Atlas MCAT Course
Physical Foundations → Newtonian Mechanics
Newtonian Mechanics
How forces produce motion.
01The idea
Everything you’ve already covered in chemistry and physics, physiology, and sensory systems rests on Newtonian mechanics: blood flow exists because forces move fluids, muscle contraction turns force into motion, and pressure, work, energy, and power all derive from force.
If you can track forces, you can predict motion, energy, and work — no memorization required.
THREAD · Energy vs. rate
02Build the model
AThe three quantities that matter
All of mechanics reduces to relationships between force (interaction), mass (resistance to acceleration), and motion (change over time). Everything else — energy, power, momentum — is derived.
BNewton’s first law: motion persists unless forced
An object maintains constant velocity unless acted upon by a net external force. This law is about frames of reference, not rest.
If velocity is changing, a force must exist — even if you don’t see it immediately.
CNewton’s second law: the engine of prediction
This is the workhorse equation of mechanics. Force causes acceleration, and acceleration depends on both force and mass.
DNet force: why direction matters
Forces are vectors, with magnitude and direction. Multiple forces can cancel, reinforce, or compete. This is why free body diagrams are essential.
EFree body diagrams: the skill that solves CP
An FBD is not optional — it is the problem. Steps: isolate the object, draw all forces acting on it, choose coordinate axes, and apply Newton’s laws.
FCommon forces you must recognize instantly
| Force | What it is |
|---|---|
| Gravitational force | Weight |
| Normal force | Constraint response |
| Tension | Force through a connector |
| Friction | Opposes relative motion |
| Applied force | Intentional input |
You do not need formulas yet — just recognition and direction.
GNewton’s third law: interaction pairs
For every force, there is an equal and opposite force. Crucial clarification: these forces act on different objects, so they do not cancel each other.
HStatic vs. dynamic situations
| Static (equilibrium) | Dynamic |
|---|---|
| Acceleration = 0 | Acceleration ≠ 0 |
| Net force = 0 | Net force ≠ 0 |
IFrom force to work
When a force causes displacement, work is done. Force without motion does no work; motion without force does no work. This bridges mechanics to energy.
For now: forces explain why motion changes; energy explains how much change occurs. Newton → work → energy → power. That’s the flow.
03See the connections
The lesson names these links itself. Read the map left to right.
Laws
- First: no net force, no acceleration
- Second: F = ma
- Third: pairs on different objects
Tool
- Free body diagram
- Net force (vectors)
Leads to
- Work
- Energy
- Power
Next: work, energy, and power.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
A puck slides across frictionless ice at constant velocity. What is the net force on it?
Think first.
The same force is applied to a light cart and a heavy cart. Which accelerates more?
Think first.
A book rests on a table. Why don’t the book’s weight and the table’s push on the book count as a third-law pair?
Think first.
A ball thrown straight up is momentarily at rest at the top. What is its acceleration there?
05MCAT lens
Predict the direction
Object speeding up → net force in direction of motion
Object slowing down → net force opposite motion
Constant velocity → forces balanced
Larger mass → smaller acceleration for same force
No equations beyond F = ma required.
06Common traps
Trap 1: Motion requires force
Misconception
Acceleration requires force.
Correct model
Velocity persists.
Trap 2: Large force means fast object
Misconception
Depends on mass.
Correct model
Acceleration matters.
Trap 3: Third-law forces cancel motion
Misconception
Different objects.
Correct model
Interaction pairs.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
Why acceleration signals force
Check against 02 · B and C: F = ma.
Why free body diagrams matter
Check against 02 · E: they are the problem.
Why direction is critical
Check against 02 · D: forces are vectors.
Why equilibrium ≠ rest
Check against 02 · H: equilibrium means zero acceleration, not zero velocity.
If something feels unclear, the missing idea is usually thisForces don’t cause motion — they cause changes in motion.
08Remember
REMEMBER · if you keep only a few things
- Forces change motion; they don’t sustain it.
- F = ma, with net force.
- Draw the free body diagram first.
- Third-law pairs act on different objects and never cancel.
- Equilibrium = zero acceleration, not zero velocity.
- Work is how forces transfer energy.
09Forward hook
Now that force and motion are grounded, the natural next step is work, energy, and power.
Is there a faster way to solve mechanics problems than chasing every force?
That leads to work, energy, and power: a more efficient way to solve many MCAT mechanics problems.
Next lessonWork, Energy, and Power→