What is the lift problem?

• The?lift?problem?involves objects (particles) that are?directly?in?contact?with each other – typically a?person?or?crate?in a?lift
• If it is not a person in the lift the object is often referred to as a?load
• There may be?more?than?two?objects?involved – for example?two?crates?stacked on top of each other on a lift floor
• Vertical?motion is involved so use?g?m s-2, the?acceleration?due?to?gravity, where appropriate
  • Gravity always acts?vertically?downwards
  • Depending on the?positive?direction chosen - and which other forces are acting vertically - acceleration (a?m s-1)?may be?positive?or?negative
  • Round?final?answers based on the number of?significant?figures?used for?g
    • if?g?= 10 m s-2, round to 1 s.f.
    • if?g?= 9.8 m s-2,?round to 2 s.f.
  • Remember that?acceleration?links?F = ma(N2L) and the ‘suvat’ equations

How do I solve ‘lift problem’ type questions?

• • Lift problems will only consider motion in the?vertical?direction
  • As motion is involved?Newton’s?Laws?of?Motion?apply so use “F = ma” (N2L)
  • The steps for solving lift problems are the same as for solving?rope problems
  • As?both?the?lift?and?load?are travelling in the?same?direction the system can be?treated as one?particle (as well as separate particles)
    • There is no?reaction?force?acting on the?lift?or?load?when treating the particle as one - mathematically they cancel each other out
    • You can think of the upward as counteracting the person’s weight?and?moving the load upwards; N3L applies so there must be an equal force acting in the opposite direction; - you can think of this as the force keeping the person in contact with the lift floor whilst it is moving
  • For?constant?acceleration?the ‘suvat’ equations could be involved

How do we form the equations for problems involving tow bars and ropes?

• Form the equations as follows:
  • Treating the lift and person/load as one

(↓)?(M + m)g - T = (M + m)a

• • Treating the lift and person/load separately

Lift:?(↓)?(Mg + R) - T = Ma

Person/load:?(↓)?mg - R = ma

• You do not necessarily need all equations but if in doubt attempt all and it may help you make progress

(a)? Briefly explain how the force of 800g N arises in this problem.

(b)? Find the mass of the load,?m?kg .

(c)? Find the tension, T N, in the cable of the lift.

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