Solve Vertical Circles: Force, Velocity, Acceleration, Mass, Reaction

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The discussion revolves around solving a physics problem involving a marble on a hemispherical bowl, focusing on calculating the normal reaction force after the marble descends. The initial calculations for velocity were correct, but confusion arose regarding the direction of forces and the correct formulation of the equations. Participants identified a mistake in the setup of the force equation, specifically the relationship between the normal reaction force and gravitational force. The issue was resolved when it was clarified that the correct equation should be 0.01g - R instead of R - 0.01g. Ultimately, the problem was solved, highlighting the importance of accurately representing forces in physics problems.
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Homework Statement


F=Force, v=Velocity, a=acceleration m=mass (0.01kg) R=Normal Reaction to surface

Hi, can anyone help, I keep getting the wrong answer on this question and it is really annoying me. Here is a picture of the diagram:

q7.jpg


"A Marble of mass 0.01kg is on top of a smooth hemispherical bowl and given an initial speed of 1 m/s. The bowl has a radius of 0.5m and centre o.
a) The marble descends 0.1m, find the speed. got this part correct and figured out that v^2=1+0.2g

b) "Find the normal reaction of the bowl at this point"


Homework Equations


I have a solution for Velocity^2=1+0.2g


The Attempt at a Solution


So I have v^2=1+0.2g, now trying to resolve in the direction of R using F=ma I got:
R-0.01gcos(alpha)=m x (v^2/r)
=>R=0.01x(1+0.2g/0.5)
Now, trying to find alpha I thought of the height raise as 0.4m and the radius 0.5 to create a right angled triangle, getting cos(alpha)=0.4/0.5, and subsequently alpha to equal 36.86... deg.

When I sub this is my value that I get for R=0.1376, however in the book it says it should = 0.0192.

I have tried re doing it from scratch twice, faffing about with different bits and I just cannot get it right :/
 
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RJWills said:
So I have v^2=1+0.2g, now trying to resolve in the direction of R using F=ma I got:
R-0.01gcos(alpha)=m x (v^2/r)
This looks OK.
=>R=0.01x(1+0.2g/0.5)
What happened to the 0.01gcos(alpha) term? (But it looks like you include that term in your calculation.)
Now, trying to find alpha I thought of the height raise as 0.4m and the radius 0.5 to create a right angled triangle, getting cos(alpha)=0.4/0.5, and subsequently alpha to equal 36.86... deg.
OK.

When I sub this is my value that I get for R=0.1376, however in the book it says it should = 0.0192.
I get the same answer that you do.

Ah... Here's the problem: The marble's rolling down the inside of a concave bowl. Your diagram is not correct.
 
Doc Al said:
This looks OK.

What happened to the 0.01gcos(alpha) term? (But it looks like you include that term in your calculation.)

I get the same answer that you do.

Ah... Here's the problem: The marble's rolling down the inside of a concave bowl. Your diagram is not correct.

As you say, I have included it in my working, i just miss typed what I have written on paper.
The diagram is included in the textbook question. Still really confused and I cannot see how to solve this.

Edit: HURRAY SOLVED IT! I was thinking of it as R-0.01g= xyz where as it should be 0.01g-R!

Thanks :)
 
RJWills said:
As you say, I have included it in my working, i just miss typed what I have written on paper.
Good.
The diagram is included in the textbook question. Still really confused and I cannot see how to solve this.
Yes, my bad. I was making the same mistake as you! :redface:

You have the direction of the acceleration and the reaction force wrong.
 
RJWills said:
Edit: HURRAY SOLVED IT! I was thinking of it as R-0.01g= xyz where as it should be 0.01g-R!
Yay! Sorry for not spotting that earlier. :rolleyes:
 
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