Electric Potential related question

AI Thread Summary
The discussion revolves around calculating the potential difference between the starting point and the top of a trajectory for a charged ball on planet Tehar, which has both gravitational and electric forces acting on it. The initial approach involved using kinematic equations, but the participants struggled with incorporating the effects of the electric field into their calculations. They debated the need for a free-body diagram and the relevance of the time of flight in solving the problem. Despite attempts to derive a single equation with one unknown, confusion persisted regarding the correct acceleration due to the combined forces. Ultimately, the original poster expressed frustration and decided to tackle the problem independently.
kfan321
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On planet Tehar, the free-fall acceleration is same as that on Earth, but there is also a strong downward electric field that is uniform close to the planet's surface. A 2 kg ball having a charge of 5 uC is thrown upward at a speed of 20.1 m/s. It hits the ground after an interval of 4.1 sec. What is the potential difference between the starting point and the top of the trajectory?
Well my plan was to use V = E d
So I need to find E and d and I'll have the problem solved.
I used V2^2 = V1^2 + 2ad
v2 is 0, v1 is 20.1, and i used 9.8 for a.
So d came out to 20.6. But I don't think that's correct because I didnt account for the acceleration due to the electric field? So how do I do that?
To solve for E, I wanted to use F/q but not enough info given. Thought about (KQ)/r^2 but not sure if this scenario can be considered point charges...
 
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Due to the electric field, now effective accelaration is changed. Use Newton's second low to find a. (Hint: What are the two forces now acting on the object?)
 
Well you really haven't said anything I don't already know. I've said force gravity and electric field will deccelerate the ball... and that's where I am stuck
 
kfan321 said:
Well you really haven't said anything I don't already know. I've said force gravity and electric field will deccelerate the ball... and that's where I am stuck

Did you draw a free-body diagram?
As you have determined, a is an unknown. d is also unknown. So, your single kinematic equation is not sufficient to find d. Note that you have not used the information about the time of flight. Is there another applicable kinematic equation that can be used here?
 
i've tried d = v1t + .5 at^2
still have 2 variables. So I did a system of equations with the first kinematic equation. Subbing d into a, hoping to find the new a and it came out to -9.8. lol...

...I'm all for not doing other peoples homework for them, but you guys are hiding the info like it's some top FBI confidential document.
 
kfan321 said:
i've tried d = v1t + .5 at^2
still have 2 variables. So I did a system of equations with the first kinematic equation. Subbing d into a, hoping to find the new a and it came out to -9.8. lol...
If you do things algebraically, you might find a single equation with one unknown. (That is, there may be a simpler starting point.)

kfan321 said:
...I'm all for not doing other peoples homework for them, but you guys are hiding the info like it's some top FBI confidential document.

In "Homework, Coursework, & Textbook Questions", you are nudged toward (but not given) the answer... but you have to show your work.
 
using the 2 kinematic equations previously

0 = v1^2 + 2*a*v1*t + a^2 * t^2

this is what I've done on post #5, i knew everything except a in that equation, so I solved for it and it came out to -9.8. Which can't be right b/c of the field, so where did I go wrong?
 
kfan321 said:
v2 is 0

Why is this true?
 
b/c at the top of the trajectory, it's 0 m/s, and I use time as 2.05 instead of 4.1...
But now I think about it, time shouldn't be half... Great, back to square one.

You know what, nvm the thread. I think I am better off figuring it out myself.
 
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