AQA A Level Maths: Mechanics复习笔记3.2.4 Connected Bodies - Pulleys

• In all pulley questions the particles are moving in different directions so it is best if they are considered separately as opposed to treating them as one object
• If a particle is in motion in the direction being considered then Newton’s Laws of Motion apply so use “F = ma” (N2L)
• For constant acceleration the ‘suvat’ equations could be involved
• Step 1. Draw a series of diagrams
  • • Label the forces and the positive direction of motion for each particle.
    • Colour coding forces acting on each particle may help.

• Step 2. Write equations of motion, using “F = ma"
  • • Equations 1 and 2: Treating each particle separately

(↑) T - m1g = m1a
(↓) m2g - T = m2a

• Step 3. Solve the relevant equation(s) and answer the question
  • • Some trickier problems may lead to simultaneous equations

• Sketch a diagram or add to a diagram given in a question.
• All pulleys are smooth and in most (but not all) the pulley itself can be ignored.
• In pulley questions the particles will be moving in different directions - so each particle will need to be considered separately.
• If one particle is on a horizontal surface (such as a desk or table) then the weight only need be considered if friction is involved (since F = μR and R is related to weight).
• Unless told otherwise, use g = 9.8 m s-2 and round your final answer to two significant figures.
• Some questions may direct you to use g = 10 m s-2 in which case round your final answer to one significant figure.