# Trajectory of a proton as it moves between the plates

Cicicicici

## Homework Equations

What makes the shape circular/ parabolic?
what determines the direction?

## The Attempt at a Solution

Because the furthermost plate is positive, the proton would be repelled towards the screen, so i or iii. How do I know the shape?

#### Attachments

• Screen Shot 2018-05-25 at 1.44.19 pm.png
31.8 KB · Views: 767

## Answers and Replies

Science Advisor
Homework Helper
Gold Member
2021 Award
How do I know the shape?
What kind of acceleration generates circular motion? What about parabolic motion? Which do you have here?

Cicicicici
What kind of acceleration generates circular motion? What about parabolic motion? Which do you have here?
Hi, is it because the proton is accelerating along the field lines not perpendicular to them? so it is a parabolic motion?

Science Advisor
Homework Helper
Gold Member
2021 Award
You provided a valid argument for parabolic motion. To answer with certainty, you need to provide an argument for excluding circular motion. What is that argument?

Cicicicici
You provided a valid argument for parabolic motion. To answer with certainty, you need to provide an argument for excluding circular motion. What is that argument?
ummmm... I'm not sure. Has it got something to do with the proton reaching terminal velocity before the plates?

Science Advisor
Homework Helper
Gold Member
2021 Award
ummmm... I'm not sure. Has it got something to do with the proton reaching terminal velocity before the plates?
Nope. That's there to indicate that the vertical component of the velocity is constant. What kind of acceleration does an object in a circular path have? What's the difference between that kind of acceleration and the kind you have here?

Cicicicici
Nope. That's there to indicate that the vertical component of the velocity is constant. What kind of acceleration does an object in a circular path have? What's the difference between that kind of acceleration and the kind you have here?
Centripetal acceleration? And this one is just uniform?

Science Advisor
Homework Helper
Gold Member
2021 Award
Correct on both accounts. Centripetal acceleration changes direction to stay perpendicular to the velocity. Here the direction of the acceleration is fixed in space and is perpendicualr to the velocity instantaneously only when the proton enters the plate region.

Cicicicici
Correct on both accounts. Centripetal acceleration changes direction to stay perpendicular to the velocity. Here the direction of the acceleration is fixed in space and is perpendicualr to the velocity instantaneously only when the proton enters the plate region.
I am a bit confused - isn't the acceleration zero here? (because it is at terminal velocity?)

Science Advisor
Homework Helper
Gold Member
2021 Award
How many different forces act on the proton?

Cicicicici
How many different forces act on the proton?
gravity and repulsion from the positive plate?

Science Advisor
Homework Helper
Gold Member
2021 Award
And some kind of air resistance otherwise the proton would not have reached terminal velocity before entering the plate area. The acceleration is zero before the proton enters the plate area. When it enters the plate area, the acceleration is no longer zero.

Cicicicici
And some kind of air resistance otherwise the proton would not have reached terminal velocity before entering the plate area. The acceleration is zero before the proton enters the plate area. When it enters the plate area, the acceleration is no longer zero.
Ah! right. Thank you!