# Question about Terminal Velocity and Net Force

1. Oct 18, 2007

### STS816

My physics teacher told my class that in order for there to movement, there must be a net force on an object. He also said that when an object reaches terminal velocity the Fnet is zero because air resistance equals that of the object's weight. Then, he said that the object continues to fall because of Newton's First Law.

Now my problem here is that I understand Newton's first law works the way it does because of the object's inertia. But if there is no Fnet on an object and the object is moving solely on its inertia through the air, wouldn't air resistance continue to slow it down?

I think the problem is coming from the first thing my teacher said. I think he meant to say that in order for there to be acceleration, there needs to be a net force. I probably just answered my own question in some way but I just want to be sure. Thanks.

2. Oct 18, 2007

### nicksauce

In the simplest model, Air resistance will be proportional to velocity, and points opposite to gravity. Thus there will be some velocity, let's call it terminal velocity, such that weight = air resistance. Thus force = 0. Since force = 0, acceleration = 0, thus the velocity of the object does not change, thus the air resistance force stays the same and does not slow the object down any more, since gravity balances it.

"My physics teacher told my class that in order for there to movement, there must be a net force on an object. " That is false. An object can be stationary in one frame of reference, but moving in another frame with no additional forces added. "I think he meant to say that in order for there to be acceleration, there needs to be a net force. " That is correct.

3. Oct 18, 2007

### Staff: Mentor

I don't think you understand what "fnet" is. Fnet is the net force on the object. It is the weight minus the air resistance. When moving at terminal velocity, the net force is zero, so there is no acceleration or deceleration. If it is going slower than terminal velocity, fnet is negative, and it accelerates downward.

4. Oct 19, 2007

### STS816

All right thanks you guys. I was pretty sure he was talking about acceleration, not movement, but I wanted to clear it up a little bit. The thing is that he just kept repeating that if there is no fnet then there is no movement and I really didn't think he could be wrong that many times but apparently he was.

It's good to know that my education is in good hands....

5. Oct 19, 2007

### clustro

Yes, what russ_waters says is correct.

It is simply Newton's first law of motion.

"An object in motion will remain in motion unless acted upon by a net force."

Where "motion" is the velocity of the object. Any object with constant velocity has zero net force on it, whether that object is moving at 0 mph or 2,000 mph.

oh btw new poster -_-
hello world!