Question about Terminal Velocity and Net Force

In summary, my physics teacher said that in order for there to be 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.
  • #1
STS816
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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.
 
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  • #2
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
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
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
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!
 

What is terminal velocity?

Terminal velocity is the maximum velocity that an object can reach when falling through a fluid, such as air or water. It occurs when the upward force of air resistance is equal to the downward force of gravity, resulting in a constant velocity.

How is terminal velocity calculated?

The formula for calculating terminal velocity is Vt = √(2mg/ρAC), where Vt is the terminal velocity, m is the mass of the falling object, g is the acceleration due to gravity, ρ is the density of the fluid, A is the cross-sectional area of the object, and C is the drag coefficient.

What factors affect terminal velocity?

The factors that affect terminal velocity include the mass and shape of the object, the density and viscosity of the fluid, and the force of gravity. Objects with larger surface areas and lower densities will reach terminal velocity at a slower rate, while objects with smaller surface areas and higher densities will reach terminal velocity faster.

How does net force affect terminal velocity?

Net force is the sum of all the forces acting on an object, including the force of gravity and air resistance. When the net force is zero, the object will reach terminal velocity. If the net force is greater than zero, the object will accelerate and its velocity will increase. If the net force is less than zero, the object will decelerate and its velocity will decrease.

Can terminal velocity be exceeded?

Terminal velocity is the maximum velocity that an object can reach when falling through a fluid. It is theoretically possible to exceed terminal velocity, but it would require a significantly higher air resistance or a change in the object's mass or shape. In most cases, terminal velocity is a constant and cannot be exceeded.

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