DP Physics (first assessment 2025)

Question EXE.2.HL.TZ0.9

Date	Example questions Example questions	Marks available	[Maximum mark: 7]	Reference code	EXE.2.HL.TZ0.9
Level	HL	Paper	2	Time zone	TZ0
Command term	Calculate, Explain, State	Question number	9	Adapted from	N/A

[Maximum mark: 7]

EXE.2.HL.TZ0.9

The propellor of an model plane is driven by an electric motor that is mounted inside the plane. This propellor is modelled as a rod rotating about an axis through the centre of the rod at right angles to the length of the rod. The moment of inertia for such a rod is $\frac{1}{12} M L^{2}$ where $M$ is the mass of the rod and $L$ is the total length of the rod.

(a)

For the propellor, $L = 5.0 cm$ and $M = 0.035 kg$ .

Calculate the moment of inertia of the propellor.

[1]

Markscheme

$7.3 \times 10^{- 6} {kg m}^{2}$ ✓

(b)

The propellor is at rest when the electric motor is switched on. The net average torque acting on the propellor due to the motor and resistive forces is $3.5 \times 10^{- 2} kg m^{2} s^{- 2}$ . The final speed of the propellor is 190 revolutions per second.

(b.i)

Calculate the angular impulse that acts on the propellor.

[2]

Markscheme

190 rev s⁻¹ = 1190 rad s⁻¹✓

$Δ L = Δ (I ω) = 7.3 \times 10^{- 6} \times 1190 = 8.7 \times 10^{- 3} {kg m}^{2} s^{- 1}$ ✓

(b.ii)

Calculate, using your answer to (b)(i), the time taken by the propellor to attain this rotational speed.

[2]

Markscheme

$Δ t = \frac{Δ L}{τ}$ used ✓

0.25 s ✓

(b.iii)

State and explain the effect of the angular impulse on the body of the aeroplane.

[2]

Markscheme

As the motor is internal, angular momentum is conserved (ignoring the torques due to resistive forces) ✓

The body of the plane will (try to) rotate in the opposite direction to the propellor ✓

Syllabus sections

A. Space, time and motion

A. Space, time and motion » A.4 Rigid body mechanics

Question EXE.2.HL.TZ0.9

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