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1.4.3States of Matter

Rate of Change of Momentum — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 4 of 30 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Force and Momentum

Newton's second law can be written in terms of momentum, which links force to how quickly momentum changes.

Force as rate of change
  • Force as rate of change The resultant force equals the change in momentum divided by the time taken, F = Δp ÷ Δt, which matches F = m × a.
Change in momentum (Δp)
  • Change in momentum (Δp) The final momentum minus the initial momentum, measured in kg m/s, with its sign showing the direction.
Contact time (Δt)
  • Contact time (Δt) The time between two objects first touching and the moment they lose contact or come to rest.
Rate of change
  • Rate of change How quickly momentum changes, so a larger rate of change means a larger resultant force.

Part 2 of 5Calculating the Force

Signs are needed, because a rebound reverses the direction of the momentum.

1
Calculate the initial momentum using p = m × v, with the chosen positive direction.
2
Calculate the final momentum, remembering that a rebound gives a negative velocity in the positive direction.
3
Find Δp as the final momentum minus the initial momentum, keeping the sign of the answer.
4
Divide Δp by the contact time to find the force, and state its direction from the sign.

Part 3 of 5Reducing Impact Force

Reducing Impact Force
Longer contact time
  • Longer contact time For the same change in momentum, a longer contact time gives a smaller force.
Crumple zones
  • Crumple zones Crumple zones deform in a controlled way, which lengthens the time the car takes to come to rest.
Airbags and seat belts
  • Airbags and seat belts They stretch or cushion the passenger, increasing the time over which the passenger's momentum falls to zero.
Crash mats
  • Crash mats A thick, soft mat increases the contact time with a falling person, reducing the force on them.

Part 4 of 5Worked Example

Question

  • A 0.06 kg ball hits a wall at 8 m/s and rebounds at 6 m/s.
  • The contact time with the wall is 0.02 s.
  • Calculate the average force on the ball and state its direction.

  1. Take the initial direction, towards the wall, as positive.
  2. Initial momentumpi = 0.06 × 8 = +0.48 kg m/s.
  3. Final momentumpf = 0.06 × (−6) = −0.36 kg m/s.
  4. Δp = pf − pi = −0.36 − 0.48 = −0.84 kg m/s.
  5. F = Δp ÷ Δt = −0.84 ÷ 0.02 = −42 N.

Answer The average force on the ball is 42 N, acting away from the wall, opposite to its initial motion.

Part 5 of 5Exam Checklist & Tips

1.4.3 – Key Points for the Exam

  • Rate of change of momentum The change in momentum divided by the time taken, which equals the resultant force.
  • Change in momentum (Δp) The final momentum minus the initial momentum.
  • Contact time The time for which two colliding objects are in contact.
  • Crumple zone A part of a vehicle designed to deform in a controlled way during a collision.
  • Newton (N) The unit of force, equal to 1 kg m/s².

Common Mistakes

  • Using the mass alone as the change in momentum; Δp depends on the velocities as well.
  • Forgetting the sign for a rebound, which gives the wrong direction for the force.
  • Stating that airbags reduce the momentum of the passenger;
    • they reduce the force by increasing the time taken for the momentum to change.
  • Describing contact time as the time in the air before the impact;
    • it is the time when the objects are in contact.

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