What does Forces and Momentum cover in IB Physics?

This topic covers forces, momentum, and circular motion, including Newton's laws and energy considerations in interactions. Newton's three laws describe forces as interactions between bodies. Free-body diagrams represent forces, including normal force, friction F_f ≤ μ_S F_N or F_f = μ_D F_N, tension, elastic restoring force F_H = -kx, viscous drag F_d = 6πeta rv, buoyancy F_b = ρ Vg, gravitational force F_g = mg, electric, and magnetic forces.

Is Forces and Momentum SL or HL?

Both. SL and HL students study Forces and Momentum to the same depth.

How do I revise Forces and Momentum for IB Physics?

Start from the core idea: this topic covers forces, momentum, and circular motion, including Newton's laws and energy considerations in interactions. In the exam: heavily examined in both papers. Free-body diagrams are drawn or annotated. Then practise exam-style questions, easiest first, writing out every step of your working before you check it.

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FourtyFive has 48 Forces and Momentum questions. Every answer you write is marked mark by mark, IB-style, and you see where each mark was won or lost. Every part has a hint, the AI tutor helps you through the step you are stuck on, and your Study Profile picks what to practise next.

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Topic A.2 · SL and HL

Forces and Momentum: notes and practice questions

12 min read Jump to 20 practice questions

Summary

This topic covers forces, momentum, and circular motion, including Newton's laws and energy considerations in interactions.
Newton's three laws describe forces as interactions between bodies.
Free-body diagrams represent forces, including normal force, friction $F_f \le \mu_S F_N$ or $F_f = \mu_D F_N$ , tension, elastic restoring force $F_H = -kx$ , viscous drag $F_d = 6\pi\eta rv$ , buoyancy $F_b = \rho Vg$ , gravitational force $F_g = mg$ , electric, and magnetic forces.
Linear momentum $p = mv$ is conserved in the absence of external resultant forces.
Impulse $J = F\Delta t = \Delta p$ .
Centripetal acceleration $a = \frac{v^2}{r} = \omega^2 r$ for circular motion, caused by a centripetal force.
Energy is conserved in elastic collisions, but not necessarily in inelastic collisions or explosions.

How it is examined

Heavily examined in both papers. Free-body diagrams are drawn or annotated. Drag force appears in unit-analysis form: May 2025 SL and HL Paper 2 TZ1 question 1(b) gave $F = cv^2$ and asked candidates to determine c and state the fundamental SI unit, for 3 marks. Command terms: draw, determine, explain, show. A generated question that treats a banked corner quantitatively, or that needs simultaneous momentum-and-energy equations, is out of syllabus at either level.

Given in the booklet

$F_\text{f} \le \mu_\text{s} F_\text{N}$ , $F_\text{f} = \mu_\text{d} F_\text{N}$ , $F_\text{H} = -kx$ , $F_\text{d} = 6\pi\eta r v$ , $F_\text{b} = \rho V g$ , $F_\text{g} = mg$ , $p = mv$ , $J = F\Delta t$ , $F = \Delta p/\Delta t$ , $a = v^2/r = \omega^2 r = 4\pi^2 r/T^2$ , $v = 2\pi r/T = \omega r$ . Newton's three laws as statements are not equations and must be known in words. $F = ma$ is a special case the student is expected to know.

Key ideas

Newton's three laws of motion
forces as interactions between bodies
that forces on a body can be represented in a free-body diagram, and that free-body diagrams can be analysed to find the resultant force on a system
the nature and use of these contact forces: - normal force $F_\text{N}$ , the component of the contact force perpendicular to the surface - surface frictional force $F_\text{f}$ parallel to the plane of contact, for a stationary body as given by $F_\text{f} \le \mu_\text{s} F_\text{N}$ and for a body in motion as given by $F_\text{f} = \mu_\text{d} F_\text{N}$ , where $\mu_\text{s}$ and $\mu_\text{d}$ are the coefficients of static and dynamic friction - tension - elastic restoring force $F_\text{H}$ following Hooke's law as given by $F_\text{H} = -kx$ , where k is the spring constant - viscous drag force $F_\text{d}$ on a small sphere moving through a fluid as given by $F_\text{d} = 6\pi\eta r v$ , where $\eta$ is the fluid viscosity, r the radius of the sphere and v its velocity - buoyancy $F_\text{b}$ due to displacement of fluid as given by $F_\text{b} = \rho V g$ , where V is the volume of fluid displaced

Not assessed

The use of simultaneous equations involving conservation of momentum and energy in collisions is not required.
Analysis of forces on bodies in non-uniform circular motion in a vertical plane at points other than the top or bottom is not required.
Quantitative treatment of problems involving banked surfaces is not required.

Guiding questions

How can forces acting on a system be represented both visually and algebraically?
How can Newton's laws be modelled mathematically?
How can knowledge of forces and momentum be used to predict the behaviour of interacting bodies?

Linking questions

How do collisions between charge carriers and the atomic cores of a conductor result in thermal energy transfer?
How can knowledge of electrical and magnetic forces allow the prediction of changes to the motion of charged particles?
How does the application of a restoring force acting on a particle result in simple harmonic motion?
How are concepts of equilibrium and conservation applied to understand matter and motion from the smallest atom to the whole universe?
Why is no work done on a body moving along a circular trajectory?
In which way is conservation of momentum relevant to the workings of a nuclear power station?
If experimental measurements contain uncertainties, how can laws be developed based on experimental evidence? (NOS)
What assumptions about the forces between molecules of gas allow for ideal gas behaviour? (NOS)

Practice questions

48 questions · 7 easy · 37 medium · 4 hard

Showing 20 of 20

Question 1

EasyPaper 1A · calculator1 mark

A moving railway carriage collides with an identical stationary carriage on a straight, level track. The two carriages couple together and move off as one unit. Assume frictional forces are negligible.

What are the changes in the total kinetic energy and the total momentum of the two-carriage system as a result of the collision?

	Total kinetic energy	Total momentum
A.	no change	decreases
B.	decreases	decreases
C.	no change	no change
D.	decreases	no change

Question 2

MediumPaper 1A · calculator1 mark

In a simplified classical model of an atom, an electron orbits the nucleus in a circular path. Electron A orbits the nucleus at a radius $R$ . Electron B orbits the same nucleus at a radius $2R$ . Assume the only force acting on the electrons is the electrostatic force from the nucleus, which provides the centripetal force for the orbit. This electrostatic force follows an inverse square law with distance.

What is the ratio $\frac{\text{centripetal acceleration of electron A}}{\text{centripetal acceleration of electron B}}$ ?

A. $\frac{1}{4}$

B. $\frac{1}{2}$

C. 2

D. 4

Question 3

HardPaper 2 · calculator18 marks

(a)

An electric ski lift is powered by a motor at the base station. The motor is connected to a 750 V DC power supply by a cable with a total resistance of 0.15 Ω. When operating at full capacity, the motor draws a constant current of 400 A.

(a) Determine the potential difference across the terminals of the motor.

[2]

(b)

(b) The motor has an efficiency of 92%. Calculate the useful mechanical power output of the motor.

[2]

(c)

The ski lift carries skiers up a slope of length 1800 m that rises by a vertical height of 500 m. There are 50 chairs on the ascending side. Each empty chair has a mass of 25 kg and carries, on average, 1.5 skiers of average mass 75 kg. A constant resistive force of 12 kN opposes the motion.

[3]

(d)

(d) Estimate the maximum speed, $v$ , of the ski lift.

[2]

(e)

(e) The lift operates continuously. Estimate the maximum number of skiers that can be transported to the top station in one hour.

[3]

(f)

(f) In an emergency stop, a brake is applied to a large solid steel disc, bringing the lift to a halt from its maximum speed. The total mass of the moving system (chairs, skiers, and cable) is 18 000 kg. Assume all the kinetic energy of the system is converted into thermal energy in the brake disc.

Calculate the temperature rise of the disc.

Data for this question:
Brake disc radius = 0.75 m
Brake disc thickness = 0.10 m
Density of steel = 7850 kg m⁻³
Specific heat capacity of steel = 450 J kg⁻¹ K⁻¹

[4]

(g)

(g) The speed of a chair is monitored using a radar device at the base station that emits microwaves of frequency 30 GHz. It measures the waves reflected from a chair as it moves away. The frequency shift, $Δf$ , for a source moving directly away from a stationary observer can be approximated by the formula $Δf/f ≈ v/c$ . In this radar measurement, this formula gives a good approximation for the shift detected.

Calculate the expected frequency shift.

[2]

Question 4

EasyPaper 1A · calculator1 mark

A block is suspended at rest by a string attached to a ceiling. The string exerts an upward force of tension $T$ on the block.

Which of the following describes the reaction force to $T$ according to Newton's third law?

A. The downward gravitational force of the Earth on the block.

B. The upward gravitational force of the block on the Earth.

C. The downward force of the block on the string.

D. The downward force of the string on the ceiling.

Question 5

MediumPaper 2 · calculator6 marks

(a)

(a) A delivery driver places a package on the flat trunk of their car. The car then accelerates forward.

State the nature and direction of the force that causes the package to accelerate with the car.

[1]

(b)

(b) The mass of the package is 25 kg, and the mass of the car is 1500 kg. The coefficient of static friction between the package and the car's trunk is 0.65.

Determine the maximum acceleration the car can have without the package sliding relative to the trunk.

[3]

(c)

(c) Calculate the maximum engine force that can be applied to the car for the package to remain stationary relative to the trunk. Assume no other horizontal forces act on the car.

[2]

Question 6

HardPaper 2 · calculator12 marks

(a)

In a specialized medical imaging technique, high-energy X-rays are directed at a target to probe its atomic structure. During this process, a photon with an initial wavelength of $4.00 \times 10^{-12}$ m collides with a stationary electron in the target material. After the collision, the photon's wavelength is observed to have increased by exactly $1.21 \times 10^{-12}$ m.

(a) Calculate the wavelength of the photon after the collision.

[2]

(b)

(b) Deduce the angle through which the photon has been deflected in this collision.

[3]

(c)

(c) Explain whether the angle between the original direction of the photon and the final direction of the electron is greater, smaller or equal to your answer in (b).

[3]

(d)

(d) Determine the kinetic energy of the electron after the collision. Express your answer in keV.

[4]

Question 7

EasyPaper 1A · calculator1 mark

A soccer ball of mass $0.45 \text{ kg}$ is initially at rest on the ground. A player kicks the ball, imparting an impulse of $7.5 \text{ N s}$ to it.

What is the final kinetic energy of the soccer ball immediately after the kick?

A. $7.5 \text{ J}$

B. $8.3 \text{ J}$

C. $62.5 \text{ J}$

D. $125 \text{ J}$

Question 8

MediumPaper 1A · calculator1 mark

An electric car moves at a constant speed of 72 km h $^{-1}$ on a level road. The car's electric motor has an output power of 20 kW. What is the total resistive force acting on the car?

A. 280 N

B. 400 N

C. 1000 N

D. 400000 N

Question 9

HardPaper 2 · calculator14 marks

(a)(i)

In a controlled environment, a technician is studying the behavior of microscopic charged dust particles. One such particle, with two excess electrons, is observed to be held perfectly stationary between two horizontal parallel metal plates. The uniform electric field between the plates is $2.5 \times 10^5\,\text{V}\,\text{m}^{-1}$ . The density of the dust particle is $\rho = 1200\,\text{kg}\,\text{m}^{-3}$ .

(a) (i) Calculate the radius of the dust particle.

[3]

(a)(ii)

(a) (ii) State one significant assumption made in your calculation in (a)(i).

[1]

(b)(i)

(b) (i) The electric field is suddenly switched off, and the same dust particle begins to fall, reaching a constant terminal speed of $1.5 \times 10^{-4}\,\text{m}\,\text{s}^{-1}$ . Explain why the particle reaches a constant terminal speed.

[2]

(b)(ii)

(b) (ii) Using the data, estimate the dynamic viscosity of the air in the controlled environment.

[4]

(c)

(c) If the particle were to acquire only one excess electron and the original electric field was re-established, determine the new terminal velocity of the particle, stating its direction.

[4]

Question 10

EasyPaper 1A · calculator1 mark

A ball is projected vertically upwards from the ground. Air resistance is negligible. What is correct about the motion of the ball as it is rising towards its maximum height?

A. The displacement for each successive second is constant.

B. The net force on the ball decreases as it rises.

C. The change in velocity for each successive second is constant.

D. The acceleration of the ball is zero at the highest point.

Question 11

MediumPaper 1A · calculator1 mark

A roller coaster cart is designed to travel around a banked curve of radius $r$ . The track is banked at an angle $θ$ to the horizontal. The maximum safe speed for the cart depends on this angle and the coefficient of friction $μ$ between the wheels and the track.

Which combination of changes to the banking angle $θ$ and the coefficient of friction $μ$ is most effective for allowing the cart to travel at a greater maximum speed?

A. $θ$ is increased and $μ$ is increased.

B. $θ$ is increased and $μ$ is decreased.

C. $θ$ is decreased and $μ$ is increased.

D. $θ$ is decreased and $μ$ is decreased.

Question 12

HardPaper 2 · calculator4 marks

(a) A glass bead of density $2.5 \times 10^3 \text{ kg m}^{-3}$ falls through a column of oil. The oil has a density of $9.0 \times 10^2 \text{ kg m}^{-3}$ and a viscosity of $0.085 \text{ Pa s}$ . The bead reaches a terminal velocity of $9.2 \text{ cm s}^{-1}$ .

Determine the radius of the bead.

Question 13

EasyPaper 1A · calculator1 mark

A wooden block of mass $m$ rises vertically through water at its terminal speed. The buoyancy force acting on the block has magnitude $F_{\text{b}}$ . What is the magnitude of the viscous drag force acting on the block?

A. $F_{\text{b}} + mg$

B. $F_{\text{b}}$

C. $mg - F_{\text{b}}$

D. $F_{\text{b}} - mg$

Question 14

MediumPaper 2 · calculator4 marks

A small spherical dust particle of density $2.6 \times 10^3 \text{ kg m}^{-3}$ falls through still air. The viscosity of air is $1.8 \times 10^{-5} \text{ Pa s}$ . The particle reaches a terminal velocity of $5.0 \text{ cm s}^{-1}$ .

(a) Determine the radius of the dust particle.

Question 15

EasyPaper 1A · calculator1 mark

An astronaut of mass $75\text{ kg}$ is initially stationary in space. The astronaut throws a tool of mass $1.5\text{ kg}$ with a speed of $8.0\text{ m s}^{-1}$ . The net external force on the system is zero.

What is the magnitude of the impulse on the astronaut and the total momentum of the system just after the tool is thrown?

	Magnitude of impulse on the astronaut / $\text{N s}$	Total momentum of the system / $\text{kg m s}^{-1}$
A.	12	12
B.	12	0
C.	600	12
D.	600	0

Question 16

MediumPaper 2 · calculator4 marks

A cylindrical buoy of mass $m$ and cross-sectional area $A$ floats vertically in calm water of density $\rho$ . The buoy is pushed down a small distance $x$ from its equilibrium position and released.

(a) By considering the forces acting on the buoy, explain why its subsequent oscillation is approximately simple harmonic.

Question 17

EasyPaper 1A · calculator1 mark

A student measures the mass of a metal block as $(250 \pm 5)$ g and its volume as $(50 \pm 2)$ cm $^3$ . What is the fractional uncertainty in the calculated density of the block?

A. 0.02

B. 0.04

C. 0.06

D. 0.08

Question 18

MediumPaper 2 · calculator7 marks

(a)

A simple pendulum is used to demonstrate simple harmonic motion inside the cabin of a vertical drop tower ride.

(a) The cabin is moving upwards at a constant speed. State and explain the effect on the period of oscillation of the pendulum.

[2]

(b)

(b) The cabin is then accelerated upwards at a constant rate of $a = 4.9 \text{ m s}^{-2}$ . State and explain the effect on the period of oscillation.

[3]

(c)

(c) At the top of the ride, the cabin is released and undergoes free-fall. Describe the motion of the pendulum bob, as observed from within the cabin, immediately after release.

[2]

Question 19

MediumPaper 2 · calculator10 marks

(a)

A small laboratory cart of mass $250\text{ g}$ is attached to a horizontal spring with a spring constant of $5.0\text{ N m}^{-1}$ . The cart is placed on a frictionless track and displaced by $0.15\text{ m}$ from its equilibrium position, then released. Its subsequent motion is simple harmonic.

(a) Calculate the maximum value of stored elastic potential energy.

[2]

(b)

(b) Calculate the maximum speed of the cart.

[2]

(c)

[2]

(d)

(d) Calculate the frequency of vibration.

[2]

(e)

(e) Calculate the displacement when the stored elastic potential energy equals the kinetic energy.

[2]

Question 20

MediumPaper 1A · calculator1 mark

A soccer ball of mass $0.45 \text{ kg}$ is initially at rest on the ground. A player kicks the ball, imparting an impulse of $3.6 \text{ Ns}$ to it.

What is the final kinetic energy of the soccer ball immediately after the kick?

A. $3.6 \text{ J}$

B. $8.0 \text{ J}$

C. $14.4 \text{ J}$

D. $28.8 \text{ J}$

28 more Forces and Momentum questions in the app

Every answer is marked mark by mark, IB-style, and the AI tutor helps when you are stuck.

Where marks are lost

Believing that a constant net force is required to maintain a constant velocity.
Confusing the normal force with the Newton's third law pair to the gravitational force.
Treating centripetal force as a new, separate force rather than the resultant of existing physical forces.
Forgetting that momentum is a vector quantity and ignoring signs in one-dimensional collisions.
Assuming kinetic energy is conserved in all collisions rather than just elastic ones.

Forces and Momentum: notes and practice questions

How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

28 more Forces and Momentum questions in the app

Where marks are lost

Practise this topic as a session

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How it is examined

Guiding questions

Linking questions

Practice questions

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Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

28 more Forces and Momentum questions in the app

Where marks are lost

Questions,answered.

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Questions,
answered.

Start with the IB question
bank built for you.