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

[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 the answer wrong? Did I do it wrong?

 

[OlderDan]

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 the 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.

 

[muna580]

So how do I calculate the Kenetic enegery at that point?

 

[OlderDan]

So how do I calculate the 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.