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Work and Energy

Chapter 2: Work and Energy

3 min read Updated Mar 26, 2026
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🎯 Diagnostic: Test Your Starting Level 24 questions (2 per section). No prior reading required, see what you already know.

Aim to answer every question before checking. Missed questions point you to the sections you need most.

1. (2.1) Work done by a constant force F on an object through displacement d is:
D. Perpendicular forces (e.g., centripetal force on an orbiting object) do no work.
2. (2.1) The SI unit of work is:
A. Both work and energy share the joule.
3. (2.2) For a variable force F(x), the work done over a displacement is:
B. Graphically, the area under the F-x curve equals the work done.
4. (2.3) The kinetic energy of mass m at speed v is:
C. Doubling speed quadruples the KE, which is why high-speed impacts are so damaging.
5. (2.3) The work-energy theorem states:
D. All forces contribute to net work; friction does negative work and lowers KE.
6. (2.4) Gravitational potential energy near Earth's surface is:
A. For larger distance scales, use the full U = -GMm/r.
7. (2.5) Hooke's law for a spring is:
B. The negative sign tells you the force always pushes back toward equilibrium.
8. (2.5) Elastic potential energy stored in a spring stretched by x is:
C. Integrating F = -kx from 0 to x gives ½kx².
9. (2.6) In an isolated system with only conservative forces:
D. Non-conservative forces like friction change total mechanical energy (usually producing heat).
10. (2.6) When a ball falls from height h in the absence of air resistance:
A. Set mgh = ½mv² to get v = √(2gh).
11. (2.7) Power is defined as:
B. Equivalent expressions: P = F × v (for constant force parallel to motion).
12. (2.7) Which statement about a 100-W bulb is correct?
C. 1 watt = 1 joule per second.
13. (2.8) The magnitude of the torque about a pivot is:
D. Maximum torque occurs when F is perpendicular to the moment arm (sin θ = 1).
14. (2.8) An object is in rotational equilibrium when:
A. For static equilibrium, BOTH net force AND net torque must be zero.
15. (2.9) The mechanical advantage of a simple machine is:
B. Trading distance for force: more MA means less input force, but proportionally greater input distance.
16. (2.9) For an ideal lever with the input arm twice the output arm, the MA is:
C. Friction and mass of the lever reduce the MA in a real setup.
17. (2.10) Efficiency of a real machine is:
D. Second-law consequence: heat engines, for example, are fundamentally limited below the Carnot efficiency.
18. (2.10) A real pulley system has efficiency less than 100% because:
A. Any machine with moving parts loses some energy to non-conservative forces.
19. (2.11) An energy (PE vs. position) diagram:
B. The sign of the force is F = -dU/dx, so slope of the curve gives the local force direction.
20. (2.11) A particle sitting at a local maximum of a PE curve is in:
C. Local minimum → restoring force → stable. Local max → force pushes away → unstable.
21. (2.12) Muscle converts chemical energy (ATP) into mechanical work with efficiency of roughly:
D. Exercise heat production is why vigorous activity raises body temperature.
22. (2.12) Basal metabolic rate (BMR) represents:
A. Adult BMR averages about 1500-2000 kcal/day, depending on sex, age, and body composition.
23. (2.12) Work is biologically important because:
B. Even against weak concentration gradients, cellular work adds up to a major fraction of the daily energy budget.
24. (2.1) The work done by gravity on a ball falling straight down is:
C. W = Fd cos θ = Fd when θ = 0°. This positive work converts PE to KE.

You’re shoving a stalled car across a parking lot. Your muscles burn. Your shoes scrape. Sweat drips. You’re clearly putting in effort.

But are you doing work in the physics sense?

That depends on whether your push actually moves the car — and in what direction. Hold the same car in place against a hill (you’re straining hard but it doesn’t move) and you’ve done zero physics work. Push it 5 meters across flat asphalt and you’ve done quite a lot. Same effort. Different physics.

Energy is one of the most powerful problem-solving tools on the entire MCAT. When a question gives you heights and speeds but never mentions time, energy methods let you skip kinematics entirely and jump straight to the answer. When a passage describes a roller coaster, a pendulum, a falling object, or a ball on a ramp, energy conservation is almost always the fastest path.

This chapter builds your energy toolkit from the ground up: what work means in physics (and what it doesn’t), how energy is stored and transferred, when it’s conserved and when it’s not, how to read the energy diagrams the MCAT loves, plus torque, simple machines, efficiency, and the biological energy systems that appear in crossover passages.

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