Help with Tension Homework: Calculate Net Upward Force & Tension

[saber1357]

Homework Statement

A 15,000 kg helicopter lifts a 4500 kg truck with an upward acceleration of 1.4 m/s2. Calculate (a) the net upward force on the helicopter blades from the air and (b) the tension in the cable between helicopter and truck.

Homework Equations

F=ma
F = (m1+m2)a

The Attempt at a Solution

I do not know how to even begin. Calculating the net upward force, I understand that an upward acceleration is given and that can be simply plugged into F=ma, however, what about gravity?

 

[Doc Al]

In F=ma, F stands for the net force. What forces act on the helicopter? I count three.

 

[saber1357]

Aha! So you can simply add the force of gravity, upward acceleration, and tension together?

 

[Doc Al]

The "upward acceleration" is not a force, but is the result of all those forces acting. You mentioned gravity and tension--what's the third force on the helicopter? (Yes you can add up all the forces.)

 

[saber1357]

Ah woah, if the upward acceleration doesn't produce the third force, then what does?

 

[Doc Al]

Ah woah, if the upward acceleration doesn't produce the third force, then what does?

The air! (Acceleration is not a force!) As a result of those three forces, there's a net force which produces an upward acceleration. Obviously, the upward force of the air must be larger than the sum of the two downward forces.

You might also want to analyze the forces on the truck.

 

[saber1357]

But isn't a force created due to the upward acceleration..? F=ma, with "a" being this upward acceleration?
Do the forces have to be analyzed separately for the truck? I thought I could just add the mass of the truck to the helicopter.

 

[Doc Al]

But isn't a force created due to the upward acceleration..? F=ma, with "a" being this upward acceleration?

The fact that there's an upward acceleration allows you to deduce that there's a net upward force on the helicopter. That net force is due to the actual forces acting on the system. (Real forces have agents: things that exert the force.)

Do the forces have to be analyzed separately for the truck? I thought I could just add the mass of the truck to the helicopter.

You are welcome to treat the helicopter + truck as a single combined "object", but that will not allow you to figure out the tension. But you can use it to find the force due to the air. (Realize that the rope tension acts on both helicopter and truck, so if you treat them as a single object, the tension cancels out.)

 

[saber1357]

Gotcha! I really appreciate all the help you provided.
You are the thx.
<3
(If you ever need help finding someone to ask you loads of physics questions, I'm your man)