JEE · Physics · Laws of Motion doubts

What does “Inertia” actually mean in Laws of Motion, and why does it matter for JEE?

the tendency of a body to resist any change in its state of rest or uniform motion (Newton's first law).

I keep getting “Inertia” questions wrong in Laws of Motion. What's the trap?

thinking a moving body needs a continuous force to keep moving — it doesn't, unless friction acts.

What does “Newton's second law” actually mean in Laws of Motion, and why does it matter for JEE?

the net force equals mass times acceleration, F=ma, in the direction of the net force.

I keep getting “Newton's second law” questions wrong in Laws of Motion. What's the trap?

using a single force instead of the net (vector sum) force.

What does “Newton's third law” actually mean in Laws of Motion, and why does it matter for JEE?

every action has an equal and opposite reaction, and the two forces act on different bodies.

I keep getting “Newton's third law” questions wrong in Laws of Motion. What's the trap?

thinking action and reaction cancel out — they can't, because they act on different objects.

What does “Friction” actually mean in Laws of Motion, and why does it matter for JEE?

static friction adjusts itself up to a maximum μₛN to prevent motion; kinetic friction μₖN opposes sliding.

I keep getting “Friction” questions wrong in Laws of Motion. What's the trap?

assuming friction always equals μN — static friction is only as big as needed, up to its limit.

What does “Normal force” actually mean in Laws of Motion, and why does it matter for JEE?

the contact push a surface exerts perpendicular to itself.

I keep getting “Normal force” questions wrong in Laws of Motion. What's the trap?

assuming the normal force always equals mg.

What is the difference between static friction and kinetic friction?

Static friction acts on a stationary object and self-adjusts up to a limit; kinetic friction acts on a sliding object and is roughly constant (and usually smaller).

What is the difference between mass and weight?

Mass is the amount of matter (kg, same everywhere); weight is the gravitational force on it (N, changes with g).

What is the difference between inertial frame and non-inertial frame?

An inertial frame moves at constant velocity (Newton's laws hold directly); a non-inertial frame accelerates (you must add pseudo-forces).

When do I use fₛ(max) = μₛN in Laws of Motion, and what does it mean?

the maximum static friction before an object starts to slip.

When do I use fₖ = μₖN in Laws of Motion, and what does it mean?

kinetic friction once the object is sliding.