# Newton's 2nd Law confusing!

1. Sep 25, 2008

### PhysicFailure

A helicopter is moving horizontally to the right at a constant velocity. The weight of the helicopter is W = 57600 N. The lift force L generated by the rotating blade makes an angle of 21.0° with respect to the vertical.

(a) What is the magnitude of the lift force in N?

(b) Determine the magnitude of the air resistance R that opposes the motion.

I used $$\Sigma$$Fx= max--> 0 (because constant velocity means 0 acceleration)= Lsin(21)

and $$\Sigma$$Fy= may-->Lcos21-57600

now I dont know what to do. What did i do wrong?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 25, 2008

### bchandler

Hi PhysicsFailure!

I am a new member and somewhat new to physics compared to most here but I think I can help you out on this one.

First, we know that the helicopter is not accelerating in either the X OR Y directions (it is moving strictly horizontally). For this reason, the sum of all forces in each direction is 0, not just X.

Have you tried drawing a simple diagram, with the helicopter represented as a point at the origin, the weight acting downward, and the force of lift acting upward at an angle of 21 degrees to the Y axis?

I think if you set the Y equation you already have to 0, the force of lift will become apparent. From here, figure what component of that lift on the X axis must be matched by the air resistance for the copter to not have any acceleration (sum of X forces=0)

Let me know if you get it!

3. Sep 25, 2008

### Mattowander

So far so good! It looks as though you've done everything right.

"I used $$\Sigma$$Fx= max--> 0 (because constant velocity means 0 acceleration)= Lsin(21)"

So then what is the force due to air resistance?

"and $$\Sigma$$Fy= may-->Lcos21-57600"

Part a is asking for L. How would you solve for L in this case?

Keep in mind that because there is no vertical acceleration, the net force in the y direction will also be zero.

4. Sep 26, 2008

### PhysicFailure

haha thanks so much bchandler and mattowander. I actually figured out what I was not doing. I could just solve it! and for some reason i was just not thinking...THANK YOU!!!!!!!!