Questions where students often pick the tempting wrong answer — make sure you know the right one:
If a car doubles its speed, what happens to its kinetic energy?✗ Kinetic energy doubles because energy is proportional to speed.✓ It quadruples (because KE = 1/2 mv^2 — kinetic energy is proportional to v^2).
When energy is 'lost' as heat due to friction, what has really happened?✗ Energy has been used up and no longer exists.✓ Energy has been transferred to the thermal store of the surroundings — it's not destroyed, just dissipated.
Can a device ever be 100 percent efficient?✗ A well-designed device can be 100 percent efficient by eliminating all friction.✓ In practice, no — some energy is always transferred to less useful stores (e.g. heat to surroundings).
What is the difference between energy and power?✗ Energy and power are the same — both describe how much fuel something uses.✓ Energy is measured in joules; power is the rate of energy transfer, measured in watts (joules per second).
Which is the best way to reduce energy wasted as heat in a moving machine?✗ Painting it black✓ Adding lubrication
Practise Energy — free games
Test yourself with these quick revision games for this topic:
