A Level H2 Physics Energy Power Quiz

A-Level Physics H2 Quiz - Energy Power

Answer Key

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Question 1 [2 marks]

Answer:
The work-energy theorem states that the net work done on an object is equal to the change in its kinetic energy.

$W_{\text{net}} = \Delta KE = \frac{1}{2}mv^2 - \frac{1}{2}mu^2$

Marking:

• 1 mark for stating that net work equals change in kinetic energy.
• 1 mark for the correct mathematical expression.

Teaching note: This is a fundamental theorem linking the concept of work (a process quantity) to kinetic energy (a state quantity). It tells us that if positive net work is done on an object, it speeds up; if negative net work is done, it slows down.

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Question 2 [4 marks]

(a) [2 marks]

Answer:
Net force: $F_{\text{net}} = 12 - 4.0 = 8.0$ N

$W_{\text{net}} = F_{\text{net}} \times d = 8.0 \times 5.0 = 40 \text{ J}$

Marking:

• 1 mark for calculating net force correctly.
• 1 mark for correct work done with unit.

(b) [2 marks]

Answer:
Using the work-energy theorem:

$W_{\text{net}} = \frac{1}{2}mv^2 - \frac{1}{2}mu^2$

Since $u = 0$:

$40 = \frac{1}{2}(2.0)v^2$

$v^2 = 40$

$v = \sqrt{40} = 6.3 \text{ m s}^{-1}$

Marking:

• 1 mark for correct substitution into work-energy theorem.
• 1 mark for correct final answer with unit.

Common mistake: Students may forget that the block starts from rest ($u = 0$) and try to include an initial kinetic energy term.

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Question 3 [2 marks]

Answer:
Power is defined as the rate of doing work (or the rate of energy transfer).

$P = \frac{W}{t} = \frac{\Delta E}{t}$

The SI unit of power is the watt (W), where $1 \text{ W} = 1 \text{ J s}^{-1}$.

Marking:

• 1 mark for correct definition (rate of doing work / rate of energy transfer).
• 1 mark for correct SI unit (watt, W).

Teaching note: Power measures how quickly energy is transferred or work is done. A more powerful machine does the same amount of work in less time.

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Question 4 [2 marks]

Answer:
At constant speed, the tension in the cable equals the weight of the load:

$T = mg = 50 \times 9.81 = 490.5 \text{ N}$

Output power:

$P = F \times v = 490.5 \times 0.80 = 392 \text{ W}$

Marking:

• 1 mark for calculating the tension/weight correctly.
• 1 mark for correct power with unit.

Alternative method: $P = \frac{mgh}{t} = mgv$ since $h/t = v$.

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Question 5 [3 marks]

Answer:
A conservative force is one where the work done in moving an object between two points is independent of the path taken. The total work done by a conservative force around any closed path is zero. Example: gravitational force (or electrostatic force, spring force).

A non-conservative force is one where the work done depends on the path taken. Mechanical energy is dissipated (usually as thermal energy). Example: friction (or air resistance, drag).

Marking:

• 1 mark for correct definition of conservative force.
• 1 mark for correct definition of non-conservative force.
• 1 mark for one valid example of each.

Teaching note: The key distinction is path-independence. For gravity, lifting a box from the floor to a table requires the same work regardless of the path. For friction, a longer path means more work is dissipated as heat.

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Question 6 [4 marks]

(a) [1 mark]

Answer:

$a = \frac{v - u}{t} = \frac{25 - 0}{10} = 2.5 \text{ m s}^{-2}$

(b) [1 mark]

Answer:

$F = ma = 1200 \times 2.5 = 3000 \text{ N}$

(c) [2 marks]

Answer:
Average velocity:

$\bar{v} = \frac{u + v}{2} = \frac{0 + 25}{2} = 12.5 \text{ m s}^{-1}$

Average power:

$P = F \times \bar{v} = 3000 \times 12.5 = 3.75 \times 10^4 \text{ W} = 37.5 \text{ kW}$

Marking:

• 1 mark for using average velocity (or equivalent method using distance).
• 1 mark for correct final answer with unit.

Alternative method: Distance $s = \frac{1}{2}(u+v)t = \frac{1}{2}(25)(10) = 125$ m. Work $= Fs = 3000 \times 125 = 375000$ J. Power $= W/t = 375000/10 = 37500$ W.

Common mistake: Students may use final velocity instead of average velocity in $P = Fv$, which gives the instantaneous power at $t = 10$ s, not the average power.

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Question 7 [2 marks]

Answer:
The principle of conservation of energy states that energy cannot be created or destroyed; it can only be transferred from one form to another. The total energy of an isolated/closed system remains constant.

Marking:

• 1 mark for stating energy cannot be created or destroyed.
• 1 mark for stating total energy of an isolated system is constant (or equivalent).

Teaching note: This is one of the most fundamental laws in all of physics. In mechanics problems, we often use the special case of conservation of mechanical energy (KE + PE), which holds only when no non-conservative forces do work.

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Question 8 [3 marks]

(a) [1 mark]

Answer:

$PE = mgh = 0.50 \times 9.81 \times 10.0 = 49.05 \text{ J} \approx 49 \text{ J}$

(b) [2 marks]

Answer:
By conservation of mechanical energy (taking ground as reference):

$PE_{\text{top}} = KE_{\text{bottom}}$

$mgh = \frac{1}{2}mv^2$

$v = \sqrt{2gh} = \sqrt{2 \times 9.81 \times 10.0} = \sqrt{196.2} = 14.0 \text{ m s}^{-1}$

Marking:

• 1 mark for correct energy conservation equation.
• 1 mark for correct final answer with unit.

Note: Mass cancels out — the final speed is independent of mass when air resistance is negligible.

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Question 9 [5 marks]

(a) [2 marks]

Answer:
Using conservation of energy from A to B (track is frictionless):

$mgh = \frac{1}{2}mv_B^2$

$v_B = \sqrt{2gh} = \sqrt{2 \times 9.81 \times 2.5} = \sqrt{49.05} = 7.0 \text{ m s}^{-1}$

Marking:

• 1 mark for correct energy conservation equation.
• 1 mark for correct answer with unit.

(b) [3 marks]

Answer:
On the rough horizontal surface, friction does work to bring the object to rest. Using the work-energy theorem from B to C:

$W_{\text{friction}} = \Delta KE$

$-f \times d = 0 - \frac{1}{2}mv_B^2$

$0.80 \times d = \frac{1}{2}(0.20)(7.0)^2 = \frac{1}{2}(0.20)(49) = 4.9$

$d = \frac{4.9}{0.80} = 6.125 \text{ m} \approx 6.1 \text{ m}$

Marking:

• 1 mark for correct work-energy setup (friction force × distance = kinetic energy at B).
• 1 mark for correct substitution.
• 1 mark for correct answer with unit.

Common mistake: Forgetting the negative sign for friction work, or using the wrong value for speed at B.

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Question 10 [6 marks]

(a) [2 marks]

Answer:

$F_{\text{down slope}} = mg\sin\theta = 1500 \times 9.81 \times \sin 15°$

$= 1500 \times 9.81 \times 0.2588 = 3808 \text{ N} \approx 3810 \text{ N}$

Marking:

• 1 mark for correct formula ($mg\sin\theta$).
• 1 mark for correct answer with unit.

(b) [2 marks]

Answer:
At constant speed, net force = 0, so:

$F_{\text{driving}} = F_{\text{down slope}} + F_{\text{resistive}} = 3808 + 500 = 4308 \text{ N} \approx 4310 \text{ N}$

Marking:

• 1 mark for understanding that constant speed means zero net force.
• 1 mark for correct answer.

(c) [2 marks]

Answer:

$P = F_{\text{driving}} \times v = 4308 \times 20 = 86160 \text{ W} \approx 86.2 \text{ kW}$

Marking:

• 1 mark for correct formula ($P = Fv$).
• 1 mark for correct answer with unit.

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Question 11 [7 marks]

(a) [1 mark]

Answer:

$a = \frac{\Delta v}{\Delta t} = \frac{4.0 - 0}{2.0} = 2.0 \text{ m s}^{-2}$

(b) [2 marks]

Answer:
Applying Newton's second law during the acceleration phase:

$T - mg = ma$

$T = m(g + a) = 300(9.81 + 2.0) = 300 \times 11.81 = 3543 \text{ N} \approx 3540 \text{ N}$

Marking:

• 1 mark for correct Newton's second law equation.
• 1 mark for correct answer with unit.

(c) [2 marks]

Answer:
Total height = area under the v-t graph:

$h = \text{Area} = \frac{1}{2}(2.0)(4.0) + (4.0)(4.0) + \frac{1}{2}(2.0)(4.0)$

$= 4.0 + 16.0 + 4.0 = 24.0 \text{ m}$

Marking:

• 1 mark for recognising area under v-t graph gives displacement.
• 1 mark for correct calculation.

(d) [2 marks]

Answer:
During constant speed phase: $T = mg = 300 \times 9.81 = 2943$ N
During deceleration phase: $T = m(g-a) = 300(9.81 - 2.0) = 300 \times 7.81 = 2343$ N

Work done = work in phase 1 + phase 2 + phase 3:

$W_1 = 3543 \times 4.0 = 14172 \text{ J}$ $W_2 = 2943 \times 16.0 = 47088 \text{ J}$ $W_3 = 2343 \times 4.0 = 9372 \text{ J}$

$W_{\text{total}} = 14172 + 47088 + 9372 = 70632 \text{ J} \approx 70.6 \text{ kJ}$

Alternative (simpler) method: The net work done by tension equals the gain in gravitational potential energy (since it starts and ends at rest):

$W_{\text{total}} = mgh = 300 \times 9.81 \times 24.0 = 70632 \text{ J}$

Marking:

• 1 mark for correct method.
• 1 mark for correct answer with unit.

Note: The simpler method works because the change in kinetic energy is zero (starts and ends at rest), so the work done by tension equals the gain in gravitational potential energy.

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Question 12 [5 marks]

(a) [2 marks]

Answer:
Conservation of linear momentum:

$m_1 u_1 + m_2 u_2 = (m_1 + m_2)v$

$0.40 \times 3.0 + 0.60 \times 0 = (0.40 + 0.60)v$

$1.20 = 1.0v$

$v = 1.20 \text{ m s}^{-1}$

Marking:

• 1 mark for correct conservation of momentum equation.
• 1 mark for correct answer with unit.

(b) [2 marks]

Answer:

$KE_{\text{initial}} = \frac{1}{2}(0.40)(3.0)^2 = 1.80 \text{ J}$

$KE_{\text{final}} = \frac{1}{2}(1.0)(1.20)^2 = 0.72 \text{ J}$

$\Delta KE = 0.72 - 1.80 = -1.08 \text{ J}$

Kinetic energy lost = $1.08$ J

Marking:

• 1 mark for calculating both kinetic energies correctly.
• 1 mark for correct energy lost.

(c) [1 mark]

Answer:
This is an inelastic collision because kinetic energy is not conserved (it decreases). In a perfectly inelastic collision, the objects stick together and the maximum possible kinetic energy is lost.

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Question 13 [4 marks]

(a) [1 mark]

Answer:

$P_{\text{out}} = \eta \times P_{\text{in}} = 0.75 \times 2500 = 1875 \text{ W}$

(b) [3 marks]

Answer:
The output power is used to increase the gravitational potential energy of the water:

$P_{\text{out}} = \frac{mgh}{t}$

Rearranging for mass per minute ($t = 60$ s):

$\frac{m}{t} = \frac{P_{\text{out}}}{gh} = \frac{1875}{9.81 \times 8.0} = \frac{1875}{78.48} = 23.9 \text{ kg per minute}$

Marking:

• 1 mark for correct relationship $P = mgh/t$.
• 1 mark for correct substitution.
• 1 mark for correct answer with unit.

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Question 14 [4 marks]

(a) [2 marks]

Answer:

$W = mgh = 70 \times 9.81 \times 12.0 = 8240.4 \text{ J} \approx 8240 \text{ J}$

Marking:

• 1 mark for correct formula.
• 1 mark for correct answer with unit.

(b) [1 mark]

Answer:

$P = \frac{W}{t} = \frac{8240}{15.0} = 549 \text{ W}$

(c) [1 mark]

Answer:
The athlete also gains kinetic energy (they are moving at the top of the stairs), and additional energy is expended overcoming friction/air resistance in the muscles and joints. The calculation in (b) only accounts for the gain in gravitational potential energy.

Accept any valid reason: e.g., "The athlete also has kinetic energy at the top" or "Energy is lost to heat in the body/muscles" or "Work is done against internal friction."

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Question 15 [6 marks]

(a) [2 marks]

Answer:
At maximum height, $v = 0$. Using conservation of energy:

$\frac{1}{2}mv_0^2 = mgh_{\text{max}}$

$h_{\text{max}} = \frac{v_0^2}{2g} = \frac{(15)^2}{2 \times 9.81} = \frac{225}{19.62} = 11.47 \text{ m} \approx 11.5 \text{ m}$

Marking:

• 1 mark for correct energy conservation equation.
• 1 mark for correct answer with unit.

(b) [2 marks]

Answer:
Using conservation of energy from launch to height 5.0 m:

$\frac{1}{2}mv_0^2 = mgh + \frac{1}{2}mv^2$

$\frac{1}{2}(15)^2 = 9.81 \times 5.0 + \frac{1}{2}v^2$

$112.5 = 49.05 + 0.5v^2$

$v^2 = 2(112.5 - 49.05) = 126.9$

$v = 11.3 \text{ m s}^{-1}$

Marking:

• 1 mark for correct energy conservation equation.
• 1 mark for correct answer with unit.

(c) [2 marks]

Answer:
The maximum height would be less than the value calculated in (a). This is because air resistance does negative work on the ball during its flight, dissipating some of the mechanical energy as thermal energy. Therefore, not all of the initial kinetic energy is converted to gravitational potential energy, and the ball reaches a lower maximum height.

Marking:

• 1 mark for stating the height would be less.
• 1 mark for correct explanation involving energy dissipation/air resistance doing negative work.

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Question 16 [6 marks]

(a) [1 mark]

Answer:
Efficiency is defined as the ratio of useful output energy (or power) to the total input energy (or power), often expressed as a percentage:

$\eta = \frac{\text{useful output power}}{\text{input power}} \times 100\%$

(b) [2 marks]

Answer:

$\eta = \frac{6.5}{13.0} \times 100\% = 50.0\%$

Marking:

• 1 mark for correct formula.
• 1 mark for correct answer.

(c) [1 mark]

Answer:
Energy is lost as heat due to resistance in the motor's coils (Joule heating / $I^2R$ losses), friction in the moving parts of the motor, and sound energy.

Accept any one valid reason.

(d) [2 marks]

Answer:
The efficiency increases as the mass lifted increases. Calculating efficiencies:

• 0.50 kg: $2.0/4.8 = 41.7\%$
• 1.00 kg: $4.2/8.5 = 49.4\%$
• 1.50 kg: $6.5/13.0 = 50.0\%$
• 2.00 kg: $8.8/18.0 = 48.9\%$
• 2.50 kg: $11.0/24.0 = 45.8\%$

The efficiency initially increases, reaches a maximum around 1.00–1.50 kg, then decreases. This is because at low loads, a significant fraction of the input power is used to overcome constant losses (friction, $I^2R$ losses in the motor). As the load increases, the useful output power increases relative to these fixed losses, so efficiency increases. At very high loads, the current increases significantly, causing $I^2R$ losses to increase rapidly, reducing efficiency.

Marking:

• 1 mark for identifying the trend (increases then decreases / peaks at intermediate load).
• 1 mark for a reasonable explanation involving fixed losses and/or $I^2R$ losses.

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Question 17 [6 marks]

(a) [3 marks]

Answer:
Using conservation of energy from A to B:

$mgh_A = mgh_B + \frac{1}{2}mv_B^2$

$v_B^2 = 2g(h_A - h_B) = 2 \times 9.81 \times (30.0 - 16.0) = 2 \times 9.81 \times 14.0 = 274.68$

$v_B = \sqrt{274.68} = 16.6 \text{ m s}^{-1}$

Marking:

• 1 mark for correct energy conservation equation.
• 1 mark for correct height difference ($h_A - h_B = 14.0$ m).
• 1 mark for correct answer with unit.

(b) [3 marks]

Answer:
At point C (ground level), all initial PE has been converted to KE:

$mgh_A = \frac{1}{2}mv_C^2$

$v_C = \sqrt{2gh_A} = \sqrt{2 \times 9.81 \times 30.0} = \sqrt{588.6} = 24.26 \text{ m s}^{-1}$

Using work-energy theorem for the braking section:

$W_{\text{brake}} = \Delta KE$

$-F_{\text{brake}} \times d = 0 - \frac{1}{2}mv_C^2$

$2000 \times d = \frac{1}{2}(400)(24.26)^2 = 200 \times 588.6 = 117720$

$d = \frac{117720}{2000} = 58.9 \text{ m}$

Marking:

• 1 mark for calculating speed at C correctly.
• 1 mark for correct work-energy setup for braking section.
• 1 mark for correct answer with unit.

Alternative: Directly from energy conservation: initial PE = work done by brake:

$mgh_A = F_{\text{brake}} \times d \implies d = \frac{mgh_A}{F_{\text{brake}}} = \frac{400 \times 9.81 \times 30.0}{2000} = 58.9 \text{ m}$

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Question 18 [6 marks]

(a) [3 marks]

Answer:
Initial mechanical energy (at top, rest):

$E_i = mgh = 60 \times 9.81 \times 15.0 = 8829 \text{ J}$

Final mechanical energy (at bottom):

$E_f = \frac{1}{2}mv^2 = \frac{1}{2}(60)(8.0)^2 = 30 \times 64 = 1920 \text{ J}$

Energy lost:

$\Delta E = E_i - E_f = 8829 - 1920 = 6909 \text{ J} \approx 6910 \text{ J}$

Marking:

• 1 mark for correct initial energy.
• 1 mark for correct final energy.
• 1 mark for correct energy lost with unit.

(b) [2 marks]

Answer:
The energy lost equals the work done against the average resistive force:

$F_{\text{resistive}} \times d = \Delta E$

$F_{\text{resistive}} = \frac{6909}{120} = 57.6 \text{ N}$

Marking:

• 1 mark for correct relationship.
• 1 mark for correct answer with unit.

(c) [1 mark]

Answer:
Gravitational potential energy is converted into kinetic energy and thermal energy (due to friction/air resistance).

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Question 19 [6 marks]

(a) [4 marks]

Answer:
At maximum speed, the driving force equals the resistive force, and power = $F \times v$:

$P = F_{\text{driving}} \times v_{\text{max}} = F_{\text{resistive}} \times v_{\text{max}}$

$P = (200 + 0.40v_{\text{max}}^2) \times v_{\text{max}}$

$90000 = 200v_{\text{max}} + 0.40v_{\text{max}}^3$

Testing $v_{\text{max}} = 50$ m s⁻¹:

$200(50) + 0.40(50)^3 = 10000 + 0.40(125000) = 10000 + 50000 = 60000$

This gives 60000, not 90000. Let me solve properly:

$0.40v^3 + 200v = 90000$

$0.40v^3 + 200v - 90000 = 0$

Dividing by 0.40:

$v^3 + 500v - 225000 = 0$

Testing $v = 50$: $125000 + 25000 - 225000 = -75000$ (too low)

Testing $v = 55$: $166375 + 27500 - 225000 = -31125$

Testing $v = 58$: $195112 + 29000 - 225000 = -888$

Testing $v = 58.1$: $196122 + 29050 - 225000 = 172$

So $v_{\text{max}} \approx 58$ m s⁻¹.

Revised question intent: The question asks students to verify. Let me re-check the numbers. Actually, let me re-derive with the given numbers more carefully.

$90000 = (200 + 0.40v^2)v = 200v + 0.40v^3$

At $v = 50$: $200(50) + 0.40(125000) = 10000 + 50000 = 60000 \neq 90000$

The numbers don't give exactly 50 m/s. Let me adjust the answer to show the proper method:

Answer (corrected):
At maximum speed, net force = 0, so driving force = resistive force:

$F_{\text{driving}} = 200 + 0.40v_{\text{max}}^2$

Since $P = F_{\text{driving}} \times v_{\text{max}}$:

$90000 = (200 + 0.40v_{\text{max}}^2) \times v_{\text{max}}$

$90000 = 200v_{\text{max}} + 0.40v_{\text{max}}^3$

$0.40v_{\text{max}}^3 + 200v_{\text{max}} - 90000 = 0$

Dividing by 0.40:

$v_{\text{max}}^3 + 500v_{\text{max}} - 225000 = 0$

By trial: $v = 58$ gives $195112 + 29000 - 225000 = -888$
$v = 58.1$ gives $196122 + 29050 - 225000 = +172$

So $v_{\text{max}} \approx 58$ m s⁻¹ (to 2 s.f.).

Note: The question states "approximately 50 m s⁻¹" but with the given numbers, the answer is approximately 58 m s⁻¹. The marking scheme should accept the correct mathematical derivation. If the question intended $v_{\text{max}} \approx 50$ m s⁻¹, the power should be 60 kW. For this answer key, I'll mark based on correct method.

Marking:

• 1 mark for stating that at max speed, driving force = resistive force.
• 1 mark for using $P = F \times v$.
• 1 mark for setting up the cubic equation correctly.
• 1 mark for solving to find $v_{\text{max}} \approx 58$ m s⁻¹ (or verifying the given value if numbers are adjusted).

(b) [2 marks]

Answer:
At $v = 25$ m s⁻¹:

$P = F_{\text{driving}} \times v$

$F_{\text{driving}} = \frac{P}{v} = \frac{90000}{25} = 3600 \text{ N}$

Marking:

• 1 mark for using $P = Fv$.
• 1 mark for correct answer with unit.

Note: The driving force at 25 m/s is 3600 N, while the resistive force at 25 m/s is $200 + 0.40(25)^2 = 200 + 250 = 450$ N. The net force is $3600 - 450 = 3150$ N, so the car is accelerating at this speed.

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Question 20 [8 marks]

(a) [2 marks]

Answer:

$v_x = v_0 \cos\theta = 10 \cos 30° = 10 \times 0.866 = 8.66 \text{ m s}^{-1}$

$v_y = v_0 \sin\theta = 10 \sin 30° = 10 \times 0.50 = 5.0 \text{ m s}^{-1}$

Marking:

• 1 mark for correct horizontal component.
• 1 mark for correct vertical component.

(b) [3 marks]

Answer:
Taking upward as positive, the vertical displacement is $-20.0$ m:

$s_y = v_y t - \frac{1}{2}gt^2$

$-20.0 = 5.0t - \frac{1}{2}(9.81)t^2$

$4.905t^2 - 5.0t - 20.0 = 0$

Using the quadratic formula:

$t = \frac{5.0 \pm \sqrt{(-5.0)^2 + 4(4.905)(20.0)}}{2(4.905)}$

$t = \frac{5.0 \pm \sqrt{25.0 + 392.4}}{9.81} = \frac{5.0 \pm \sqrt{417.4}}{9.81} = \frac{5.0 \pm 20.43}{9.81}$

Taking the positive root:

$t = \frac{25.43}{9.81} = 2.59 \text{ s}$

Marking:

• 1 mark for correct kinematic equation with correct signs.
• 1 mark for correct substitution into quadratic formula.
• 1 mark for correct positive root with unit.

(c) [3 marks]

Answer:
Horizontal velocity remains constant: $v_x = 8.66$ m s⁻¹

Vertical velocity at impact:

$v_y' = v_y - gt = 5.0 - 9.81 \times 2.59 = 5.0 - 25.41 = -20.41 \text{ m s}^{-1}$

Speed:

$v = \sqrt{v_x^2 + v_y'^2} = \sqrt{(8.66)^2 + (20.41)^2} = \sqrt{75.0 + 416.6} = \sqrt{491.6} = 22.2 \text{ m s}^{-1}$

Alternative (energy method):

$\frac{1}{2}mv_0^2 + mgh = \frac{1}{2}mv^2$

$v = \sqrt{v_0^2 + 2gh} = \sqrt{100 + 2(9.81)(20.0)} = \sqrt{100 + 392.4} = \sqrt{492.4} = 22.2 \text{ m s}^{-1}$

Marking:

• 1 mark for finding vertical component at impact.
• 1 mark for combining components using Pythagoras (or energy method).
• 1 mark for correct final answer with unit.

Note: The energy method is much simpler and gives the same result. This is because air resistance is negligible, so mechanical energy is conserved. The speed at impact depends only on the initial speed and the vertical displacement, not on the launch angle.

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Total: 60 marks