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-2) may be?positive?or?negative

   

• 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 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.