Work QUICK: And Energy Problem Involving Air Resistance

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Homework Help Overview

The discussion revolves around a work and energy problem involving a projectile experiencing air resistance. The original poster presents a scenario where a projectile of mass 0.750 kg is shot vertically with an initial speed of 18.0 m/s, leading to questions about maximum height without friction and the average force due to air resistance when the projectile reaches a maximum height of 11.8 m.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the application of the work-energy theorem and the principles of conservation of mechanical energy. There is confusion regarding how to incorporate non-conservative forces, particularly air resistance, into their calculations. Questions arise about how to calculate the average force of air resistance and the relationship between work and force.

Discussion Status

The discussion is active, with participants exploring different interpretations of the problem. Some have provided guidance on computing mechanical energy loss and relating it to the average force of air resistance. There is an acknowledgment of the complexity introduced by non-conservative forces, and participants are encouraged to revisit their understanding of the concepts involved.

Contextual Notes

Participants note that the professor has not covered non-conservative forces in class, which adds to the confusion. The urgency of the assignment deadline is also mentioned, indicating time constraints on the discussion.

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QUICK: Work And Energy Problem Involving Air Resistance

a projectile mass of 0.750 kg is shot straight up with an intial speed of 18.0 m/s. (a) how high would it go if there were no friction? (b) if the projectile rises to a max height of only 11.8 m, determine the magnitude of the avg force due to air resistance.

Im confused about the 2nd part..the 1st part isn't a prob (i got hf=16.5 m)
 
Last edited:
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Use the work-energy theorem in both cases.
 
arildno said:
Use the work-energy theorem in both cases.

how does that work?

KE(initial) + PE(initial) = KE(final) + PE(final) ? like this...i get for part A--16.5m

now if i work on the 2nd part...i don't even know how to calculate the air resistance (which is a force)! where does the force come from the equation above? that's where I am stuck right now...(haha this is due in an hour ouch)
 
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No, that is the principle of conservation of mechanical energy when there are no non-conservative forces acting upon the system. In b) you DO have a non-conservative force!
 
arildno said:
No, that is the principle of conservation of mechanical energy when there are no non-conservative forces acting upon the system. In b) you DO have a non-conservative force!

WOW, what an assignment then...the prof didn't even touch the subject of non-conservative forces--although as i look ahead in the chapt it's there! let me see what i can pick up from it and i'll post back with another question..thanks arildno

so the way i solved the first part wrong?
 
Well, you HAVE learned of work, have you?

Compute the mechanical energy loss in b), then calculate the average force from air resistance against which work has been done by the system.
 
arildno said:
Well, you HAVE learned of work, have you?

Compute the mechanical energy loss in b), then calculate the average force from air resistance against which work has been done by the system.

ok, i had you when you said compute the mechanical energy loss...but how am i supposed to calculate avg force from air resistance agains work done by system? which equation do i use (or do reuse)?
 
well, work is FORCE TIMES DISTANCE, right?
 
arildno said:
well, work is FORCE TIMES DISTANCE, right?

yesireee hahah...ok, then I am makin it harder on myself! thanks, thanks...
 

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