Dabour Hub EGEgyptian Curriculum 2026/2027
5 Bodies connected by a string over a smooth pulley

5 Bodies connected by a string over a smooth pulley

Point 5 of 10 — Lesson 1: Momentum, Newtons Three Laws of Motion, Impulse and EnergyQuestion bank: 4 questions (4 with pictures)

05 5 Bodies connected by a string over a smooth pulley

Two vertical masses

  • When the pulley is small and smooth and the string is light and inextensible, the tension is the same on both sides.
  • Let m1>m2. Equations of motion:  m1a=m1g−T and m2a=T−m2g. a=m1−m2m1+m2 g,T=2m1m2m1+m2 g
  • The pressure on the pulley axis is P=2T.
  • If the masses start level, after time t each has moved s=12at2 and the vertical distance between them is 2s.

Special cases

  • Equal masses: a=0 and the system stays at rest or moves with uniform velocity.
  • Extra mass m3 added to one side: (m1+m3)a=(m1+m3)g−T and m2a=T−m2g.
  • Extra mass removed after t seconds: the system keeps moving in the same direction with the velocity gained, but with the new (smaller) acceleration, which may be a deceleration.
  • String cut after t seconds: each body continues in its direction with the velocity gained and then moves under gravity alone (a=±g).
  • If the heavier side is unknown and the system returns to its start, m1<m2; if it moves uniformly, m1=m2.
✍️ Worked examples

Example 14: Two masses start level and after 1 s the vertical distance between them is 20 cm. Find m1:m2 (m1>m2).

Solution:

  • Each moved 10 cm: 10=12a(1)2, so a=20 cm/s2.
  • a=m1−m2m1+m2 g, so 20980=m1−m2m1+m2.
  • m1+m2=49m1−49m2, so 50m2=48m1.
  • m1:m2=25:24.

📝 Question bank for this point

4 questions linked to this point only, 4 of them with pictures.