MCAT work, energy, and fluids extend Newton’s mechanics into two of the exam’s most reliable topics. Energy conservation turns hard force problems into simple bookkeeping, and fluid dynamics — pressure, buoyancy, and Bernoulli’s principle — underlies circulation and gas exchange in the living systems the MCAT cares about.
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bolt The 60-second version
check_circleWork = force × displacement along the direction of motion (W = Fd·cosθ).
check_circleThe work–energy theorem: net work equals the change in kinetic energy.
check_circleMechanical energy (KE + PE) is conserved when only conservative forces act.
check_circlePressure increases with depth: P = ρgh.
check_circleArchimedes: the buoyant force equals the weight of displaced fluid.
check_circleContinuity (A₁v₁ = A₂v₂) plus Bernoulli: where a fluid speeds up, its pressure drops.
target What you'll be able to do · aligned to AAMC content categories 4A & 4B
4AApply the work–energy theorem and conservation of mechanical energy to solve motion problems.
4ADistinguish kinetic and potential energy and calculate work and power.
4BUse pressure, density, and Archimedes’ principle to analyze buoyancy.
4BApply the continuity equation and Bernoulli’s principle to fluid flow.
How the MCAT tests work, energy, and fluids
Work and energy sit in AAMC content category 4A, and fluids in 4B — the role of fluids in circulation and gas exchange. The MCAT loves energy conservation because it sidesteps messy force analysis, and it ties fluids directly to physiology (blood flow, the alveoli). Expect conceptual reasoning over heavy computation.
workspace_premium Why energy first
When a problem gives you start and end states but a complicated path, reach for energy conservation instead of forces — it turns the problem into simple algebra.
Work and power
Work is energy transferred by a force acting through a displacement. Only the force component along the motion does work, and power is the rate of doing it.
W = F·d·cosθWorkθ is the angle between force and displacement. A force perpendicular to motion (like the normal force) does zero work.
P = W / tPowerThe rate of energy transfer, in watts (J/s). Also P = Fv for a constant force.
Energy and its conservation
Energy comes in forms that convert into one another. The two mechanical forms are kinetic energy (motion) and potential energy (position). The work–energy theorem says the net work on an object equals its change in kinetic energy.
Conservation of energy. At the top the cart has maximum potential energy; at the bottom that has converted entirely to kinetic energy. Without friction, PE + KE stays constant.
KE = ½mv² · PE = mghMechanical energyKinetic energy depends on speed squared; gravitational potential energy depends on height. Their sum is conserved when only conservative forces act.
functions Worked example — speed at the bottom of a drop
A cart starts from rest and drops a height h = 5 m on a frictionless track. What is its speed at the bottom? (g = 10 m/s²)
Conserve energy: all PE at the top becomes KE at the bottom.
mgh = ½mv² — the mass cancels, giving v² = 2gh.
v = √(2·10·5) = √100 = 10 m/s.
Answer. v = 10 m/s. Notice mass does not matter — only the height and g.
Fluids at rest: pressure and buoyancy
A static fluid exerts pressure that increases with depth. That depth-dependent pressure is what produces buoyancy.
Buoyancy. The upward buoyant force equals the weight of the fluid the object displaces. A floating object displaces exactly its own weight in fluid.
P = P₀ + ρghHydrostatic pressurePressure rises linearly with depth h; ρ is fluid density. This is why deep water and deep veins carry higher pressure.
workspace_premium Float or sink
An object floats if its average density is less than the fluid’s and sinks if greater. Archimedes’ principle (F_buoyant = ρ_fluid·V_displaced·g) makes this quantitative.
Fluids in motion: continuity and Bernoulli
For an ideal fluid, two principles govern flow. Continuity conserves mass; Bernoulli’s principle conserves energy.
Continuity and Bernoulli. Where the pipe narrows, the fluid must speed up (A₁v₁ = A₂v₂), and by Bernoulli’s principle its pressure drops.
Continuity (A₁v₁ = A₂v₂): a narrower cross-section means faster flow, because the same volume must pass each second.
Bernoulli’s principle: along a streamline, faster-moving fluid has lower pressure — the trade-off between speed and pressure.
Together they explain everything from a garden-hose nozzle to blood flow through a constricted artery.
Energy in fluids and physiology
The same energy accounting applies to circulation. Bernoulli and continuity describe how blood accelerates through narrowed vessels and why pressure changes across the circulatory system.
These ideas connect directly to cardiac output and gas exchange covered in the biology section — the physics behind physiology.
How to study it — and the traps to avoid
Work, energy, and fluids reward choosing the right tool. Practice recognizing when energy methods beat force methods.
Reach for energy conservation when start and end states are given.
Remember perpendicular forces (like the normal force) do zero work.
For buoyancy, compare densities and use the displaced-fluid weight.
Pair continuity with Bernoulli for any flowing-fluid problem.
error Highest-yield trap
Bernoulli’s principle is counterintuitive: faster fluid has LOWER pressure, not higher. In a narrowed vessel or pipe the fluid speeds up and its pressure falls — the opposite of what many students first guess.
Straight from the Progress Note question bank, each with a cited explanation. No account needed.
12
A 2.0 kg object initially at rest is pushed by a constant net force that does 16 J of work on it. Using the work–energy theorem, what is the object's final speed?
Answer: B. The work–energy theorem states that the net work done on an object equals its change in kinetic energy: W = ΔKE = ½mv_f² − ½mv_i². Starting from rest, 16 J = ½(2.0 kg)v_f² = (1.0)v_f², so v_f² = 16 and v_f = 4.0 m/s. A results from forgetting to take the square root or mishandling the factor of ½, while C, D, and E come from other arithmetic errors. The correct final speed is 4.0 m/s.
A 2-kilogram object starts from rest, and the net work done on it as it moves is 16 joules. Using the work–energy theorem (net work equals the change in kinetic energy), what is the object's final speed?
Answer: B. The work–energy theorem gives net work = ½mv² − 0, so 16 J = ½(2 kg)v², which means v² = 16 and v = 4 m/s. A mistakes the work value for the speed, C and E come from arithmetic errors, and D uses the mass rather than solving for speed. Solving ½(2)v² = 16 yields a final speed of 4 m/s.
Source: Young, H. D., & Freedman, R. A. (2016). University physics with modern physics (14th ed.). Pearson.
Your next questions are ready.
The question bank picks up from here: it adapts to what you just missed and brings it back right before you'd forget it.
The work–energy theorem states that the net work done on an object equals its change in kinetic energy (W_net = ΔKE). It links force and motion through energy, so if you know the net work you know how the object’s speed changed, without tracking the detailed forces along the way.
When is mechanical energy conserved?expand_more
Mechanical energy (kinetic plus potential) is conserved when only conservative forces, such as gravity, do work. When non-conservative forces like friction or air resistance act, some mechanical energy is converted to heat, so the total mechanical energy decreases.
How does Archimedes’ principle explain buoyancy?expand_more
Archimedes’ principle says the upward buoyant force on an object equals the weight of the fluid it displaces (F_buoyant = ρ_fluid·V_displaced·g). An object floats if it can displace its own weight in fluid before fully submerging, which happens when its average density is less than the fluid’s.
What does Bernoulli’s principle say about pressure and speed?expand_more
Along a streamline in an ideal fluid, faster-moving fluid has lower pressure and slower-moving fluid has higher pressure. Combined with the continuity equation — which says fluid speeds up in narrower regions — this explains why pressure drops where a pipe or blood vessel constricts.
Why does the mass often cancel in energy problems?expand_more
In gravitational problems, both potential energy (mgh) and kinetic energy (½mv²) are proportional to mass, so when you set them equal the mass divides out. That is why a heavy and a light object reach the same speed sliding down the same frictionless height.
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