A Level Physics 3.1 Momentum and Newton’s Laws MCQs – 30 Practice Questions with Answers (Cambridge 9702)
Free practice MCQs for Cambridge International AS & A Level Physics (9702), Section 3.1 Momentum and Newton’s Laws of Motion. Answer all 30 questions, then open the worked solutions to check your reasoning and calculations.
Formula recap: resultant force F = ma; momentum p = mv (unit kg m s−1); weight W = mg. Keep these beside you while you work – and review the full list on the A Level Physics Formula Sheet.
Newton’s First & Second Laws
Q1. A puck slides across a frictionless ice surface in a straight line at constant speed. What is the resultant force acting on the puck?
- (A) Equal to the weight of the puck
- (B) In the direction of motion, to keep it moving
- (C) Zero
- (D) Small, because it balances the puck’s inertia
Show Answer
Answer: C – By Newton’s first law, an object continues at constant velocity (constant speed in a straight line) when the resultant force is zero. No force is needed to maintain motion; friction is absent, so weight and the normal contact force cancel and the resultant is zero.
Q2. A resultant force of 24 N acts on an object of mass 6.0 kg. What is the acceleration of the object?
- (A) 0.25 m s−2
- (B) 4.0 m s−2
- (C) 30 m s−2
- (D) 144 m s−2
Show Answer
Answer: B – F = ma, so a = F / m = 24 / 6.0 = 4.0 m s−2.
Q3. A car of mass 1200 kg has a driving force of 3600 N. Air resistance and friction together exert a force of 600 N opposing the motion. What is the acceleration of the car?
- (A) 2.5 m s−2
- (B) 3.0 m s−2
- (C) 3.5 m s−2
- (D) 0.50 m s−2
Show Answer
Answer: A – Resultant force = 3600 − 600 = 3000 N. Then a = F / m = 3000 / 1200 = 2.5 m s−2. (Using 3600 N alone gives the trap answer 3.0.)
Q4. An astronaut takes a tool from Earth to the Moon, where the gravitational field strength is smaller. Which statement about the tool is correct?
- (A) Its mass and its weight both decrease
- (B) Its mass stays the same but its weight decreases
- (C) Its mass decreases but its weight stays the same
- (D) Its mass and its weight both stay the same
Show Answer
Answer: B – Mass is the amount of matter in the tool and does not change with location. Weight W = mg depends on g, so the weight is smaller on the Moon.
Q5. A heavy truck and a small car are each to be given the same acceleration. The truck needs a much larger resultant force mainly because of its larger
- (A) inertia
- (B) weight
- (C) volume
- (D) speed
Show Answer
Answer: A – Inertia is the resistance of an object to a change in its motion, and it increases with mass. From F = ma, for the same a a larger mass m needs a proportionally larger resultant force F.
Q6. Two blocks of mass 2.0 kg and 4.0 kg rest together on a smooth horizontal surface. A horizontal force of 12 N pushes the pair so that they move together. What is the acceleration of the blocks?
- (A) 6.0 m s−2
- (B) 3.0 m s−2
- (C) 1.0 m s−2
- (D) 2.0 m s−2
Show Answer
Answer: D – Treat the pair as one system of mass 2.0 + 4.0 = 6.0 kg. a = F / m = 12 / 6.0 = 2.0 m s−2.
Q7. A book rests on a horizontal table and remains at rest. Which explanation is given by Newton’s first law?
- (A) The book has no forces acting on it
- (B) The resultant force on the book is zero, so its velocity (zero) does not change
- (C) The table cancels the book’s inertia
- (D) Friction holds the book in place
Show Answer
Answer: B – The weight of the book and the normal contact force from the table are equal and opposite, so the resultant force is zero. With zero resultant force there is no acceleration, and the book stays at rest. Note the forces act – they simply balance.
Q8. A resultant force of 45 N produces an acceleration of 3.0 m s−2 in an object. What is the mass of the object?
- (A) 135 kg
- (B) 5.0 kg
- (C) 15 kg
- (D) 0.33 kg
Show Answer
Answer: C – m = F / a = 45 / 3.0 = 15 kg.
Q9. An astronaut has mass 80 kg. On a planet where g = 3.7 N kg−1, what is the astronaut’s weight?
- (A) 784 N
- (B) 80 N
- (C) 21.6 N
- (D) 296 N
Show Answer
Answer: D – W = mg = 80 × 3.7 = 296 N. (784 N would be the weight on Earth, where g = 9.8 N kg−1.)
Q10. A bus brakes suddenly and a standing passenger lurches forwards. The best explanation is that
- (A) a forward force throws the passenger forwards
- (B) the passenger’s feet stop with the bus but this force acts forwards
- (C) the floor pushes the passenger forwards
- (D) the passenger’s body tends to keep moving forwards at the bus’s original speed because of inertia, while friction at the feet slows the lower body
Show Answer
Answer: D – This is Newton’s first law: the passenger’s body continues in its state of motion unless acted on by a resultant force. Friction between shoes and floor decelerates the feet with the bus, but the upper body continues forwards until a force (for example from a handrail) acts on it.
Newton’s Third Law & Force Pairs
Q11. Which statement is a correct statement of Newton’s third law?
- (A) When two bodies interact, they exert equal and opposite forces of the same type on each other
- (B) Every force on a body is balanced by another force on the same body
- (C) Action and reaction forces act on the same body and cancel
- (D) The forces in a pair are equal because the bodies have the same mass
Show Answer
Answer: A – Third-law pairs are equal in magnitude, opposite in direction, of the same type (both gravitational, both contact, and so on) and they act on two different bodies.
Q12. A book lies on a table. The Earth pulls the book downwards with a gravitational force (the book’s weight). The Newton’s-third-law partner of this force is
- (A) the normal contact force of the table on the book
- (B) the push of the book on the table
- (C) the friction between the book and the table
- (D) the gravitational pull of the book on the Earth, acting upwards on the Earth
Show Answer
Answer: D – The pair must be the same type of force (gravitational) acting on the other body: the book attracts the Earth with an equal and opposite gravitational force. The normal contact force is the partner of the book pushing on the table, not of the weight.
Q13. The two forces in a Newton’s-third-law pair never cancel each other out because
- (A) one of them is always slightly larger
- (B) they act on different bodies
- (C) they act at different times
- (D) friction always acts between the bodies
Show Answer
Answer: B – Forces can only cancel (give a zero resultant) when they act on the same body. Each force of a third-law pair acts on a different body, so each body can still accelerate under its own resultant force.
Q14. A horse pulls a cart and the system accelerates forwards, even though the cart pulls backwards on the horse with an equal and opposite force. The system accelerates because
- (A) the pull of the horse on the cart is larger than the pull of the cart on the horse
- (B) third-law pairs do not apply when there is friction
- (C) the horse pushes backwards on the ground, and the ground pushes the horse forwards with a force that exceeds the backward pull of the cart
- (D) the wheels remove all backward forces
Show Answer
Answer: C – Considering the horse alone: the ground’s forward push (the reaction to the horse pushing the ground backwards) is greater than the cart’s backward pull, so the horse has a forward resultant force. The horse-cart pair of forces is internal to the combined system and cannot accelerate its centre of mass.
Q15. A rocket accelerates in space by burning fuel. The forward force on the rocket is provided by
- (A) the exhaust gases pushing on the rocket
- (B) the exhaust gases pushing on the air behind the rocket
- (C) the pressure of space on the front of the rocket
- (D) the rocket’s engines pulling it forwards
Show Answer
Answer: A – The rocket pushes the exhaust gases backwards at high speed; by Newton’s third law the gases push the rocket forwards with an equal force. This works in a vacuum – no air is needed.
Q16. A book rests on a table. The table exerts an upward normal contact force on the book. The Newton’s-third-law partner of THIS force is
- (A) the weight of the book
- (B) the downward push of the book on the table
- (C) the pull of the Earth on the table
- (D) the friction between the book and the table
Show Answer
Answer: B – The pair must be the same type (contact force) on the other body: the book pushes down on the table. The weight is a gravitational force on the book; it balances the normal force but is not its third-law partner – a very common examination trap.
Q17. A swimmer pushes the water backwards with her hands and feet and moves forwards. This is explained by Newton’s third law because
- (A) the water becomes less dense behind the swimmer
- (B) the swimmer’s mass decreases as she pushes
- (C) the water pushes the swimmer forwards with an equal-sized force
- (D) friction between the swimmer and the water is zero
Show Answer
Answer: C – The swimmer exerts a backward force on the water; the water exerts an equal and opposite (forward) force on the swimmer, which accelerates her forwards against drag.
Q18. A magnet attracts an iron nail with a force of 0.40 N. What is the force with which the nail attracts the magnet?
- (A) Zero, because the nail is not a magnet
- (B) Less than 0.40 N, because the nail is smaller
- (C) More than 0.40 N, because the nail accelerates more
- (D) 0.40 N
Show Answer
Answer: D – The attraction is a third-law pair: the forces are equal in magnitude and opposite in direction, whatever the sizes of the two bodies. The nail accelerates more because its mass is smaller (a = F / m), not because the force on it is bigger.
Q19. When a person walks forwards, the foot pushes backwards and downwards on the ground. The force that actually accelerates the person forwards is
- (A) the forward friction force exerted by the ground on the foot
- (B) the weight of the person
- (C) the normal contact force of the ground on the foot
- (D) the push of the foot on the ground
Show Answer
Answer: A – The foot pushes backwards on the ground, so by Newton’s third law the ground pushes the foot (and hence the person) forwards. This forward force is friction between shoe and ground – which is why walking on ice is difficult.
Q20. Two ice skaters of different masses stand facing each other and push each other apart. Which statement is correct?
- (A) The heavier skater exerts the larger force
- (B) The forces are equal and opposite, but the lighter skater gains the larger acceleration
- (C) Both skaters gain the same acceleration
- (D) Only the skater who pushes moves backwards
Show Answer
Answer: B – The pushes form a third-law pair: equal in magnitude, opposite in direction. Since a = F / m, the skater with the smaller mass gets the larger acceleration and moves away faster.
Momentum & F = ma Calculations
Q21. A ball of mass 0.15 kg moves at a speed of 20 m s−1. What is its momentum?
- (A) 133 kg m s−1
- (B) 0.0075 kg m s−1
- (C) 3.0 kg m s−1
- (D) 13.3 kg m s−1
Show Answer
Answer: C – p = mv = 0.15 × 20 = 3.0 kg m s−1.
Q22. A ball of mass 0.20 kg travelling at 8.0 m s−1 strikes a wall at right angles and rebounds at 6.0 m s−1. What is the magnitude of the change in momentum of the ball?
- (A) 2.8 kg m s−1
- (B) 0.40 kg m s−1
- (C) 1.6 kg m s−1
- (D) 2.0 kg m s−1
Show Answer
Answer: A – Momentum is a vector, so the rebound velocity is in the opposite direction. Taking the initial direction as positive: Δp = m(v − u) = 0.20 × (−6.0 − 8.0) = 0.20 × (−14) = −2.8 kg m s−1, a change of magnitude 2.8 kg m s−1. (0.40 comes from wrongly treating momentum as a scalar: 0.20 × (8.0 − 6.0).)
Q23. An object has momentum 240 kg m s−1 and mass 60 kg. What is its speed?
- (A) 0.25 m s−1
- (B) 40 m s−1
- (C) 14 400 m s−1
- (D) 4.0 m s−1
Show Answer
Answer: D – v = p / m = 240 / 60 = 4.0 m s−1.
Q24. A person of mass 70 kg stands in a lift that accelerates upwards at 2.0 m s−2. Taking g = 9.8 m s−2, what is the force exerted on the person by the floor of the lift?
- (A) 686 N
- (B) 826 N
- (C) 546 N
- (D) 140 N
Show Answer
Answer: B – The forces on the person are the floor’s push R (up) and the weight mg (down). Resultant upward = ma, so R − mg = ma and R = m(g + a) = 70 × (9.8 + 2.0) = 70 × 11.8 = 826 N. The person feels heavier.
Q25. A person of mass 70 kg stands in a lift that accelerates downwards at 1.5 m s−2. Taking g = 9.8 m s−2, what is the force exerted on the person by the floor of the lift?
- (A) 686 N
- (B) 826 N
- (C) 581 N
- (D) 105 N
Show Answer
Answer: C – With downward acceleration, mg − R = ma, so R = m(g − a) = 70 × (9.8 − 1.5) = 70 × 8.3 = 581 N. The person feels lighter than the true weight of 686 N.
Q26. A block of mass 5.0 kg on a smooth horizontal surface is pulled by a horizontal force of 24 N. A light rope joins the block to a second block of mass 3.0 kg behind it, so both blocks accelerate together. What is the tension in the rope?
- (A) 9.0 N
- (B) 15 N
- (C) 24 N
- (D) 3.0 N
Show Answer
Answer: A – System: a = F / (m1 + m2) = 24 / (5.0 + 3.0) = 3.0 m s−2. The rope only has to accelerate the 3.0 kg block behind: T = ma = 3.0 × 3.0 = 9.0 N.
Q27. Three horizontal forces act on an object of mass 2.0 kg along a straight line: 12 N and 7.0 N to the right, and 5.0 N to the left. What is the acceleration of the object?
- (A) 12 m s−2
- (B) 2.0 m s−2
- (C) 5.0 m s−2
- (D) 7.0 m s−2
Show Answer
Answer: D – Resultant force = 12 + 7.0 − 5.0 = 14 N to the right. a = F / m = 14 / 2.0 = 7.0 m s−2.
Q28. The unit of momentum, kg m s−1, is equivalent to which of the following?
- (A) N
- (B) N s
- (C) N s−1
- (D) kg m s−2
Show Answer
Answer: B – 1 N = 1 kg m s−2, so 1 N s = 1 kg m s−2 × s = 1 kg m s−1. This is why impulse (force × time) has the same unit as change of momentum.
Q29. A constant resultant force of 500 N acts on a stationary object of mass 250 kg. What is the speed of the object after the force has acted for 4.0 s?
- (A) 2.0 m s−1
- (B) 4.0 m s−1
- (C) 8.0 m s−1
- (D) 16 m s−1
Show Answer
Answer: C – a = F / m = 500 / 250 = 2.0 m s−2. Starting from rest, v = at = 2.0 × 4.0 = 8.0 m s−1.
Q30. Which object has the greatest momentum?
- (A) A car of mass 1200 kg moving at 12 m s−1
- (B) A truck of mass 2000 kg moving at 5.0 m s−1
- (C) A train of mass 50 000 kg moving at 0.20 m s−1
- (D) A cyclist of total mass 90 kg moving at 8.0 m s−1
Show Answer
Answer: A – Compute p = mv for each: car = 1200 × 12 = 14 400 kg m s−1; truck = 2000 × 5.0 = 10 000; train = 50 000 × 0.20 = 10 000; cyclist = 90 × 8.0 = 720. The car has the greatest momentum – a very large mass moving slowly can have less momentum than a smaller mass moving fast.
Keep practising: work through the other AS Level topics and boards on the Physics MCQs hub, and keep the A Level Physics Formula Sheet open while you revise.
All questions are original practice questions written for QuaidianPoint – not Cambridge past-paper questions. QuaidianPoint is not affiliated with Cambridge Assessment International Education.