# Emergency Homework Help: Forces

#### 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

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

#### 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?

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.

#### viola_85

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

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.

#### ShawnD

I think cookie is saying this

F = (4500)(9.8 + 2)

#### Chen

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

#### ShawnD

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

#### 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.

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