A force is a push or pull on an object that can change its state of motion or shape. Unit: newton (N).

The SI unit of force is newton (N). 1 N = 1 kg·m/s².

Mass is the amount of matter in a body and a measure of its inertia. Unit: kilogram (kg).

Inertia is the tendency of an object to resist change in its state of motion; quantitatively measured by mass.

Momentum is the product of mass and velocity: p = m·v. Unit: kg·m/s.

Impulse is the product of force and time during which it acts; impulse equals change in momentum: J = F·Δt = Δp.

Net force is the vector sum of all forces acting on an object (resultant force) that determines acceleration.

A free-body diagram is a sketch showing all forces acting on a single object, used to analyze motion or equilibrium.

Newton's first law: A body remains at rest or in uniform motion in a straight line unless acted upon by a net external force (law of inertia).

Newton's second law: Net force on a body = mass × acceleration (F = m·a). It relates force, mass and acceleration.

Newton's third law: For every action, there is an equal and opposite reaction. Action and reaction act on different bodies.

If net force is zero, acceleration is zero; the object is either at rest or moves with constant velocity.

For the same net force, larger mass gives smaller acceleration (a = F/m), so more massive objects accelerate less.

They act on different bodies, so they cannot cancel; each affects the motion of the body it acts on.

Example: When you jump, your legs push the ground (action) and the ground pushes you upward (reaction), propelling you off the ground.

Equilibrium is the state when net force is zero and the object has zero acceleration (either at rest or moving uniformly).

Linear momentum of a closed system is conserved if no external force acts (conservation of momentum).

The 2 kg stone has greater inertia because inertia increases with mass.

No. Inertia is a property of matter (resistance to change in motion), not a force.

Mass is the quantitative measure of inertia—the larger the mass, the larger the inertia.

No. Mass is amount of matter (kg). Weight is the gravitational force on the mass: W = m·g (unit N).

Passengers tend to remain in motion due to inertia; the bus stops but passengers continue forward until seat belts/handholds exert force to stop them.

F = m·a (vector equation).

a = F/m = 10 / 2 = 5 m/s².

Yes. Forces may be present but balance out (net force zero), producing zero acceleration (equilibrium).

Centripetal force is the net force directed toward the center that keeps an object moving in a circular path.

Doubling the net force doubles the acceleration (a ∝ F) if mass is unchanged.

Doubling the mass halves the acceleration (a = F/m) if force is constant.

Acceleration is in the direction of the net force acting on the body.

Friction is the resistive force between two surfaces in contact that opposes relative motion or tendency to move.

Static friction (prevents start of motion) and kinetic/rolling friction (opposes motion while sliding or rolling).

Maximum static friction f_s(max) = μ_s · N, where N is normal reaction and μ_s is coefficient of static friction.

Coefficient of static friction μs is usually larger than coefficient of kinetic friction μk.

Polishing usually reduces roughness and therefore reduces friction (both static and kinetic) between surfaces.

Frictional force is proportional to normal reaction N; greater N typically increases maximum friction (f ∝ N).

Momentum p = m·v, vector quantity along direction of motion; unit kg·m/s.

Impulse J = F·Δt equals the change in momentum Δp (J = Δp).

Δp = F·Δt increases proportionally with time; longer duration gives larger change in momentum for same force.

Cushioning increases contact time Δt for same Δp, reducing average force F = Δp/Δt experienced, lowering injury risk.

Yes. Total momentum of a closed system is conserved in both elastic and inelastic collisions (energy distribution differs).

Impulse unit is N·s (newton-second), equivalent to kg·m/s.

Action: book exerts downward force (weight) on table. Reaction: table exerts equal upward normal force on book.

Rocket expels gas downward (action); gas exerts equal and opposite reaction force upward on rocket, propelling it up.

No. Action and reaction forces are equal in magnitude and opposite in direction (Newton III).

Swimmer pushes water backward (action); water pushes swimmer forward (reaction) enabling forward motion.

Not necessarily; reaction is opposite to action force, not always opposite to motion direction. Context determines effect on motion.

Draw a clear free-body diagram showing all forces, choose a coordinate system and list knowns/unknowns.

Resolving simplifies vector sum calculations and lets you apply F = m·a separately along chosen axes (usually horizontal & vertical).

When problem states 'frictionless' or when frictional forces are negligible compared to other forces; otherwise do not neglect without justification.

Always state sign convention, draw the free-body diagram and convert units to SI before applying F = m·a or momentum relations.