Practice work, energy, the work-energy theorem, springs, and power. Quiz one topic at a time or take the whole unit.
The idea each topic turns on, and the mistake students make most often.
Key ideaWork is the force component along the displacement times the displacement (Fd cos θ). On a force–position graph, work is the area under the curve.
Common mistakeSaying a force does work when it is perpendicular to the motion, such as the normal force on level ground or the tension in uniform circular motion.
Key ideaKinetic energy is ½mv², gravitational potential energy is mgh (h measured from a chosen reference level), and spring potential energy is ½kx². Only changes in potential energy matter.
Common mistakeForgetting that kinetic energy depends on v², so doubling the speed quadruples the kinetic energy.
Key ideaThe net work done on an object equals its change in kinetic energy: Wnet = ΔK.
Common mistakeUsing the work of one force instead of the net work, or forgetting that friction does negative work.
Key ideaSpring force is proportional to the stretch or compression, F = kx, and the stored energy is ½kx². x is measured from the spring’s natural length.
Common mistakeUsing the spring’s total length for x, or forgetting that doubling the compression quadruples the stored energy.
Key ideaPower is the rate of energy transfer: P = W/t, or P = Fv for a force along the velocity.
Common mistakeConfusing power with energy. Doing the same work faster takes more power, not more energy.
This is one of the 23 questions in the Unit 3 quiz.
A 10 N force acts in the same direction as an object's 3 m displacement. How much work does the force do?
Work depends on the force, the displacement, and the angle between them. What is that angle here? Check the units of your answer too.
Answer: C. 30 J
Work is W = Fd cos θ. Here the force is parallel to displacement, so θ = 0° and W = (10 N)(3 m)cos 0° = 30 J.
AP-style FRQs for this unit. Students score themselves point by point against a rubric and see a full solution.
