Edexcel A Level Physics · Topic 2 · 6-mark prose

“With reference to Newton’s laws, explain…”

These 5- and 6-mark questions come up again and again, and the mark scheme wants the same six points every time. Learn the chain once, then fill in the objects from the question.

Print it out

The notes: the chain, how to mark up a question and every worked answer. Or just the questions, with the chain checklist at the top and the answers at the back.

⬇ Notes (PDF) ⬇ Questions & answers (PDF)

1. How to spot one

The trigger words

“With reference to Newton’s laws of motion, explain…”

…followed by a scenario and a picture: a rocket, a balloon car, a passenger holding a pole, ice whizzing across water. Object A pushes on object B, and object A accelerates. That’s your cue for the Newton chain.

2. The Newton chain: six links, six marks

THE NEWTON CHAIN 3 → 2 Third law first, then the second law

Say it until it sticks: A on B · B on A · equal & opposite · resultant · accelerates · so…

1
A on BObject A exerts a force on object B. Name them from the question.
2
B on A N3By Newton’s third law, B exerts a force on A.
3
Equal & oppositeSame size, opposite direction. Say which way it pushes A.
4
ResultantThis is the resultant (unbalanced) force on A.
5
Accelerates N2By Newton’s second law, A accelerates: it changes speed, direction or momentum.
6
So…Link it back to the question: which way, how fast, and why.

The 0-mark trap. Writing out Newton’s laws word for word scores nothing. The mark scheme says so. Every link must be about A and B in this question: “the gas exerts a force on the ice”, never just “every action has an equal and opposite reaction”.

Six links for five marks? Good. The mark scheme usually has one more point than the marks available, so the extra link is your safety net if an examiner doesn’t accept one of the others.

3. Before you write: mark up the question

Ring three things in the question. They’re the only parts of your answer that change from question to question.

Ring AThe object that accelerates

The thing the question asks about: the ice, the car, the passenger.

Ring BThe object it interacts with

The gas, the air, the pole. A pushes B, so B pushes A back.

UnderlineWhat object A does (F = ma)

starts moving
speeds up
turns
keeps going at a steady speed

Either it accelerates, or its acceleration is zero.

If the question says A…	Resultant force on A	So your link 5 says
changes speed (starts moving, speeds up, slows down)	Not zero, along the line of motion	A accelerates (or decelerates): its velocity and momentum change
changes direction (turns, spins, curves)	Not zero, at an angle to the motion	A accelerates even at a steady speed, because its velocity changes direction
moves at constant velocity (or stays still)	Zero: the forces are balanced	By Newton’s first law, A does not accelerate

4. Worked example: ice on water

Solid carbon dioxide on waterEdexcel A Level Physics past paper · 5 marks

Solid carbon dioxide^A changes state directly from solid to gas. This process is called sublimation. Solid carbon dioxide, when placed on water, will move rapidly across the surface due to jets of ejected gas^B.

The diagram below shows the direction of movement for a large piece of solid carbon dioxide placed on water.

Redrawn from the exam photo, with the two forces added. They are the same size and point opposite ways, and each one acts on a different object.

(a) When placed at rest on water, piece X begins to move rapidly in the direction shown.

With reference to Newton’s laws of motion explain the motion of piece X. (5)

Newton’s lawsExplain Newton’s third law, then Newton’s second law. Run the chain.

A and BA = the solid carbon dioxide (piece X). B = the gas it pushes out.

What A does (F = ma)

“at rest”
“begins to move rapidly”
it changes speed, so there’s a resultant force and it accelerates

Your turn Have a go before you look

Your answer stays on this device, so you can come back to it.

Step 1 Run the chain in note form

1ice on gas 2N3: gas on ice 3equal & opposite (pushes X the way shown) 4resultant force (little friction) 5N2: accelerates from rest 6F = ma: small mass → large acceleration

Step 2 Write it in full sentences

1The solid carbon dioxide exerts a force on the gas, pushing the jet of gas out backwards.✓ 1 mark

2By Newton’s third law, the gas exerts a force on the solid carbon dioxide.✓ 1 mark

3This force is equal in size and opposite in direction to the force on the gas, so it pushes piece X in the direction shown.✓ 1 mark

4There is very little friction from the water, so this is a resultant (unbalanced) force on piece X.✓ 1 mark

5By Newton’s second law, a resultant force causes an acceleration, so piece X accelerates from rest: its velocity increases in the direction shown.✓ 1 mark

6Using F = ma: as the piece of ice has a small mass, there is a large acceleration for the same resultant force, so it moves rapidly.✓ 1 mark

Six marking points, five marks available: full marks with one to spare.

Check it against the mark scheme

Mark scheme says	Link
Solid (CO₂) exerts a force on the gas (CO₂)	1 A on B	(1)
N3 means (gas exerts) a force on the solid/X	2 B on A	(1)
Force is in opposite direction on the solid/gas	3 Equal & opp.	(1)
There is a resultant/unbalanced force (on the solid)	4 Resultant	(1)
N2/1 means the (solid) accelerates (accept changes velocity/speed)	5 Accelerates	(1)
Rapid because mass/friction is small	6 So…	(1)
Max 5. No mark for a statement of Newton’s laws. Quality of written communication is assessed, so write it in order: the chain does that for you.

5. Your turn: practise the chain

Same chain, different objects. Mark up the question first (ring A, ring B, underline what A does), write your answer, then open the answer to see the mark-up, the chain and the mark scheme.

Practice 1 · Holding on in an accelerating trainEdexcel A Level Physics past paper · 5 marks

(b) As the train leaves the next station the passenger^A holds on to a vertical support^B as the train accelerates. This prevents the passenger falling backwards.

The passenger pulls the pole backwards; the pole pulls the passenger forwards.

With reference to Newton’s laws of motion, explain why holding on to a vertical support prevents the passenger falling backwards. (5)

Newton’s lawsThird law, then second law. Run the chain.

A and BA = the passenger. B = the vertical support (pole).

What A does (F = ma)

“the train accelerates”
the passenger must speed up with the train, so there’s a resultant force and they accelerate

Your turn Write your answer

Your answer stays on this device.

Step 1 Run the chain in note form

1passenger pulls pole backwards 2N3: pole on passenger 3equal & opposite (forwards) 4resultant force 5N2: accelerates forwards 6same acceleration as the train

Step 2 Write it in full sentences

1The passenger exerts a force on the support, pulling it backwards.✓ 1 mark

2By Newton’s third law, the support exerts a force on the passenger.✓ 1 mark

3This force is equal in size and opposite in direction, so it pulls the passenger forwards, the same way as the train accelerates.goes with 2

4This forward force is the resultant (unbalanced) force on the passenger.✓ 1 mark

5By Newton’s second law, a resultant force causes an acceleration, so the passenger accelerates forwards.✓ 1 mark

6The passenger accelerates at the same rate as the train, so they move with the train and do not fall backwards.✓ 1 mark

Check it against the mark scheme

Mark scheme says	Link
Man pulls (backward) on the support	1 A on B	(1)
Due to N3 the support exerts an (opposite) force on the man	2–3 B on A	(1)
This force is a resultant/unbalanced/net force on man	4 Resultant	(1)
Due to N1/N2 the man will accelerate	5 Accelerates	(1)
With the same acceleration/speed/velocity as the train	6 So…	(1)
5 marks. Here “equal and opposite” shares a mark with the third law, so link 6 is the one you can’t skip: say same acceleration as the train.

Practice 2 · The balloon carEdexcel A Level Physics past paper · 3 marks

The diagram shows a toy car^A driven by air^B from a deflating balloon.

The car pushes the air backwards; the air pushes the car forwards.

When the air in the inflated balloon is released, the car starts to move forwards.

(a) Use Newton’s first and third laws of motion to explain why the air coming out of the balloon causes this. (3)

Newton’s lawsThey ask for the first and third laws. Run the chain, and use the first law at link 5.

A and BA = the car (with its balloon). B = the air pushed out.

What A does (F = ma)

“starts to move forwards”
it changes speed from rest, so there’s a resultant force and it accelerates

Your turn Write your answer

Your answer stays on this device.

Step 1 Run the chain in note form

1car pushes air backwards 2N3: air on car 3equal & opposite (forwards) 4resultant force (bigger than friction) 5N1: can’t stay at rest, accelerates 6starts moving forwards

Step 2 Write it in full sentences

1The balloon exerts a force on the air, pushing it out backwards.✓ mark 1

2By Newton’s third law, the air exerts a force on the balloon and car.✓ mark 1

3This force is equal in size and opposite in direction, so it pushes the car forwards.✓ mark 1

4The forward force is bigger than the friction and drag, so there is a resultant (unbalanced) force on the car.✓ mark 2

5By Newton’s first law, the car can only stay at rest if there is no resultant force, so the car accelerates.✓ mark 2

6So the car starts to move forwards, in the opposite direction to the air coming out.✓ mark 3

Only 3 marks, but the chain still works: links 1–3 earn mark 1, links 4–5 earn mark 2, and naming both laws in context earns mark 3.

Check it against the mark scheme

Mark scheme says	Link
Uses N3: force (backward) on air by balloon/car, (so/=) force (forward) on balloon/car by air	1–3	(1)
Uses N1: resultant force / forces unbalanced / force on balloon > drag, (so) there is an acceleration / moves from rest / starts moving	4–5	(1)
Identifies the use of N1 or N3 (by name or description) correctly, linking it to the context	2 and 5	(1)

6. Spot the difference: who gets pushed, and who keeps going

Some questions have two groups. The people a seat pushes on get the chain: third law, then second law, so they slow down. The person nothing pushes on gets Newton’s first law: the resultant force is zero, so they keep going. The clue: a vehicle slows down suddenly and someone carries on moving.

PUSHED OR NOT? 3 → 2, then 1Who gets a force, and who doesn’t?

1
Pushed N3They push on their seat; the seat pushes them back, equal and opposite.
2
Slow down N2That’s a resultant force on them, so they decelerate with the vehicle.
3
Keeps going N1No horizontal force on them: resultant force zero, so they carry on at the same speed. Never “thrown forwards”.
4
So…Who hits whom, and so which seat is safest.

Practice 3 · Which seat is safest?Edexcel A Level Physics past paper · 4 marks

The diagram shows three passengers sitting on a train that is travelling at a high speed in the direction shown. Seat belts are not used on trains.

With reference to one of Newton’s laws of motion, explain why seat C is the safest seat for a passenger to be sitting on in the event of a rapid deceleration. You may assume that the seats all remain fixed firmly to the floor and do not break. (4)

Newton’s lawsTwo groups. B and C get pushed by their seats: third law, then second law. A gets no horizontal force: first law.

Who gets a force?B and C: their seat backs are in front of them. A: nothing in front of them.

What happens (F = ma)

“rapid deceleration”
B and C decelerate with the train; resultant force on A is zero, so A keeps going

Your turn Write your answer

Your answer stays on this device.

Step 1 Run the check in note form

1B and C push chair to the right 2N3: seats push B and C to the left 3resultant force: B and C decelerate 4no horizontal force on A 5resultant force zero (N1) 6A continues at same speed 7A hits B 8nobody hits C

Step 2 Write it in full sentences

1When the train decelerates, passengers B and C push on the backs of their seats, to the right.goes with 2

2By Newton’s third law, seats B and C push the passengers to the left, with an equal and opposite force.✓ 1 mark

3This is a resultant force on B and C, so by Newton’s second law they decelerate (slow down) with the train and stay in their seats.✓ 1 mark

4No horizontal force acts on passenger A: their seat back is behind them, not in front.goes with 5

5The resultant force on A is zero, so by Newton’s first law A’s velocity does not change.✓ 1 mark

6So A continues to move forwards at the same speed while the train slows down.✓ 1 mark

7Passenger A hits passenger B.✓ 1 mark

8Nobody hits C, so seat C is the safest.answers the question

Five marking points, four marks available. Don’t write that A is “thrown forwards”: nothing throws them, they just keep going. The mark scheme refuses that mark.

Check it against the mark scheme

Mark scheme says	Link
(B and) C will stay in their seats	3	(1)
Resultant force acts / chair exerts force on (B and) C, or (B and) C will decelerate	1–3	(1)
Passenger A continues to move (at the same speed). Not if A is “thrown/thrust/pushed” forward	6	(1)
Identifies movement of passenger A as Newton’s first law, in the context of the question	4–5	(1)
A will collide with B	7	(1)
Max 4. Just quoting N1 doesn’t score: it has to be about passenger A.