Atlas MCAT Course
Physical Foundations → Work, Energy, and Power
Work, Energy, and Power
Tracking change without chasing forces.
01The idea
Newton’s laws tell you why motion changes, but they can be cumbersome: many forces, changing directions, complex paths. Energy methods answer a different question.
On the MCAT, this often turns a multi-step force problem into one line.
THREAD · Energy vs. rate
02Build the model
AWork: how forces transfer energy
Work is the transfer of energy by a force acting through a displacement. Force without displacement does no work; displacement without force does no work; and only the component of force along the displacement matters.
BSign of work: direction matters
| Force relative to displacement | Work |
|---|---|
| With it | Positive |
| Against it | Negative |
| Perpendicular to it | Zero |
This explains why the normal force often does no work, and why centripetal forces do no work in circular motion.
CKinetic energy: energy of motion
Kinetic energy reflects how fast mass is moving. It depends on mass, depends on speed (not direction), and is always ≥ 0.
This is why energy methods ignore vector headaches.
DPotential energy: energy of configuration
Potential energy reflects stored energy due to position or arrangement: gravitational potential (height) or elastic potential (stretch or compression).
EThe work–energy theorem: the bridge
This is one of the most powerful relationships in physics. Do positive net work and speed increases; do negative net work and speed decreases. No forces need to be tracked individually.
FConservative vs. non-conservative forces
GConservation of mechanical energy
When only conservative forces act, total mechanical energy stays constant; energy simply changes form between potential and kinetic. This explains falling objects, pendulums, and springs.
HWhen energy is not conserved
Mechanical energy is not conserved when friction is present, energy is converted to heat, or external work is done. But total energy (including thermal) is always conserved.
IPower: how fast energy changes
Power measures the rate of energy transfer. Same work, higher power → done faster. Same force, higher speed → higher power.
JWhy energy methods dominate MCAT CP
Energy methods excel when motion is complex, forces vary, or geometry is messy. Force methods excel when acceleration is required or forces are explicitly asked.
Knowing which tool to use is the real skill.
03See the connections
The lesson names these links itself. Read the map left to right.
Transfer
- Work (sign by direction)
- Work–energy theorem
Stores
- Kinetic (speed)
- Potential (configuration, reference point)
Accounting
- Conservative: conserved
- Non-conservative: to heat
- Power: rate
Next: oscillations and simple harmonic motion.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
A box slides across a level floor. How much work does the normal force do on it?
Think first.
Two identical balls are dropped from different heights, ignoring air resistance. Which is faster at the bottom?
Think first.
A sled slides down a rough hill and arrives slower than energy conservation predicts. Where did the energy go?
Think first.
Two people do the same work lifting boxes, but one finishes in half the time. Who has more power?
05MCAT lens
Predict the direction
Object speeds up → net work positive
Object stops → net work negative
Higher drop height → higher speed at bottom
Same work, shorter time → higher power
No memorization — just tracking energy.
06Common traps
Trap 1: Normal force does work
Misconception
Usually perpendicular.
Correct model
No displacement along force.
Trap 2: Potential energy is absolute
Misconception
Only differences matter.
Correct model
Reference-dependent.
Trap 3: Energy conservation ignores friction
Misconception
Must include non-conservative work.
Correct model
Expand system or track losses.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
What work actually measures
Check against 02 · A: energy transferred by a force along a displacement.
Why energy simplifies mechanics
Check against 02 · C and E: scalars, and the work–energy theorem.
When energy is conserved
Check against 02 · F through H: only conservative forces, or losses tracked.
How power relates to performance
Check against 02 · I: rate of energy transfer.
If something feels unclear, the missing idea is usually thisEnergy tracks what forces accomplish, not how they act.
08Remember
REMEMBER · if you keep only a few things
- Work = energy transferred by a force along a displacement.
- Perpendicular force → zero work.
- W_net = ΔKE.
- Only changes in potential energy matter.
- Friction moves mechanical energy into heat; total energy is conserved.
- Power = rate of energy transfer.
09Forward hook
Now that force-based and energy-based mechanics are unified, the next step is oscillations and simple harmonic motion.
What happens when energy continuously trades forms in a predictable way?
That leads to oscillations and simple harmonic motion.
Next lessonElectrostatics→