Motion and Laws of Motion

For uniformly accelerated motion, the three equations are: (1) v = u + at, (2) s = ut + (1/2)at^2, (3) v^2 = u^2 + 2as. Here u = initial velocity, v = final velocity, a = acceleration, t = time, s = displacement. Memory aid: 'V-U-A-S' — each equation links four of the five variables, leaving one out. Equation 1 omits s, equation 2 omits v, equation 3 omits t. For a freely falling body, a = g = 9.8 m/s^2 (often taken as 10). For a body thrown up, a = -g. SI unit of acceleration is m/s^2. These are valid ONLY when acceleration is constant.

First Law (Law of Inertia): A body stays at rest or in uniform motion unless an external force acts on it. Explains why passengers jerk forward when a bus brakes. Second Law: Force = mass x acceleration (F = ma); rate of change of momentum equals applied force. Third Law: To every action there is an equal and opposite reaction — e.g. recoil of a gun, rocket propulsion, swimming. Memory aid: 'Inertia, Force, Reaction' = 1-2-3. SI unit of force is newton (N); 1 N = 1 kg.m/s^2. The second law is the most quantitative and is the real definition of force.

A car moving at 20 m/s decelerates at 4 m/s^2. Find the distance before it stops. Using v^2 = u^2 + 2as with v = 0, u = 20, a = -4: 0 = 400 + 2(-4)s, so 8s = 400, giving s = 50 m. Shortcut for stopping distance: s = u^2 / (2a) = 400/8 = 50 m. Time to stop: v = u + at gives 0 = 20 - 4t, t = 5 s. Note: stopping distance is proportional to the SQUARE of speed — doubling speed quadruples the distance, a key road-safety fact often asked in RPF exams.

Cover the answer, recall, then check. 12 cards on the must-know facts of motion for RPF SI.

Q1. Difference between distance and displacement?
A1. Distance is total path length (scalar, always positive); displacement is the shortest straight-line change in position (vector, can be zero or negative). Both have SI unit metre.

Q2. State the three equations of motion for uniform acceleration.
A2. v = u + at; s = ut + ½at²; v² = u² + 2as (u = initial velocity, v = final, a = acceleration, s = distance, t = time).

Q3. State Newton's First Law of Motion.
A3. A body stays at rest or in uniform motion in a straight line unless acted on by an external unbalanced force. Also called the Law of Inertia.

Q4. State Newton's Second Law and give the SI unit of force.
A4. Force = rate of change of momentum; F = ma. SI unit is the newton (N); 1 N = 1 kg·m/s².

Q5. State Newton's Third Law with one example.
A5. To every action there is an equal and opposite reaction. Example: recoil of a gun, or a rocket moving up as gases are pushed down.

Q6. What is inertia, and on what does it depend?
A6. The tendency of a body to resist a change in its state of rest or motion. It depends only on the body's mass — more mass means more inertia.

Q7. Define momentum and give its SI unit.
A7. Momentum = mass × velocity (p = mv). It is a vector. SI unit is kg·m/s. Momentum is conserved in the absence of external force.

Q8. Why do passengers jerk forward when a moving bus suddenly stops?
A8. Due to inertia of motion — the lower body stops with the bus but the upper body tends to keep moving forward.

Q9. What is the acceleration due to gravity (g) near the Earth's surface?
A9. About 9.8 m/s² (often taken as 10 m/s² in calculations). It acts downward and is independent of the body's mass.

Q10. State the law of conservation of momentum.
A10. In an isolated system (no external force), total momentum before an event equals total momentum after it. It explains gun recoil and rocket propulsion.

Q11. What is friction and name its types?
A11. A force that opposes relative motion between surfaces in contact. Types: static, sliding (kinetic), and rolling friction; rolling friction is the smallest.

Q12. Which physical quantity does the second law use to define force — and what stays constant in uniform circular motion?
A12. Force is defined via rate of change of momentum. In uniform circular motion, speed stays constant but velocity keeps changing (direction changes), so there is acceleration (centripetal).

Motion is the single highest-yield Physics chapter for RPF SI General Science, typically worth 2–4 marks. Questions are conceptual and formula-based — Newton's three laws, inertia, momentum and the equations of motion appear almost every year. Master this and you lock in easy, predictable marks.

Why it matters

Everyday railway situations — a jerk when a train starts or stops, the recoil of a coupling, why heavier wagons are harder to move — all apply these laws. Examiners love one-line conceptual questions here.

Core facts and laws

• Distance is a scalar (total path); displacement is a vector (shortest path). SI unit: metre.
• Speed = distance/time; velocity = displacement/time; acceleration = change in velocity/time (m/s²).
• Newton's laws: 1st = inertia; 2nd = F = ma; 3rd = action–reaction (equal and opposite).
• Momentum p = mv (kg·m/s), and it is conserved when no external force acts.
• g ≈ 9.8 m/s², independent of mass.

Quantity	Formula	SI unit
Velocity	displacement/time	m/s
Acceleration	Δv/time	m/s²
Force	m × a	newton (N)
Momentum	m × v	kg·m/s

Exam Tricks & Tips

• 🎯 "1-2-3" mnemonic: 1st law = Inertia, 2nd law = F=ma, 3rd law = action–reaction. Remember the order and you never mix them up.
• 🎯 Recoil, rocket, jet, swimming, walking are ALL Newton's 3rd law + momentum conservation — spot the keyword and pick that answer.
• 🎯 Inertia depends only on MASS, never on speed. If an option says "inertia depends on velocity", reject it.
• 🎯 v = u + at, s = ut + ½at², v² = u² + 2as — memorise as "V-U-A / S-U-half-A / V²-U²-2AS". The equation without time is v² = u² + 2as.
• 🎯 Unit of force = newton = kg·m/s²; a 1 kg body needs 1 N to accelerate at 1 m/s².
• ❌ Common mistake: thinking a body in uniform circular motion has constant velocity. Speed is constant, but velocity (direction) changes, so it is accelerating.

Expected exam pattern

Expect direct one-liners: "Which law explains gun recoil?", "SI unit of momentum?", "Inertia depends on?", or a small numerical using v = u + at. Usually 2–4 questions, all scoring if the laws and units are memorised.

Quick recap

Newton's three laws (inertia, F=ma, action–reaction), momentum = mv and its conservation, the three equations of motion, and g ≈ 9.8 m/s². Distance is scalar, displacement is vector. These few facts cover almost every question set from this topic.