Please help with tension

[charevans]

Hi, can someone please explain to me how you work out tension. Here is an example of the problems I have.

A lift of mass 480kg is designed to carry a maximum load of 3000N. Calculate the tension in the lift cable at maximum load when:

a)The lift moves down at a steady speed.

b)The lift moves downward, accelerating at 1.5ms-2

c)The lift moves upward, accelerating at 1.5ms-2

Thank you so much!

[insane_alien]

so this is a home work question yeah?

well, your not going to just get an answer. we'll still make you think about it.

 

a useful way of envisioning the question is to draw a freebody diagram. Do you know how do draw these? is pretty simple in this case and i'll assume you don't know how to draw one to save me posting again.

 

start of with a nice big dot. this is your lift. then draw arrows which will represent the forces. the arrows MUST point in the direction of the force. but the size doesn't matter too much as long as you label them.

 

so my first question to you is, what forces are acting upon the lift?

 

i'll give you a hint, there are two. (although, it might be considered 3 but we'll get to that later. and if you don't come up with 3 forces(or the wrong 3), i'll explain it to you.

 

I hope this starts you off.

[charevans]

are the forces gravity (downwards) and suppourt (upwards)

[insane_alien]

that'll do. the upwards force is actually tension in the cables holding it up.

 

What i said about 3 forces is that the weight of the lift and the weight of the load could be considered seperate forces. but for this situation we can take them as a single combined force.

 

Now, you need to calculate the total weight of the lift and its load. you know how to do this yeah? Weight=mass*g

[charevans]

yea got the mass of the lift 4708.8N

[insane_alien]

now add the maximum load. tot get the total weight.

[charevans]

whoops mean weight

[charevans]

yea 7708.8

[insane_alien]

okay. this force is constantly acting upon the lift so it will stay the same in all situations.

 

if you use F=ma then you can calculate the total force acting upon the lift in all three scenarios.

 

you can also say that the total force F is equal to the tension force plus the weight (the forces act in opposite directions so one will be a negative number)

 

so you can say that tension = F - weight

 

should be easy from there.

[charevans]

thank you so much.

[insane_alien]

no problem. its one of the many purposes of this forum. stick around. you might learn other things.