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Back to A: Space, time and motion

A.4 Rigid body mechanics

Practice exam-style IB Physics questions for Rigid body mechanics, aligned with the syllabus and grouped by topic.

Question 1
HL • Paper 1A
Easy
Non Calculator

A force of 18 N is applied to a spanner at a distance of 0.25 m from the nut. The angle between the spanner and the force is 60°.

A.

7.8 N m

B.

3.9 N m

C.

4.5 N m

D.

2.3 N m

Question 2
HL • Paper 1A
Easy
Non Calculator

A uniform beam is in rotational equilibrium about a pivot.

A.

The beam has zero angular momentum.

B.

The resultant force on the beam is zero only.

C.

The resultant torque about the pivot is zero.

D.

All forces act through the centre of mass.

Question 3
HL • Paper 1A
Easy
Non Calculator

A constant resultant torque acts on a rigid body whose moment of inertia remains constant.

A.

Constant angular acceleration

B.

Constant angular displacement

C.

Constant angular velocity

D.

Constant angular momentum

Question 4
HL • Paper 1A
Easy
Non Calculator

A wheel rotates at 240 revolutions per minute.

A.

120π rad s⁻¹

B.

8.0π rad s⁻¹

C.

4.0 rad s⁻¹

D.

480π rad s⁻¹

Question 5
HL • Paper 1A
Easy
Non Calculator

Two point masses of 0.20 kg are fixed to a light rod, each 0.30 m from an axis through the rod’s centre and perpendicular to the rod.

A.

0.12 kg m²

B.

0.036 kg m²

C.

0.24 kg m²

D.

0.018 kg m²

Question 6
HL • Paper 1A
Easy
Non Calculator

A resultant torque of 6.0 N m acts on a flywheel of moment of inertia 1.5 kg m².

A.

9.0 rad s⁻²

B.

4.0 rad s⁻²

C.

7.5 rad s⁻²

D.

0.25 rad s⁻²

Question 7
HL • Paper 1A
Easy
Non Calculator

A disc of moment of inertia 0.40 kg m² rotates clockwise at 12 rad s⁻¹.

A.

−4.8 kg m² s⁻¹

B.

30 kg m² s⁻¹

C.

0.033 kg m² s⁻¹

D.

4.8 kg m² s⁻¹

Question 8
HL • Paper 1A
Easy
Non Calculator

A flywheel has moment of inertia 2.0 kg m² and angular speed 5.0 rad s⁻¹.

A.

25 J

B.

50 J

C.

10 J

D.

100 J

Question 9
HL • Paper 2
Easy
Calculator

A horizontal force of 35 N is applied to the end of a door at a distance of 0.82 m from its hinges. The force makes an angle of 70° to the surface of the door.

Top-view diagram of a rectangular door hinged at one side, showing the point of application of a force at the far edge, the distance from hinge to force, and the angle between the door surface and the force.
1.

Calculate the torque about the hinges.

[1]
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2.

State how the torque would change if the same force were applied at the hinge.

[1]
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Question 10
HL • Paper 2
Easy
Calculator

A pair of equal and opposite forces act on a rigid plate along parallel lines of action that do not coincide.

Plan view of a rigid plate with two equal and opposite parallel forces acting at separated lines of action, with the perpendicular separation indicated.
1.

State the name given to this pair of forces.

[1]
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2.

Explain why the plate can rotate even though the resultant force is zero.

[1]
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Question 11
HL • Paper 1A
Medium
Non Calculator

A turntable starts from rest and has a uniform angular acceleration of 3.0 rad s⁻² for 4.0 s.

A.

12 rad

B.

48 rad

C.

18 rad

D.

24 rad

Question 12
HL • Paper 1A
Medium
Non Calculator

Two wheels have the same mass and radius. Wheel X has most of its mass near the axle; wheel Y has most of its mass near the rim.

A.

X has a larger moment of inertia than Y.

B.

They have the same moment of inertia because mass and radius are the same.

C.

Y has a larger moment of inertia than X.

D.

Their moments of inertia are zero if they rotate steadily.

Question 13
HL • Paper 1A
Medium
Non Calculator

A skater rotates with arms extended. The skater then pulls the arms in so that the moment of inertia decreases by a factor of 2. No external torque acts.

A.

It decreases by a factor of 2.

B.

It increases by a factor of 2.

C.

It increases by a factor of 4.

D.

It remains unchanged.

Question 14
HL • Paper 1A
Medium
Non Calculator

A constant torque of 0.80 N m acts on a rotor for 5.0 s.

A.

0.16 N m s

B.

4.0 N m s

C.

0.80 N m s

D.

6.25 N m s

Question 15
HL • Paper 1A
Medium
Non Calculator

Two equal and opposite horizontal forces of magnitude 12 N act on a rigid body. Their parallel lines of action are separated by 0.40 m.

A.

0 N m

B.

2.4 N m

C.

4.8 N m

D.

9.6 N m

Question 16
HL • Paper 2
Medium
Calculator

A uniform metre rule of weight 1.2 N is pivoted at the 40 cm mark. A 2.0 N weight is hung at the 10 cm mark.

Diagram of a horizontal metre rule with pivot at 40 cm, a downward 2.0 N force at 10 cm, the rule’s weight acting at 50 cm, and an unknown downward force at 90 cm.
1.

State where the weight of the metre rule acts.

[1]
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2.

Determine the force that must be applied vertically downward at the 90 cm mark to keep the rule in rotational equilibrium.

[1]
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Question 17
HL • Paper 2
Medium
Calculator

A centrifuge rotor increases its angular speed uniformly from 120 rad s⁻¹ to 300 rad s⁻¹ in 6.0 s.

1.

Calculate its angular acceleration.

[1]
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2.

Calculate the tangential acceleration of a point 0.080 m from the axis.

[1]
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Question 18
HL • Paper 2
Medium
Calculator

Four small masses are fixed at the corners of a light square frame of side 0.40 m. Each mass is 0.15 kg. The axis of rotation passes through the centre of the square and is perpendicular to its plane.

Square frame with equal point masses at all four corners and an axis through the centre perpendicular to the square; side length labelled.
1.

Determine the distance of each mass from the axis.

[1]
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2.

Calculate the moment of inertia of the system.

[1]
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Question 19
HL • Paper 2
Medium
Calculator

A pulley has moment of inertia 0.12 kg m². A constant frictional torque of 0.18 N m opposes its rotation while a driving torque of 0.66 N m acts in the opposite sense.

1.

Calculate the resultant torque.

[1]
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2.

Calculate the angular acceleration of the pulley.

[1]
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Question 20
HL • Paper 2
Medium
Calculator

A rotor has moment of inertia 0.75 kg m² and rotates anticlockwise at 20 rad s⁻¹. Clockwise is taken as positive.

1.

State the sign of the angular velocity.

[1]
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2.

Calculate the angular momentum of the rotor.

[1]
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Question 21
HL • Paper 2
Medium
Calculator

A disc rotates with constant angular speed 8.0 rad s⁻¹. A point P is 0.25 m from the axis.

1.

Calculate the linear speed of P.

[1]
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2.

Calculate the centripetal acceleration of P.

[1]
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3.

State the tangential acceleration of P.

[1]
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Question 22
HL • Paper 1B
Medium
Non Calculator

A student records the angular speed of a turntable as it starts from rest.

Angular speed of a turntable as it starts from rest and approaches a steady value.
1.

Use the graph to determine the angular acceleration during the first part of the motion.

[1]
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2.

Estimate the angular displacement during the whole time shown.

[1]
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3.

Suggest one reason why the angular acceleration may not remain constant at later times.

[1]
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Question 23
HL • Paper 1B
Medium
Non Calculator

A torque sensor records the resultant torque on a wheel during a short collision with a brake pad.

Torque recorded as a function of time during the wheel–pad contact.
1.

State what physical quantity is represented by the signed area under the graph.

[1]
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2.

Estimate the change in angular momentum of the wheel.

[1]
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3.

State whether the angular speed increases or decreases, using the sign of the area.

[1]
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Question 24
HL • Paper 1A
Medium
Non Calculator

A solid cylinder with (I=\frac{1}{2}MR^2) rolls without slipping with centre-of-mass speed (v).

A.

\(\frac{1}{4}\)

B.

\(\frac{1}{3}\)

C.

\(\frac{1}{2}\)

D.

\(\frac{2}{3}\)

Question 25
HL • Paper 1A
Medium
Non Calculator

Two coaxial discs are brought into contact and rotate together after slipping. Disc 1 has (I) and angular speed (+3\omega). Disc 2 has (2I) and angular speed (-\omega).

A.

\(+\omega\)

B.

\(-\frac{1}{3}\omega\)

C.

\(+\frac{5}{3}\omega\)

D.

\(+\frac{1}{3}\omega\)

Question 26
HL • Paper 1A
Medium
Non Calculator

A point on the rim of a wheel has tangential speed 6.0 m s⁻¹ and centripetal acceleration 18 m s⁻².

A.

0.33 m

B.

3.0 m

C.

2.0 m

D.

108 m

Question 27
HL • Paper 2
Medium
Calculator

A wheel rotating at 15 rad s⁻¹ is brought uniformly to rest in 5.0 revolutions.

1.

Convert the angular displacement to radians.

[1]
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2.

Determine the angular acceleration.

[1]
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3.

Determine the time taken to stop.

[1]
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Question 28
HL • Paper 2
Medium
Calculator

A rotating platform of moment of inertia 5.0 kg m² rotates freely at 1.8 rad s⁻¹. A child standing on it moves inward, reducing the total moment of inertia of the child-platform system from 8.0 kg m² to 6.0 kg m². External torque is negligible.

1.

State the conserved quantity.

[1]
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2.

Determine the final angular speed.

[1]
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3.

State whether the rotational kinetic energy increases, decreases or remains the same.

[1]
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Question 29
HL • Paper 2
Medium
Calculator

A variable braking torque acts on a wheel for 3.0 s. The average torque is 2.5 N m opposite to the positive direction. The wheel has moment of inertia 0.50 kg m² and initial angular speed +18 rad s⁻¹.

1.

Calculate the angular impulse.

[1]
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2.

Determine the final angular momentum.

[1]
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3.

Determine the final angular speed.

[1]
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Question 30
HL • Paper 2
Medium
Calculator

A hoop of mass (M) and radius (R) rolls without slipping at speed (v). The moment of inertia of a hoop is (I=MR^2).

Side-view diagram of a hoop rolling without slipping on a horizontal surface, with centre-of-mass speed v and angular speed omega labelled.
1.

State the rolling condition linking (v) and (\omega).

[1]
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2.

Show that its total kinetic energy is (Mv^2).

[1]
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Question 31
HL • Paper 2
Medium
Calculator

A horizontal force is applied to a block whose centre of mass is above the line of action of the force. The block rests on a rough horizontal surface and may start to tip.

Side-view diagram of a rectangular block on a rough horizontal surface, with centre of mass marked and a horizontal force applied below the centre of mass so its line of action does not pass through the centre.
1.

State why the force may produce both translational and rotational motion.

[1]
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2.

State the condition for no angular acceleration about the centre of mass.

[1]
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Question 32
HL • Paper 2
Medium
Calculator

A wheel is accelerated from rest by a constant torque of 3.0 N m through an angular displacement of 20 rad. Resistive torques are negligible.

1.

Calculate the work done by the torque.

[1]
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2.

State the increase in rotational kinetic energy.

[1]
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3.

If the wheel’s moment of inertia is 1.2 kg m², determine its final angular speed.

[1]
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Question 33
HL • Paper 1B
Medium
Non Calculator

A rigid rotor is acted on by different known torques. Its angular acceleration is measured for each torque.

Angular acceleration measured for different applied torques on a rigid rotor.
1.

Describe the relationship shown by the graph.

[1]
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2.

Use the graph to determine the moment of inertia of the rotor.

[1]
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3.

The line does not pass exactly through the origin. Suggest what this indicates about the experiment.

[1]
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Question 34
HL • Paper 1B
Medium
Non Calculator

A class investigates rolling without slipping by releasing objects from the same height on a ramp. The table gives repeated descent times for a hoop, a solid cylinder and a solid sphere of the same mass and radius.

ObjectTrial 1 t / sTrial 2 t / sTrial 3 t / sMean t / sTiming uncertainty / s
Hoop1.941.911.931.93±0.03
Solid cylinder1.671.641.681.66±0.03
Solid sphere1.611.641.621.62±0.03
1.

Identify which object has the smallest mean descent time.

[1]
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2.

Explain the order of the mean descent times using moment of inertia.

[1]
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3.

Evaluate whether the data are sufficient to distinguish between the cylinder and the sphere.

[1]
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Question 35
HL • Paper 1B
Medium
Non Calculator

A student attaches identical small masses at different distances from the centre of a light rotating frame. The table shows the measured angular acceleration for the same applied torque.

Distance / mα trial 1 / rad s^-2α trial 2 / rad s^-2α trial 3 / rad s^-2
0.05012.412.112.3
0.1008.68.48.5
0.1505.75.95.8
0.2004.04.24.1
0.2503.02.93.1
1.

Describe how angular acceleration changes as the masses are moved farther from the axis.

[1]
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2.

Explain this trend using moment of inertia.

[1]
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3.

Suggest one improvement to reduce random uncertainty in angular acceleration.

[1]
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Question 36
HL • Paper 1B
Medium
Non Calculator

A rolling ball is filmed as it moves along a horizontal track without slipping. The graph shows the position of its centre of mass against time.

Position of the rolling ball's centre of mass as a function of time; the ball radius is included in the title.
1.

Determine the speed of the centre of mass from the graph.

[1]
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2.

Determine the angular speed of the ball using its radius.

[1]
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3.

Explain how the graph supports the assumption that the ball rolls at constant speed.

[1]
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Question 37
HL • Paper 2
Medium
Calculator

A solid sphere of mass 0.60 kg and radius 0.050 m rolls without slipping. Its centre of mass has speed 2.0 m s⁻¹. For a solid sphere, (I=\frac{2}{5}MR^2).

1.

Calculate the angular speed.

[1]
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2.

Calculate the rotational kinetic energy.

[1]
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3.

Calculate the total kinetic energy.

[1]
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Question 38
HL • Paper 2
Medium
Calculator

Two satellites orbit the same planet in elliptical orbits. At one point a satellite is closer to the planet than at another point. Assume the torque about the planet’s centre is negligible.

Elliptical orbit around a planet with two labelled positions, one close to the planet and one farther away, and radius lines from the planet to the satellite at both positions.
1.

State the conservation law that applies about the planet’s centre.

[1]
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2.

Explain why the satellite’s angular speed is greater when it is closer to the planet.

[1]
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Question 39
HL • Paper 1B
Hard
Non Calculator

Two coaxial flywheels are coupled by a clutch. The table gives their moments of inertia and angular speeds immediately before coupling. After coupling, they rotate together.

FlywheelI / kg m²ωᵢ / rad s⁻¹
A0.64+18.0
B0.36−10.0
1.

Determine the initial total angular momentum, taking clockwise as positive.

[1]
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2.

Determine the common final angular speed.

[1]
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3.

State why rotational kinetic energy is not necessarily conserved during coupling.

[1]
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Question 40
HL • Paper 1B
Hard
Non Calculator

A non-uniform horizontal beam is supported by two vertical supports. The table gives measured upward support forces for different positions of a hanging load.

x_load / mLoad / NR_A / NR_B / N
0.25020.024.08.1
0.40020.020.311.7
0.55020.016.415.6
0.70020.012.919.2
0.85020.08.923.1
1.

For one load position, check whether vertical forces are consistent with translational equilibrium.

[1]
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2.

Use moments about one support to determine the position of the beam’s centre of mass.

[1]
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3.

Evaluate one limitation of using these data to locate the centre of mass.

[1]
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Question 41
HL • Paper 1B
Hard
Non Calculator

A turntable is programmed to rotate through a fixed angle. The graph shows angular acceleration against time.

Angular acceleration of a turntable as it starts from rest, rotates, and stops.
1.

Identify the time interval during which the angular speed is constant.

[1]
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2.

Determine the change in angular speed during the acceleration phase.

[1]
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3.

Determine the total angular displacement during the motion.

[1]
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Question 42
HL • Paper 2
Hard
Calculator

A solid cylinder rolls without slipping from rest down a ramp of vertical height (h). Its moment of inertia about its central axis is (I=\frac{1}{2}MR^2).

Inclined ramp with a solid cylinder rolling down from rest through a vertical height h; radius R and centre-of-mass speed v at the bottom labelled.
1.

Show that the speed of the centre of mass at the bottom is (v=\sqrt{\frac{4gh}{3}}).

[1]
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2.

Explain why a hoop of the same mass and radius released from the same height reaches the bottom with a smaller centre-of-mass speed.

[1]
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Question 43
HL • Paper 2
Hard
Calculator

A uniform solid disc and a thin hoop have the same mass (M) and radius (R). They are each mounted on frictionless axles through their centres. The moment of inertia of the disc is (\frac{1}{2}MR^2), and that of the hoop is (MR^2). The same constant torque ( au) is applied to each from rest for the same time (t).

1.

Compare their angular accelerations.

[1]
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2.

Compare and contrast their final angular momenta and rotational kinetic energies.

[1]
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Question 44
HL • Paper 1B
Hard
Non Calculator

A flywheel is spun up by a motor. The graph shows the rotational kinetic energy stored in the flywheel against the square of its angular speed.

Rotational kinetic energy stored in a flywheel plotted against the square of angular speed.
1.

State the expected form of the relationship between (E_k) and (\omega^2).

[1]
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2.

Use the graph to determine the moment of inertia of the flywheel.

[1]
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3.

Suggest why the useful energy delivered by the motor may be greater than the increase in rotational kinetic energy.

[1]
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Question 45
HL • Paper 2
Hard
Calculator

Two coaxial flywheels are initially separated. Flywheel A has moment of inertia (0.30, ext{kg m}^2) and angular speed (+24, ext{rad s}^{-1}). Flywheel B has moment of inertia (0.20, ext{kg m}^2) and angular speed (-12, ext{rad s}^{-1}). They are then coupled and rotate together.

1.

Determine the common angular speed after coupling.

[1]
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2.

Evaluate the change in rotational kinetic energy and explain its physical origin.

[1]
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Question 46
HL • Paper 2
Hard
Calculator

A light string is wound around a uniform solid cylinder of radius (R), mass (M), and moment of inertia (I=\frac{1}{2}MR^2). The cylinder is mounted on a frictionless axle. The string is pulled with a constant force (F), causing the cylinder to rotate from rest without the string slipping.

Cylinder on a fixed axle with a string wound around its rim. A tangential force F pulls the free end; radius R and displacement s of the string end are labelled.
1.

Determine the angular acceleration of the cylinder.

[1]
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2.

Discuss how the angular speed and rotational kinetic energy after the free end of the string has moved a distance (s) can be found, and derive expressions for both.

[1]
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Question 47
HL • Paper 2
Hard
Calculator

A uniform horizontal beam of length 4.0 m and weight 120 N is hinged to a wall at one end. A cable attached to the other end makes an angle of 35° above the beam. A 200 N sign hangs from the beam 3.0 m from the hinge.

Horizontal beam hinged at a wall, cable attached at free end making 35° above the beam, uniform beam weight acting at its centre, and a hanging sign 3.0 m from the hinge.
1.

Determine the tension in the cable required for rotational equilibrium.

[1]
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2.

Explain how the hinge force can be found and why taking moments about the hinge is useful.

[1]
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Question 48
HL • Paper 2
Hard
Calculator

A wheel starts from rest and accelerates uniformly for 6.0 s until it reaches 18 rad s⁻¹. It then rotates at this constant angular speed. A point on the rim is 0.40 m from the axis.

1.

Determine the angular displacement during the acceleration phase.

[1]
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2.

Discuss the motion of the rim point during and after the acceleration phase, including tangential and centripetal accelerations.

[1]
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Question 49
HL • Paper 2
Hard
Calculator

A rotor has initial moment of inertia (I_i=0.80, ext{kg m}^2) and initial angular speed (+10, ext{rad s}^{-1}). While a motor applies a constant external torque of (+3.0, ext{N m}) for 4.0 s, movable masses in the rotor shift outward so that the final moment of inertia is (1.20, ext{kg m}^2).

Rotor with movable masses shown initially nearer the axis and finally farther from the axis while a motor applies a torque in the positive rotational sense.
1.

Determine the angular impulse delivered by the motor and the final angular momentum.

[1]
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2.

Evaluate the final angular speed and the change in rotational kinetic energy.

[1]
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Question 50
HL • Paper 2
Hard
Calculator

A ball is rolled up a ramp without slipping. It has mass (M), radius (R), initial centre-of-mass speed (v_0), and moment of inertia (I=kMR^2), where (k) is a constant for the shape.

Ball rolling without slipping up an inclined ramp, with initial centre-of-mass speed v0 at the bottom and maximum vertical height h indicated.
1.

Derive an expression for the maximum vertical height reached by the centre of mass.

[1]
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2.

Evaluate how the height depends on mass distribution and compare a hoop ((k=1)) with a solid sphere ((k=\frac{2}{5})) for the same initial speed.

[1]
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A.3 Work, energy and power