Retrieval Practice
Work, Energy & Power — AQA A-Level Physics
Q1. Define work done.
- The energy transferred when a force causes an object to move a distance.
- W = Fs (or W = Fs cos theta if the force is at an angle).
Q2. How do you find work done from a force-displacement graph?
Calculate the area under the graph.
Q3. Define power and state its unit.
- Power is the rate of doing work (or rate of energy transfer).
- P = W/t.
- Unit: watt (W).
Q4. Write the equation linking power, force and velocity.
- P = Fv.
- Valid when force is constant and in the direction of velocity.
Q5. How is efficiency calculated?
Efficiency = useful output / total input (as a ratio) or multiplied by 100 for percentage.
Q6. State the principle of conservation of energy.
Energy cannot be created or destroyed, only transferred from one form to another.
Q7. Write the equation for kinetic energy.
E_k = 0.5mv^2.
Q8. Write the equation for gravitational potential energy (uniform field).
delta-E_p = mg delta-h.
Q9. If an object falls freely through height h with no air resistance, what is its speed at the bottom?
- v = sqrt(2gh).
- Mass cancels from the energy equation.
Q10. Why is no energy transfer 100% efficient?
Some energy is always dissipated as thermal energy (heat) or sound, which is not useful for the intended purpose.
Q11. What happens to the kinetic energy if you double an object's speed?
It quadruples, because KE is proportional to \(v^{2}\).
Q12. A force at angle theta to the displacement does work W = Fs cos theta. What happens when theta = 90 degrees?
- cos 90 = 0, so no work is done.
- A force perpendicular to the displacement transfers no energy.