — the car accelerates more slowly when fully loaded (1 mark).
Short Answer 3
Model answer: A racing car has much less mass than a fully loaded truck (1 mark). If both engines produce the same force, the racing car will have far greater acceleration (1 mark) because Newton's Second Law states that acceleration depends on force and mass — less mass means more acceleration from the same force (1 mark). The loaded truck has enormous mass so even with the same engine force it barely accelerates by comparison (1 mark).