Motion help

[Inside Out]

2.3.      1)a crane on a building site lifts an 800kg load from ground level to a vertical heights of 90m at a constant speednof 2.0m/s. Ignore the mass of the cable. How much work is done by the crane in lifting the load through this distance?         Solution: W=Fx= 800(9.(90)=705600J.   < i dont understand why the acceleration is 9.8 if its at a constant speed wouldnt it mean theres 0 acceleration? Im confused. Please help

[chocolatedaddy]

work being exerted by gravity?

[b^3]

The force required to lift the mass up will be F=mg=800*9.8.
I.e. work must be done to life the mass against gravity.
Then W=Fx=800*9.8*90=705600J

Also note this is equivalent to using W=E_p=mgh (i.e. gravitational potential energy)

[chocolatedaddy]

The force required to lift the mass up will be F=mg=800*9.8.
I.e. work must be done to life the mass against gravity.
Then W=Fx=800*9.8*90=705600J

Also note this is equivalent to using W=mgh (i.e. gravitational potential energy)

This

[Inside Out]

I still dont get it 

[Lasercookie]

I still dont get it 

The load has a weight. This is given by (which is where g comes in). If you were to provide it with a force less than 7840N, you wouldn't be able to lift it up.

[b^3]

To lift the mass you must apply a force to it to oppose the force of gravity acting downwards. When they are equal as in this situation then the forces will cancel and the acceleration will be 0m/s^2, even though the mass is moving upwards.

Now work is the amoutn of energy transfered by that force moving the mass over a distance. I.e. W=Fx.

Now this Force is the force that is acting against gravity, i.e. F=mg=800*9.8 N

Then W=Fx=800*9.8*90 J

EDIT: Turns out I can't spell "lift" (I spelt it life)

[Inside Out]

Ahh! So even though the net force is zero you're only suppose to take into consideration ONLY the force that is actually going through displacement (i.e. Work)? I think i get it now thanks