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Motion, Force and Energy
Science
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Overview
Motion is described by speed, velocity and acceleration; Newton’s three laws explain how forces change motion.
Key points
- Speed is scalar; velocity is speed with direction; acceleration = change in velocity ÷ time.
- Equations: v = u + at; s = ut + ½at²; v² = u² + 2as.
- First law: a body stays at rest or in uniform motion unless a net force acts (inertia).
- Second law: F = ma; unit newton.
- Third law: every action has an equal and opposite reaction.
Explanation
- Momentum p = mv; total momentum is conserved when no external force acts.
- Impulse = force × time = change in momentum.
- Free fall: a = g ≈ 9.8 m/s².
- Friction opposes relative motion.
Step by step
- Identify the concept or law involved.
- Recall the definition, unit or formula.
- Check units and the direction of the effect before choosing.
Examples
- A car starts from rest with a = 2 m/s² for 5 s: v = 10 m/s, s = ½ × 2 × 25 = 25 m.
- F = ma: 5 kg × 4 m/s² = 20 N.
- The recoil of a gun is explained by Newton’s third law.
Traps and advanced points
- Trap: Treating velocity as scalar.
- Trap: Thinking a moving body needs a force to keep moving.
- Trap: Forgetting that action and reaction act on different bodies.
Quick revision
- Three equations of motion.
- F = ma.
- P = mv conserved.
Practice MCQs
Which option correctly answers this General Science check: Newton’s third law concerns?
- vacuum
- newton
- ½mv²
- equal and opposite action-reaction forces
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Answer: (d) equal and opposite action-reaction forces. The two forces act on different bodies.
Which option correctly answers this General Science check: basic unit of heredity?
- heart
- gene
- concave lens
- mass per unit volume
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Answer: (b) gene. Genes are hereditary units encoded in DNA.
Which option correctly answers this General Science check: vitamin deficiency classically associated with rickets?
- hertz
- vitamin D
- convex mirror
- beta cells of pancreas
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Answer: (b) vitamin D. Vitamin D is important for calcium/phosphate balance and bone mineralisation.
Speed is calculated as —
- distance × time
- distance ÷ time
- time ÷ distance
- mass × distance
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Answer: (b) distance ÷ time. Speed = distance travelled ÷ time taken.
The SI unit of force is the —
- joule
- newton
- watt
- pascal
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Answer: (b) newton. Force is measured in newtons (N).
Newton’s first law is also called the law of —
- energy
- inertia
- gravitation
- action and reaction
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Answer: (b) inertia. An object stays at rest or in uniform motion unless a net force acts — inertia.
F = ma is Newton’s —
- first law
- second law
- third law
- law of gravitation
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Answer: (b) second law. The second law relates force, mass and acceleration.
A rocket moves upward because of Newton’s —
- second law
- third law
- law of cooling
- first law
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Answer: (b) third law. Exhaust gases push back; the rocket is pushed forward (action–reaction).
Kinetic energy of a body of mass m moving with speed v is —
- mv
- ½mv²
- mgh
- m/v
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Answer: (b) ½mv². KE = ½mv².
Potential energy of a body of mass 2 kg at a height of 5 m (g = 10 m/s²) is —
- 50 J
- 100 J
- 25 J
- 10 J
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Answer: (b) 100 J. PE = mgh = 2 × 10 × 5 = 100 J.
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