9702 · 5.1
Energy conservation flashcards
Revision flashcards for Cambridge 9702 Energy conservation (syllabus 5.1). Flip, recall, then mark a real past-paper question.
Card
What is the fundamental principle of energy conservation in a closed system?
Energy cannot be created or destroyed, only transformed, so the total energy remains constant.
Card
How is kinetic energy dependent on an object's mass and velocity?
It is directly proportional to mass and the square of velocity ($KE = \frac{1}{2}mv^2$).
Card
Under what condition is work done on an object?
Work is done only if a force causes displacement in the direction of the applied force.
Card
What does an efficiency less than 1 (or 100%) imply for a real system?
It implies some input energy is inevitably lost, typically as heat or sound, rather than being converted into useful output.
Card
Can the reference point for calculating gravitational potential energy be changed?
Yes, the reference point can be chosen arbitrarily, as only the change in vertical height ($\Delta h$) is significant.
Card
What is the formula for work done when force and displacement are in the same direction?
$W = F \times d$
Card
Define power in terms of energy and time.
Power is the rate at which energy is transferred or work is performed ($P = \frac{\Delta E}{\Delta t}$).
Card
How can power be calculated if you know the force and velocity?
$P = Fv$, this is especially useful when an object moves at a constant velocity against a force.
Card
What is meant by a 'closed system' in the context of energy conservation?
A system where no matter or energy can enter or leave from its surroundings.
Card
What happens to energy 'lost' due to air resistance?
It is converted into internal energy (heat and sound) of the object and the surroundings.
Card
What is the formula for Gravitational Potential Energy (GPE)?
$GPE = mgh$
Card
When is the formula $W = Fd \cos \theta$ used to calculate work done?
When the applied force $F$ and the displacement $d$ are not in the same direction. $\theta$ is the angle between the force and displacement vectors.
Card
Why does the gravitational force do no work on a satellite in a perfectly circular orbit?
Because the gravitational force is always perpendicular to the satellite's velocity (displacement direction). The angle $\theta$ is 90°, and $\cos(90°) = 0$, so no work is done.
Card
How can efficiency be expressed in terms of power?
Efficiency can be calculated as the ratio of useful power output to total power input: $Efficiency = \frac{\text{Useful Power Output}}{\text{Total Power Input}}$.
Card
What is the relationship between the work done by the driving force of a car and the resistive forces when it moves at a constant velocity?
At constant velocity, the rate of work done by the driving force is equal to the rate at which energy is dissipated by resistive forces (like air resistance and friction). The driving force itself equals the total resistive force.