Mechanical Science

mpc4000 · Feb 4, 2011

just been given this query to do for my level 2330 any help would be much appreciated thanks

An electric hoist on the side of a building 45m lifts a quantity of electrical equipment through a vertical distance in 3.5 minutes. The motor rating of the hoist is 189.19w. Calculate the MASS of the LOAD.

I'm struggling to work this out i know the ans is nearer to 90000 but just dont know how to work it out.

any help will be much appreciated

Thanks

telectrix · Feb 4, 2011

don't forget to factor in the weight of the transit pick-up into which the pikeys have loaded all the electrical equipment whilst you have your nose buried in your calculator

j.mawer · Feb 4, 2011

Don't know how you get to 90000 (which is 90 tonnes so its a pretty big amount of kit) But I would have thought from O level physics which was a few years ago
Change in energy of load
energy = force * distance (force = mass * acceleration. acceleration =gravity = 10m/s^2)
= 10* mass * 45
= 450*mass J

For the motor
ENergy = power * time
= 189.19 * (3.5*60)
= 39729.9J

Now assuming 100% efficiency and the cable on the lift weighs nothing the energy goes from the motor to the load and nowhere else so (no noise no heat)
Energy motor = energy change of load
39729.9 = 450 * mass
mass=39730 / 450
= 88.3Kg

pushrod · Feb 4, 2011

yeah agree with above approx, 88 kgs - did it slightly differently though, depends on what you take as the number of newtons in a kg

work done = energy

energy = power x time(secs)

work = force (n) x distance (m)

so force = (power x time)/distance

then convert newtons to kg (approx 10 newton per kg)

mpc4000 · Feb 5, 2011

thanks very much for that all i think i got a bit lost with the energy formula but its all good now thanks again

i=p/u · Feb 5, 2011

good stuff

johnboy6083 · Feb 6, 2011

newtons per kg is 9.81 by the way fellas.

just anothe sad fact that i happen to remember!

just been doing some crash course revision on the above, and the answer i get is 89.95N

j.mawer · Feb 6, 2011

To be picky that depends on gravity which is not a universal constant. For example if you went to the equator where the earth bulges then gravity is less and so Netwtons per kilogram is less (or go to the moon 1Kg gives nowhere near 9.81N force) . In my original post I stated acceleration due to gravity was being taken as 10m/s^2 and therefore defined 1Kg to provide a force of 10N and all said all loses were ignored so I would argue it is actually more technically correct as it explained all the assumptions (maybe that's just a physics versus engineering difference)

Having said that you are correct that 9.81N is a much more accurate estimate and in the age of a calculator ...

I think anyone who deducted marks for either answer would be wrong to do so given all the other assumptions made along the way!

pushrod · Feb 6, 2011

johnboy6083 said:
newtons per kg is 9.81 by the way fellas.

just anothe sad fact that i happen to remember!

just been doing some crash course revision on the above, and the answer i get is 89.95N

Question asks for mass so answer must be in kg - Newton is a unit of force (load/weight)