# Formula help (1 Viewer)

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can someone give me the formula needed to work this problem?

A 4655-kg helicopter accelerated upward at 8 m/s2. What lift force is exerted by the air on the propellers?

#### chroot

Staff Emeritus
Gold Member
Newton's second law of motion.

- Warren

so i should multiply 4655 and 8 together?

#### chroot

Staff Emeritus
Gold Member
so i should multiply 4655 and 8 together?
This is correct -- but, just to make sure you understand why -- can you tell me what Newton's second law of motion says?

- Warren

net force equals the mass and aceleration.

#### chroot

Staff Emeritus
Gold Member
net force equals the mass and aceleration.
What does the word "and" mean? Why did you use that word?

- Warren

cuz the acceleration is proportonal to the magnitude or something of the force. same goes w/ mass.

but some other person is telling me the answer i got is wrong.

#### chroot

Staff Emeritus
Gold Member
Use the word "times" or "multiplied by" rather than "and."

The question is worded a little awkwardly. The lift force is experienced by the aircraft, not by the air.

Now, you know the net force on the aircraft is equal to its mass times its acceleration. What two forces does the aircraft feel? It feels a gravitational force pulling it down, and a lift force pushing it up. The sum of these two forces is equal to its mass * acceleration. Does this make sense?

- Warren

i kinda do. imma have to probably get a tutor. i dont really get what my teacher is saying, but why is the answer im getting wrong? i got 37240N as the answer.

#### chroot

Staff Emeritus
Gold Member
That is wrong.

Think about it this way: what's the first force felt by the aircraft? Gravity. It pulls the aircraft down. The magnitude of the force is

$$F_g = mg$$

What's the second force felt by the aircraft? The lift. This is the force you're asked to find. Call it $F_L$.

What's the sum of the forces? One pulls down, one pushes up -- they are in opposition. They counteract each other. Let's call the upward force positive, and the downward force negative. The net force is:

\begin{align*} F_{\textrm{net}} &= F_L - F_g\\ &= ma \end{align*}

Can you solve for $F_L$ now?

- Warren

ooo yeah i think i can get it now. lemme try

82859?

im pretty sure i get the hang of it, thanks. BUT what if all they give me is force? how do i find the acceleration w/o the mass?

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