Dabour Hub EGEgyptian Curriculum 2026/2027
2-1 (f) Momentum and a variable mass

2-1 (f) Momentum and a variable mass

Point 6 of 7 — Motion with Variable Acceleration and ForceQuestion bank: 0 questions (0 with pictures)

06 2-1 (f) Momentum and a variable mass

Rules

  • The momentum is H⃗=mv⃗ and the force is F⃗=dH⃗dt=mdv⃗dt+v⃗dmdt.
  • If the velocity is constant and the mass changes, F=vdmdt.
  • If the mass is constant, F=ma.
  • A constant rate of gain or loss gives m=m0+dmdt t.
✍️ Worked examples

Example 16: A body of mass m=(2+t) kg moves with velocity v=3t m/s along a line. Find the momentum and the force at t=2.

Solution:

  • H=mv=(2+t)(3t)=6t+3t2.
  • F=dHdt=6+6t; at t=2, F=18 N.
  • Check by the product rule: mdvdt+vdmdt=(4)(3)+(6)(1)=18 N.

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