# What is the lift force exerted by the air on the rotors?

1. Dec 11, 2005

### thschica

A 6520 kg helicopter accelerates upward at 0.57 m/s2 while lifting a 1190 kg car.
(a) What is the lift force exerted by the air on the rotors?
_________ N
(b) What is the tension in the cable (ignore its mass) that connects the car to the helicopter?
_____N

I have no idea how to do this and it is going to be on my mid-term.
Can someone please help me because I keep getting very strange numbers.
I used a kinetics equation but the answer was wrong!

2. Dec 11, 2005

### daniel_i_l

Use Newtons second law: F=ma.
For (a) find out how much force is needed to accelerate both of the masses. The tension only accelerates the car.

3. Dec 11, 2005

### lando45

If you apply Newton's Second Law to part (a) you can find the answer relatively easily; for the M value, just make sure you use the composite mass. You know a, as it's given in the question, so F can easily be solved. I think for part (b) you can just use Newton's Second Law, but remember the only mass involved is that of the suspended car. The acceleration should remain the same though...

4. Dec 11, 2005

### thschica

should a) be 4394.7
and b) be 3716.4