Determining Tension & Lift Force for Helicopter & Truck

[avocadoboy]

Homework Statement

A 15000-kg helicopter is lowering a 4000-kg truck by a cable of fixed length. The truck, helicopter, and cable are descending at 15.0 m/s and must be slowed to 5.00 m/s in the next 50.0 m of descent to prevent damaging the truck. Assume a constant rate of slowing.

Determine the tension in the cable.

Determine the lift force of the helicopter blades

Homework Equations

F=ma

The Attempt at a Solution

dont have answers but attempt was to isolate the system of truck
MG-FT=ma
4000*9.8-ft=4000*-2
ft=48000?

force of lift
LF-mg+mg =ma
LF-15000(9.8)+4000(9.8)=15000+4000(-2)
LF=152000

 

[paisiello2]

You isolated the truck but you didn't isolate the helicopter?

Also your signs are off.

 

[avocadoboy]

why do i need to isolate the helicopter to find the lift i used the entire system

 

[paisiello2]

Yes, you're absolutely right. Just fix your signs then.

 

[avocadoboy]

ohh u mean my 9.8 should be negative lol my bad

 

[paisiello2]

No, you used +ve for the truck and -ve for the helicopter.

 

[Feodalherren]

You have your gravity vector pointing in two different directions.

Also, the tension in the cable is the same everywhere, draw a FBD and cut the problem into two halves. In the top half only consider the helicopter, in the bottom half only consider the truck.