Emergency Homework Help: Forces

1. Apr 4, 2004

viola_85

A 4500-kg helicopter accelerates upward at 2m/s^2. What lift force is exerted by the air on the propellers?

Is the answer just Newtons Second Law of Motion F=ma, so F=(4500kg)*(2)
, so F= 9000N

2. Apr 4, 2004

Don't forget that gravity is also acting on the helicopter, and the propellers must overcome the force of gravity as well.

3. Apr 4, 2004

viola_85

so would I have to convert the initial mass into weight. i.e. W=MG

(4500)*(9.8)=44100
44100*2=88200N

Is that what I would have to do?

4. Apr 4, 2004

The lift must be greater than the weight enough to accelerate the helicopter 2 m/s^2.

F = aF is an equation you just made up and has no meaning. Try using Newton's 2nd Law.

5. Apr 4, 2004

viola_85

But Newton's Second Law is Force = Mass times Acceleration. Which I used in the first place.

6. Apr 5, 2004

Yes, you did a good job of finding the net force on the helicopter. But that's not the lift because you're not taking into consideration gravity.

7. Apr 5, 2004

ShawnD

I think cookie is saying this

F = (4500)(9.8 + 2)

8. Apr 5, 2004

Chen

The law is:
$$\Sigma F = ma$$
But the lift force is not the only one acting on the helicopter:
$$\Sigma F = F_{lift} - mg$$

9. Apr 5, 2004

ShawnD

Although technically correct, your post can cause a great deal of confusion for somebody who doesn't fully understand the concept.

If I'm understanding correctly, you are saying this.
ma = F - mg
F = ma + mg
F = m(a+g) <----- like my post said

10. Apr 5, 2004

Chen

Yes, and why would my post cause confusion? Your post confuses me. I just thought it would be better to show how to get to the solution you posted, so that in future problems he can do the same.