Simple Free Body Diagram question

AI Thread Summary
A user seeks help with a physics problem involving a helicopter lifting a car, specifically calculating the lift force exerted by the air on the rotors. They initially misrepresented the direction of the helicopter's weight in their free body diagram, mistakenly suggesting it pointed upward. Clarification reveals that both the helicopter's and car's weights act downward, while the lift force from the air acts upward. The correct approach involves recognizing that the net force is the lift force minus the combined weight of the helicopter and car, leading to an upward net force due to the helicopter's acceleration. The user acknowledges their misunderstanding and appreciates the guidance provided.
psycovic23
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I have a question in my textbook that I can't answer for the life of me.

"A 6500-kg helicopter accelerates upward at .6 m/s^2 while lifting a 1200-kg car. (a) what is the lift force exerted by the air on the rotors?"

Now, my diagram has a vector pointing up with a force of (6500*9.8)N. There is also a vector pointing down at (1200*9.8)N. Wouldn't the net force then be equal to the force of the pull minus the force of the weight? Fn-(6500*9.8)-(1200*9.8) = net force

Or am I completely wrong? At the moment, having read this section multiple times, I'm still having trouble grasping this whole natural force and the summation of the forces = ma. My teacher doesn't really help much as she isn't the greatest teacher either...
 
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weight acts down

psycovic23 said:
Now, my diagram has a vector pointing up with a force of (6500*9.8)N.
Why would the weight of the helicopter point up?
There is also a vector pointing down at (1200*9.8)N.
Right.
Wouldn't the net force then be equal to the force of the pull minus the force of the weight? Fn-(6500*9.8)-(1200*9.8) = net force
Yes, if by Fn you mean the force of the air on the rotors. (It looks like you changed your mind about the direction of the helicopter's weight!)


Your force diagram should show the weight of both acting down, and the force of the air acting up. The net force acts up, since we know the acceleration is up.
 
Oooohhhhhhhh...I see what I did wrong! For some reason, I was thinking the weight of the helicopter was...um...tension up or something. I don't know, but thank you very much!
 

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