# Homework Help: Ap physics c

1. Sep 12, 2015

1. The problem statement, all variables and given/known data
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

2. Relevant equations
F=ma

3. 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

2. Sep 12, 2015

### paisiello2

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

Last edited: Sep 12, 2015
3. Sep 12, 2015

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

4. Sep 12, 2015

### paisiello2

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

5. Sep 12, 2015

ohh u mean my 9.8 should be negative lol my bad

6. Sep 12, 2015

### paisiello2

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

7. Sep 12, 2015

### 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.