Why Did I Get the Physics Problem on Work and Energy Wrong?

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The discussion focuses on a physics problem related to work and energy, specifically addressing a mistake made in calculating the answer for a problem involving circular motion. The user initially used the equations N = mv^2/R and mg = N, but was informed that this approach is incorrect because the normal force (N) is greater than the gravitational force (mg) when an object is in circular motion. To correctly solve the problem, it is suggested to apply the conservation of energy principle, calculating the change in gravitational potential energy to determine the kinetic energy at different points in the motion. The key takeaway is that total energy is conserved, and the loss of potential energy translates into an increase in kinetic energy. Understanding these concepts is crucial for accurately solving problems related to work and energy.
muna580
I need hlep with some questions related to work, and energy.

http://img246.imageshack.us/img246/8616/untitled1xn8.jpg

I tried doing #8 and I got the answer \sqrt{9.8} but I got the answer wrong.

http://img149.imageshack.us/img149/8269/untitled1df2.jpg

I used this concept to solve number 8, but why did I get it wrong?

N = mv^2/R

mg = N

mg = mv^2/RBut why did I get teh answer wrong? Did I do it wrong?
 
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muna580 said:
I need hlep with some questions related to work, and energy.

I tried doing #8 and I got the answer \sqrt{9.8} but I got the answer wrong.

I used this concept to solve number 8, but why did I get it wrong?

N = mv^2/R

mg = N

mg = mv^2/R


But why did I get teh answer wrong? Did I do it wrong?
Because N does not equal mg, and because this is not the correct approach to the problem. First, if N did equal mg, the ball souild be going in a straight line, not on the arc of a circle. What is in fact true in this case is that N is greater than mg

N - mg = mv^2/R

But that will not help you find v. To find v you need to know the energy of the ball. If you say it has zero potential energy at the bottom, the energy there is all kinetic. At the top, the energy is all potential. Energy conservation will lead you to the velocity.
 
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So how do I calculate teh Kenetic enegery at that point?
 
muna580 said:
So how do I calculate teh Kenetic enegery at that point?
Calculate the change in gravitational potential energy between the high point and the low point of the motion. The loss of potential energy equals the increase in kinetic energy. Total energy is conserved.
 
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