Lesson 1:  Newton's First Law: Inertia & Why Objects Keep Moving

1. If Newton's First Law says objects keep moving, why does everything I slide across the floor eventually stop?

Newton’s First Law says objects keep moving unless acted upon by an unbalanced force. On Earth, that "unbalanced force" is almost always friction (from the floor) and air resistance. If you were in deep space and threw a ball, it would truly float away forever in a straight line because there are no invisible forces like friction to slow it down.

2. What is an "Inertial Reference Frame" and why does Newton's Second Law fail if I'm on a spinning merry-go-round?

An Inertial Reference Frame is a viewpoint that is not accelerating (it's either perfectly still or moving at a constant speed). If you are on a spinning merry-go-round, you are accelerating (changing direction constantly). inside this "accelerating frame," you feel forces pushing you outward (centrifugal force) that don't actually exist. Newton's laws only work "out of the box" in non-accelerating frames.

3. If I push on a wall and it doesn't move, the work is zero, but why do I still get tired?

In physics, Work = Force × Distance. Since the wall moved distance zero, you did zero "Physics Work." However, your muscles are doing "Biological Work." Your muscle fibers are constantly contracting and relaxing at a microscopic level just to maintain tension. That burns chemical energy (calories), which is why you feel tired, even if the wall didn't budge.

Lesson 2: Newton's Second Law: Using F=ma to Find Acceleration

4. What does "Net Force" actually mean? Is it a new force or just the answer to a math problem?

It is not a new physical force. You will never draw an arrow labeled "Net Force" on a diagram. Net Force (Sigma F) is just the mathematical sum of all the real forces (gravity, friction, tension, push) acting on the object. It is the "total score" after you add up the winners and subtract the losers.

5. If an object is moving upward, does that mean the Net Force has to be upward?

No! This is a huge trap. The direction of motion is Velocity. The direction of the push/pull is Force.

• If you throw a ball up, it moves up (velocity is up), but gravity is pulling it down (Net Force is down).
• Because the Force and Velocity are opposite, the ball slows down.

6. How do I solve F=ma problems when the forces are in 2D (at angles)? Do I just add the numbers?

You cannot simply add the numbers (scalars) because direction matters. You must break diagonal forces into X and Y components (using sine and cosine).

• Sum all X-forces: Sum Fx = max
• Sum all Y-forces: Sum Fy = may
• Solve the two equations separately.

7. When solving for Tension in a system with two blocks, why do we sometimes treat the whole system as one big mass and other times split them up?

This is a shortcut vs. detail strategy.

• System Method: Treat all blocks as one giant object to find the acceleration quickly (internal tensions cancel out).
• Individual Object Method: Isolate just one block to find the specific Tension in the rope connecting them.

8. For a pulley problem (Atwood machine), if one mass is heavier, is the Tension equal to the weight of the lighter mass?

No. If the system is accelerating, Tension is not equal to weight ($mg$).

• For the lighter mass moving up, Tension must be greater than gravity ($T > mg$) to pull it up.
• For the heavier mass moving down, Tension must be less than gravity ($T < mg$) to let it drop.

9. How do I solve for acceleration if I'm not given the mass of the object? (e.g., car skidding to a stop).

Don't panic—the mass usually cancels out!

Example: Friction stops a car.

• F_net = ma
• Friction = ma
• mu * mg = ma

Notice there is an m on both sides? Divide by m and they disappear: a = mu * g

Lesson 3: The 4 Common Forces: Weight, Normal, Tension & Friction

10. What is the actual difference between mass and weight? Can I use them interchangeably?

• Mass (kg): How much "stuff" you are made of. This never changes, even on the Moon.
• Weight (Newtons): How hard gravity pulls on you. This changes depending on where you are (W = mg).

Never use them interchangeably in physics equations.

11. What exactly is a "Normal" force, and why is it called that?

"Normal" is an old math word meaning "Perpendicular." It is the support force from a surface (like a table or floor) pushing back against an object. It always points perpendicular (90 degrees) away from the surface.

12. Does a table push back up on a book because it "knows" the book is there, or is the table smart?

The table isn't smart, but it is elastic. Imagine the table is made of millions of tiny, stiff springs (atomic bonds). When you put a book on it, the springs compress slightly (microscopically). They want to snap back to their original shape, so they push up. That "spring back" is the Normal Force.

13. How do I know which direction Friction points without guessing?

Friction is a hater. It always opposes relative motion (or intended motion).

• Ask: "If there were zero friction, which way would this slide?"
• Friction points the exact opposite way.

14. Why is Static Friction usually stronger than Kinetic Friction? Physically, what is happening?

Answer: Think of the surfaces like two pieces of jagged sandpaper interlocking.

• Static (Stationary): The jagged peaks have settled deep into the valleys. It takes a big shove to break them loose (Cold Welding).
• Kinetic (Moving): The surfaces are skimming over the top of each other, so they don't lock together as tightly.

15. If the coefficient of friction (mu) is usually less than 1, can it ever be greater than 1? Or did I do the math wrong?

Answer: It is usually between 0 and 1, but it can be greater than 1! Dragster tires or specialized glue can have a coefficient > 1 (meaning the friction force is actually stronger than the object's weight). However, in high school physics, if you get an answer of "15", you probably did the math wrong.

16. What are the units for the Coefficient of Friction? I keep losing points for not writing them.

It is a trick question! The Coefficient of Friction ($\mu$) has NO units. It is a ratio of Force divided by Force ($N/N$), so the units cancel out. It is just a raw number describing "grippiness."

17. Does the string in a Tension problem stretch? If it doesn't, how does it transmit force?

In "Textbook Physics," we assume strings are ideal (massless and unstretchable). This means the Tension is the same everywhere along the rope. In the real world, ropes do stretch slightly to transmit force, acting like very stiff springs.

18. Is Gravity actually a force? I heard Einstein said it wasn't.

In Newtonian Physics (what you are learning now), Gravity is a Force. In General Relativity (Einstein's advanced theory), Gravity is the curvature of spacetime caused by mass, not a force. For your exam, treat it as a Force!

Lesson 4: Free Body Diagrams (FBD): The Secret to Solving Force Problems

19. Should I draw "ma" as a force on my Free Body Diagram?

Never! "ma" is the result of the forces, not a force itself.

The Left Side of the equation (Gravity, Tension, Friction) goes on the Diagram.

The Right Side (ma) is what those forces equal.

Drawing "ma" as an arrow usually leads to double-counting and wrong answers.

20. Is the Normal Force always equal to gravity (mg)?

No. This is the "Flat Earth Trap."

N = mg only if the object is on a flat surface and nothing else is pushing it.

If the surface is tilted (ramp), N = mg cosθ

If someone is pushing down on the object, N increases to support the extra load

Lesson 5: How to Solve Inclined Plane Problems? Free Body Diagram

21. How do I draw the Free Body Diagram for a block sliding DOWN a ramp with friction?

1. Draw Gravity pointing straight down (not along the ramp).
2. Draw Normal Force perpendicular to the ramp (tilted).
3. Draw Friction pointing up the ramp (opposing the slide).
4. Crucial Step: Rotate your coordinate system so X is "down the ramp" and Y is "perpendicular to the ramp."

22. On an inclined plane, how do I know if the component of gravity is mg sin θ or mg cos θ? I always mix them up.

Use the "Slide vs. Stick" trick:

• Sine sounds like Slide. The component pointing down the ramp (making it slide) is mg sin(θ).
• Cos is like Close. The component "closing" or sticking the block against the ramp is mg cosθ.

Lesson 6: Apparent Weight: The "Elevator Problem" & Scale Readings

23. In the "Elevator Problem," when the elevator accelerates down, why do I feel lighter? Does my mass change?

Your mass stays the same. You feel lighter because the floor is dropping out from under you.

• A bathroom scale reads the Normal Force pushing up on you, not your weight.
• If the elevator accelerates down, the floor pushes up less than gravity (N < mg), so the scale reads a lower number ("Apparent Weight")

Lesson 7: Newton's Third Law: Identifying Action-Reaction Pairs Correctly

24. If Action and Reaction forces are equal and opposite, don't they just cancel each other out? How does anything ever move?

They don't cancel because they act on different objects.

• If you push a box: You push the Box (Force A). The Box pushes You (Force B).
• Force A moves the box. Force B moves (or slows) you.
• Forces only cancel if they act on the same object.

25. If a truck hits a fly, does the fly actually hit the truck with the exact same force? That seems impossible.

Yes, the FORCE is exactly the same (Newton's 3rd Law).

• However, the ACCELERATION is different (a = F/m).
• The fly has tiny mass, so that force creates a massive acceleration (splat).
• The truck has huge mass, so that same force creates a tiny, unnoticeable acceleration.