Dynamics: Forces and Motion: quick revision, 15 bites

Dynamics: Why Objects Move

Forces don't just push — they change motion.

**Dynamics** studies motion *and* its causes. Unlike kinematics (which only describes motion), dynamics asks **why** an object accelerates.

Takeaway. Dynamics links force → mass → acceleration to explain every motion.

What Is a Force?

Force is a push or pull — with direction.

Takeaway. Force is a vector — direction matters as much as size.

Newton's Second Law

The master equation of dynamics.

Acceleration is **directly proportional** to net force and **inversely proportional** to mass.

Takeaway. Same force, more mass → less acceleration.

Newton's Three Laws at a Glance

Three laws explain all motion.

Takeaway. Together, the three laws predict any classical motion.

Motion Doesn't Need a Force

Constant velocity ≠ zero force? Careful.

Takeaway. Force changes motion; it doesn't maintain it.

Seatbelts: Newton's First Law in Action

Why your body keeps moving in a crash.

Takeaway. Inertia explains why seatbelts save lives.

Momentum and Its Change

Momentum: mass in motion.

Takeaway. Force is really the rate of change of momentum.

Solving a Dynamics Problem

Five steps to any force problem.

Takeaway. Free-body diagram first — always.

Common Forces You'll Meet

Know your forces by name.

Takeaway. Naming forces correctly is half the solution.

Mass vs Weight

Same you, different planet, different weight.

Takeaway. Mass is intrinsic; weight depends on gravity.

Friction Force

The force that always opposes sliding.

Takeaway. Friction scales with normal force, not with speed.

Rockets: Newton's Third Law in Space

Push gas down, rocket goes up.

Takeaway. Action-reaction pairs work without any medium.

Remember the Three Laws

One line each — never forget.

Takeaway. Simple phrases lock the laws into memory.

Exam Trap: Net Force Only

Use net force, not any single force.

Takeaway. F = ma uses the *net* force — never a single force alone.

Newton Wrote It in a Plague Year

The laws were born in isolation.

Newton formulated his laws of motion in **1687** in *Principia Mathematica* — largely during a plague-driven retreat from Cambridge.

Takeaway. Some of physics' biggest ideas came from quiet, focused time.