Estimated terminal velocity significantly less than estimated impact velocity

In summary: This would happen if the parachute was more effective at higher speeds.In summary, the conversation is about an EEI (Extended Experimental Investigation) looking at the effects of increased mass, surface area, and air pressure on terminal velocity. The experiment involved dropping a parachute of 0.36m^2, with a mass of 0.01kg, from a height of 3m. The result showed an average velocity of 0.988 m/s and an estimated terminal velocity of 0.79m/s. The conversation also explores the drag formula and the attempt at calculating terminal velocity using air density and drag coefficient. There is a discussion on whether the parachute took time to fully deploy and the need for a method to measure velocity over the
  • #1
FEllen
3
0

Homework Statement



I am currently conducting an EEI in which I am looking at the effects of increased mass, surface area and air pressure on terminal velocity. In my experiment, I dropped a parachute of 0.36m^2 from a height of 3m with a mass of 0.01kg. My result was that it took 3.036 seconds to fall, giving an average velocity of 0.988 m/s. Assuming the object did not reach terminal velocity, and exprienced constant acceleration, the impact velocity should then be around 2m/s (double the average). However, the estimated terminal velocity of the object I calculated was 0.79m/s. This is even less than the average velocity - which should theoretically be impossible. This has happened with most my results! Anyone know what I could be doing wrong? Even taking error into account does not decrease impact velocity enough.



Homework Equations



Drag formula:
F= 1/2*ρCAv^2
Where F is the drag force;
ρ is the air density;
C is the drag coefficient;
A is the area of the parachute;
And v is the velocity through the air

Thus making terminal velocity:
v=√(2W/ρCA)

The Attempt at a Solution


Air density was estimated to be 1.171kg/m^3, and the drag coefficient of the parachute was 0.75.
v=√(2*9.81*0.01/(1.171*0.75*0.36))
v= 0.79m/s
 
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  • #2
Welcome to PF!,

What is an EEI?

How do you know that the drag coefficient of the parachute is 0.75?
 
  • #3
FEllen said:

Homework Statement



I am currently conducting an EEI in which I am looking at the effects of increased mass, surface area and air pressure on terminal velocity. In my experiment, I dropped a parachute of 0.36m^2 from a height of 3m with a mass of 0.01kg. My result was that it took 3.036 seconds to fall, giving an average velocity of 0.988 m/s. Assuming the object did not reach terminal velocity, and exprienced constant acceleration, the impact velocity should then be around 2m/s (double the average). However, the estimated terminal velocity of the object I calculated was 0.79m/s. This is even less than the average velocity - which should theoretically be impossible. This has happened with most my results! Anyone know what I could be doing wrong? Even taking error into account does not decrease impact velocity enough.



Homework Equations



Drag formula:
F= 1/2*ρCAv^2
Where F is the drag force;
ρ is the air density;
C is the drag coefficient;
A is the area of the parachute;
And v is the velocity through the air

Thus making terminal velocity:
v=√(2W/ρCA)

The Attempt at a Solution


Air density was estimated to be 1.171kg/m^3, and the drag coefficient of the parachute was 0.75.
v=√(2*9.81*0.01/(1.171*0.75*0.36))
v= 0.79m/s

Could it be that the parachute takes a little while to fully deploy properly?
If so the system would be simply falling, at first, with an acceleration of 9.8, but that acceleration would quickly reduce under the air resistance. Even so the falling system may reach a higher speed before the fully opened, and stable, parachute resulted in the final , terminal velocity.

A video of the experiment could be useful. You could analyse the video frame by frame to see what is going on. Be careful if you use the "high speed shutter" facility to get nice clear images. Early cameras with that feature did not "take" regularly spaced [time] images. It seemed that when operating at 25 frames per second, but with the 0.001 sec shutter, the image was taken at almost random times during each 0.02 second period.

Who knows, the modern i-phone may do an excellent job?
 
  • #4
I got the estimated coefficient of drag from http://www.aeroconsystems.com/chutes/drag_calculator.htm (as it was a parasheet made out of garbage bag). An EEI is an Extended Experimental Investigation - really just a long assignment. It is possible that it took some time to deploy, but I don't think that could account for all the difference.
 
  • #5
FEllen said:
I got the estimated coefficient of drag from http://www.aeroconsystems.com/chutes/drag_calculator.htm (as it was a parasheet made out of garbage bag). An EEI is an Extended Experimental Investigation - really just a long assignment. It is possible that it took some time to deploy, but I don't think that could account for all the difference.

The whole purpose of the parachute is to prevent the falling object from having a constant acceleration, so you estimate of the final velocity as twice the average has no basis.

Neither, I suspect, is any theoretical terminal velocity.

I think you need to devise a method of measuring the velocity over the last metre, and checking whether it is constant [ie. the terminal velocity].
 
  • #6
Okay, so terminal velocity increases as the object velocity increases, meaning it is not a constant deceleration (as the force goes up). Any ideas on how to calculate the impact velocity otherwise? Also, that does not answer why the average velocity would be greater than the terminal velocity...
 
  • #7
FEllen said:
Okay, so terminal velocity increases as the object velocity increases, meaning it is not a constant deceleration (as the force goes up). Any ideas on how to calculate the impact velocity otherwise? Also, that does not answer why the average velocity would be greater than the terminal velocity...

I think you had better address the question of what Terminal Velocity is - judging by what I highlited RED above.

Part of the design of your EEI is to measure the terminal velocity.

If an object increased its speed to 3 m/s while the parachute began to work, then slowed to 1.5 m/s before landing, it could well have a a higher average speed than its terminal velocity.
 

FAQ: Estimated terminal velocity significantly less than estimated impact velocity

1. What is terminal velocity?

Terminal velocity is the maximum speed at which an object falls through a fluid, such as air or water, due to the balance of gravitational force and air resistance. It is the point at which the object stops accelerating and falls at a constant speed.

2. How is terminal velocity calculated?

Terminal velocity is calculated using the equation v = √((2mg)/ρACd), where v is the terminal velocity, m is the mass of the object, g is the acceleration due to gravity, ρ is the density of the fluid, A is the projected area of the object, and Cd is the drag coefficient.

3. What causes a difference between estimated terminal velocity and impact velocity?

The difference between estimated terminal velocity and impact velocity can be caused by various factors such as changes in air density, variations in the object's shape, or inaccurate calculations of the variables in the terminal velocity equation.

4. How can the estimated terminal velocity be significantly less than the estimated impact velocity?

This can occur when the object is falling from a great height or when there is a significant difference in the density of the fluid it is falling through. In these cases, the object may reach a very high impact velocity due to the force of gravity, but the air resistance slows it down to a lower terminal velocity before impact.

5. What are the implications of a significant difference between estimated terminal velocity and impact velocity?

A significant difference between estimated terminal velocity and impact velocity can have important implications for safety and accuracy in scientific experiments or engineering design. It can also provide valuable insights into the behavior of objects in different fluid environments and the effect of various factors on their motion.

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