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Velocity Folding
When a car is moving forward...?
When a car is moving forward at constant speed it has zero net force on it. This means that the force exerted by the engine is just enough to counteract the backward force exerted by friction and air resistance. So f=ma and in terms of a vector sum, the car's ma = - ma exerted by the wind?
Am I right so far?
So this would mean that according to Newtons first law that the car moving is akin to an air belt that has a moving folded piece of cardboard placed on top of where the air is blowing out, it glides along.
So the engine is exerting a force when there's constant velocity, and an acceleration but this is counteracted by the backward force (and hence acceleration) of the wind/friction.
Idk is my chain of logic right or wrong, thanks a bunch 
"When a car is moving forward at constant speed it has zero net force on it. This means that the force exerted by the engine is just enough to counteract the backward force exerted by friction and air resistance."
This statement is correct. But what follows is off-target. The right way to continue the argument is to say -'Since the net force is zero, there is no acceleration; this is in accordance with Newton's first law'.
If you use this way of thinking, you can see that your final statement makes the situation more complicated than it needs to be -
"So the engine is exerting a force when there's constant velocity, and an acceleration but this is counteracted by the backward force (and hence acceleration) of the wind/friction."
You can think in terms of forces alone, and need not bother about accelerations.
However, if the driver opens the throttle, and the engine produces a force greater than friction, there will be a net forward force, and we can then use f = ma to calculate the acceleration of the car, using the difference between the engine force and the frictional force as f. Here we use f = ma, but only once - after we have found the net (or resultant) force.
A good guideline is to use f = ma only when there is a net resultant force. Any net unopposed force will create a state of disequilibrium; the mass will accelerate, and you can use f = ma to calculate the acceleration.
Hope this helps
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UMD Bird Folding Wings
Can anyone help me with these physics problems?
1. Suppose a driver springs into the air with no initial angular velocity. can the driver begin to rotate by folding into a tucked position?
2. Suppose a torque rotates your body about one of three different axes of rotation: case A, an axis through your spine; case B, an axis through your hips; case C, an axis through your ankles. Rank these three axes of rotation in increasing order of the angular acceleration produced by the torque.
3. A fish takes a bate and pulls on the line with a force of 2.1 N. The fishing reel, which rotates without friction, is a cylinder of radius 0.055 m and mass 0.84 kg. (a) What is the angular acceleration of the fishing reel? (b) How much line does the fish pull from the reel in 0.25 s?
1 and 2 are easy. I'll help with #3.
3. You need I, the rotational inertia. From a table in my physics book, I = MR^2 for the reel. You need to convert the force to torque, T = F*R.
a. Then you can get the angular acceleration, alpha, from
T = I*alpha
b. In t = 0.25 s, omega will reach alpha*t. The angular displacement, theta, will be (1/2)*omega*t. From that and the circumference you can get the line drawn from the reel.
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