9709 · 4.5
Energy, work and power flashcards
Revision flashcards for Cambridge 9709 Energy, work and power (syllabus 4.5). Flip, recall, then mark a real past-paper question.
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What is the definition of 'work done' by a constant force?
Work done is the product of the magnitude of the force and the distance moved in the direction of the force. Formula: $W = Fd \cos \theta$.
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What are the SI units for work, energy, and power?
Work and Energy: Joules (J). Power: Watts (W). Note that 1 J = 1 Nm and 1 W = 1 J/s.
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When is the work done by a force equal to zero?
When the force is perpendicular to the direction of motion (since $\cos 90^\circ = 0$), or if there is no displacement ($d=0$).
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State the formula for Kinetic Energy (KE).
$KE = \frac{1}{2}mv^2$, where $m$ is the mass in kg and $v$ is the speed in m/s.
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State the formula for Gravitational Potential Energy (GPE).
$GPE = mgh$, where $m$ is mass (kg), $g$ is acceleration due to gravity (m/s²), and $h$ is the vertical height gain (m).
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What is the Work-Energy Principle?
The net work done on an object by all forces (including non-conservative ones like friction) is equal to the change in its kinetic energy. $W_{net} = \Delta KE = KE_{final} - KE_{initial}$.
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What is the Principle of Conservation of Mechanical Energy?
In a system where only conservative forces (like gravity) do work, the total mechanical energy (KE + GPE) is constant. $KE_i + GPE_i = KE_f + GPE_f$.
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What is the definition of 'power'?
Power is the rate at which work is done or the rate at which energy is transferred. $P = \frac{dW}{dt}$.
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How is power related to driving force and velocity?
For an object moving at an instantaneous velocity $v$ due to a driving force $F$, the instantaneous power developed by that force is $P = Fv$.
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A common exam trap is using the wrong force in $P=Fv$. Which force must be used?
$F$ must be the driving force (or tractive force) that is causing the motion at velocity $v$. It is NOT the resultant force. If velocity is constant, this driving force equals the total resistance.
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What does it mean if the work done by a force is negative?
It means the force has a component that opposes the direction of motion. For example, the work done by friction is always negative, as it removes mechanical energy from the system (usually as heat).
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When calculating GPE change on a slope, what is the most common mistake?
Using the distance along the slope instead of the vertical height change. Remember, $h$ in $mgh$ is always the vertical displacement.