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1.2.3Tools and Inquiry — IB Physics skills

Newton’s Second Law — IB Physics SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 24 of 33 Unit quiz

📚 IB Physics SL Hub / Topical Revision
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Curriculum Equation Key قوانين نيوتن للحركة - Newton Laws of Motion FormulasFast Exam Solution
F = m × a | W = m × g | F₁₂ = -F₂₁
Word Equation & Conditions: القوة المحصلة = الكتلة × التسارع | Force = Mass × Acceleration
Location & Specifications: الميكانيكا الكلاسيكية والتحصيلي وفيزياء 3 ثانوي و IGCSE Physics.
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Part 1 of 5Statement of the Law

The second law links the resultant force to the change in motion that it produces.

Momentum form
  • Momentum form The resultant force on an object equals the rate of change of its momentum, F = Δp/Δt.
    What the symbols mean: F = force (N) · Δt = time the force acts (s) · Δp = change in momentum (kg m s−1); FΔt is the impulse (N s)
Constant mass
  • Constant mass For a constant mass this becomes F = ma, where a is in the same direction as the resultant force.
Units
  • Units One newton is the force that gives a mass of 1 kg an acceleration of 1 m s⁻², so 1 N = 1 kg m s⁻².
Weight
  • Weight Weight W = mg is the gravitational force on a mass m, where g is the gravitational field strength.

Part 2 of 5Solving Problems

1
Draw a free-body diagram and choose a positive direction along the direction of the acceleration.
2
Add the forces along that direction, taking forces in the opposite direction as negative, to find the resultant force.
3
Substitute into F = ma, or into F = Δp/Δt where the momentum change is given or the mass changes.
4
Solve for the unknown, and for two connected objects apply the law to each object separately.
5
Check that the units and the sign of the answer are consistent with the direction of motion.

Part 3 of 5Understanding the Relationship

Understanding the Relationship
Proportional to force
  • Proportional to force At constant mass, doubling the resultant force doubles the acceleration.
Inversely to mass
  • Inversely to mass At constant force, doubling the mass halves the acceleration.
Direction
  • Direction The acceleration has the same direction as the resultant force, not necessarily the direction of the velocity.
Impulse
  • Impulse Rearranging gives FΔt = Δp, so a force acting for a time changes the momentum by that amount.
ChangeEffect on acceleration
Force doubled, same massDoubles
Mass doubled, same forceHalves
Both doubledUnchanged

Part 4 of 5Worked Example

Question

  • A car of mass 1200 kg has a driving force of 4500 N and resistive forces of 1500 N.
  • Calculate its acceleration, and the change in momentum in 5.0 s.

  1. Resultant force = 4500 − 1500 = 3000 N.
  2. a = F/m = 3000 / 1200 = 2.5 m s⁻².
  3. Δp = FΔt = 3000 × 5.0 = 15 000 kg m s⁻¹.

Answer a = 2.5 m s⁻² and Δp = 1.5 × 10⁴ kg m s⁻¹.

Part 5 of 5Exam Checklist & Tips

1.2.3 – Key Points for the Exam

  • Resultant force The net force on an object found by vector addition of all forces.
  • Momentum The product of mass and velocity, p = mv.
  • Newton The unit of force, equal to 1 kg m s⁻².
  • Weight The gravitational force acting on an object, W = mg.
    What the symbols mean: W = weight (N) · m = mass (kg) · g = gravitational field strength (9.81 N kg−1 on Earth)

Common Mistakes

  • Using one of the forces rather than the resultant force in F = ma.
  • Confusing mass, measured in kilograms, with weight, measured in newtons.
  • Assuming the acceleration is always in the direction of motion.
  • Forgetting to add the opposing resistive force, such as drag or friction, to the free-body diagram.

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