(Mechanics)An interesting question about projectile

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Homework Help Overview

The discussion revolves around a problem in mechanics, specifically projectile motion involving an elastic ball projected on an inclined plane at a specific angle. The original poster presents calculations for the time after the 10th bounce and the distance traveled, but expresses confusion regarding discrepancies between their results and those provided by their instructor.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts to calculate the time after the 10th bounce using their own method but questions the validity of their approach compared to their instructor's solution. Other participants ask for clarification on the original poster's calculations and the equations used, indicating a focus on understanding the reasoning behind the differing results.

Discussion Status

The discussion is ongoing, with participants actively seeking clarification on the calculations and equations involved. There is a collaborative effort to identify where misunderstandings may have occurred, but no consensus has been reached regarding the correct approach or solution.

Contextual Notes

Participants are working under the constraints of the problem as stated, including the assumptions of no energy loss and the specific angles involved in the projectile motion. The original poster's confusion highlights potential gaps in understanding the application of the equations of motion in this context.

physicsisgrea
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Homework Statement


Suppose an elastic ball is set into projectile motion on an inclined plane, which makes an angle of 30 degrees to the horizontal. It is projected with an initial velocity of 5 m/s, making an angle of 20 degrees to the slope of the plane. Suppose the plane is very long, and no energy loss throughout.

a. Find the time just after the 10th bounce, assume time = 0 when the ball is projected. (g = 10 m/s^2).
b. Hence find the distance traveled by the ball.

Homework Equations


x = uxt
y = uyt - .5gt^2
and other equations about projectile motion

The Attempt at a Solution



t' = 2(5 cos 20) / (10 sin 30) = 1.8794 s
So T = 10(1.8794) = 18.8 s.

However the answer is 3.95 s, far smaller than mine.

My instructor gives the answer as follows:
t' = 2(5sin20) / (10cos30) = 0 = 0.395 s
So T = t' * 10 = 3.95 s
I don't really understand what he is doing, and can anyone explain the approach of my instructor?
 

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hi physicsisgrea! :wink:

show us how you got from …
physicsisgrea said:
x = uxt
y = uyt - .5gt^2

to …
t' = 2(5 cos 20) / (10 sin 30) = 1.8794 s

… and we'll se what went wrong :smile:
 
tiny-tim said:
hi physicsisgrea! :wink:

show us how you got from …to …… and we'll se what went wrong :smile:
oh sorry!
y = u sin 20 t - .5 gt^2 cos 30
x = u cos 20 t + .5 gt^2 sin 30

are they correct?
 
Last edited:
looks ok…

and then?​
 

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