# Physics help on plane and equilibrium

#### lilkrazyrae

an airplane is cruising along in a horizontal level flight at a constant velocity, heading due west. (a) If the weight of the plane is 2.6 x 10^4 N what is the net force on the plane? (b) with what force does the air push upward on the plane? (c) Is the plane in equilibrium? If so, what kind of equilibrium?

I got answers but i am not too sure on them so if someone could check that would help. For (a) i got 2.6 x 10^4 west. for (b) i got gravity 9.8 N/kg
for (c) i got dynamic equilibrium. Any help with this matter would be greatly appreciated!!!

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#### quasar987

Science Advisor
Homework Helper
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(a) Is the plane accelerated in any way???

(b) a force of equal magnitude as that of gravity, yes, but 9.80 is not the gravitational force on the plane, it is the gravitationnal acceleration. You're asked for a force.

(c) what is dynamical equilibrium?! Are you writing anything here? According to wiki, dynamical equilibrium has to do with temperature and stuff (http://en.wikipedia.org/wiki/Dynamic_equilibrium)

#### lilkrazyrae

the problem read just as i wrote, dynamic equilibrium is that there is as much upward force as downward force and it moves at a constant velocity. As for the gravity thing I understand that but I still cannot figure out how to find b.

#### quasar987

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lilkrazyrae said:
dynamic equilibrium is that there is as much upward force as downward force and it moves at a constant velocity.
That doesn't make sense. If there is as much upward force as downward force, then the net force is not zero, and according to newton's 2nd law, the body has an acceleration of magnitude F/m. So it can't travel at constant velocity.

For (b), what is the gravitational force on an object of mass 'm' near the surface of earth? (F = ?)

#### Fermat

Homework Helper
I offer the following submission for your consideration.

The plane is moving at constant velocity - hence no acceleration - hence no net force.
There are drag and lift components involved. The power of the plane's engines overcome, and balance the drag. The weight of the plane is balanced by the lift. If the plane were not moving, it would be in static equilibrium. Since it is moving, I imagine that would be described as dynamic equilibrium
The answers to the problem would then be,

(a) zero

(b) 2.6 x 10^4 N

(c) dynamic equilibrium

#### quasar987

Science Advisor
Homework Helper
Gold Member
Ok, here's the vocabulary: If the sum of the force on a particle is zero, we say it is in translational equilibrium. We distinguish two kind of translational equilibrium: 1) If the particle is at rest, we say it is in static equilibrium. 2) If the particle is traveling at constant speed, we say it is in dymanic(al?) equilibrium.

So you were right about the plane being in dynamical equilibrium.

Note to lilkrazyrae: Although Fermat gave you the answers, please try to understand how he got them. If you don't, request that he explains.

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#### lilkrazyrae

Thanks for all the help, and I understand. thanks so much!!

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