# Another Basic Question About Force

1. Oct 29, 2009

### jrbillbrian

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

A helicopter with a mass of 2090 kilograms accelerates up at 3.13 meters per second per second. What lift force is exerted by the air on the propellers?

m = 2090 kg
a = 3.13 m/s/s

2. Relevant equations

F = ma ?

3. The attempt at a solution
Force = 2090 kg * 3.13 m/s/s = 6541.7 N

Which is apparently incorrect, then I thought I might have read it wrong and I tried -6541.7, which is also wrong. Can anyone tell me what I am doing wrong? Did I forget to factor something in?

2. Oct 29, 2009

### Jasso

What would be the lift force if the helicopter were just hovering?

3. Oct 29, 2009

### jrbillbrian

2090 kg * 9.8 ( 9.8 N in 1 kg)
= 20482 N?

4. Oct 29, 2009

### jrbillbrian

Or would it just be 0? I'm very confused.

5. Oct 29, 2009

### Jasso

20482 N is correct.

Ftotal = Flift + mg

The lifting force pushing up on the propellers must be of equal to the weight of the helicopter in the opposite direction as gravity when it hovers (when Ftotal = 0).

So what would the lifting force be if it accelerated upwards as your original question stated?

6. Oct 29, 2009

### jrbillbrian

Ftotal = Flift + mg
Ftotal = 6541.7 N + 2090 * 9.8?
Ftotal = 27023.7 ?

I'm just so confused.

7. Oct 29, 2009

### Jasso

Two points here:

1.) Acceleration due to gravity (g) is -9.8 m/s2, you forgot that it's negative since it acts downwards.

2.) The acceleration of the helicopter is cause by the total force acting on it, not just the lifting force.

So what force would the lift have to be so that the total force accelerates the helicopter upward at 3.13 m/s2?

8. Oct 29, 2009

### jrbillbrian

so 6541.7 N + (2090 kg * -9.8)
= -13940.3 N

So you would need 13940.3 N?