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PinguNinjitsu

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## Homework Statement

1.)A particle is released from point A and

moves in the potential U(x). Suppose the

mechanical energy of the system is conserved.

At which position(s) will the kinetic energy

of the particle have its maximum value?

2.)A(n) 126 g ball is dropped from a height of

58.1 cm above a spring of negligible mass.

The ball compresses the spring to a maximum

displacement of 4.85169 cm.

The acceleration of gravity is 9.8 m/s2 .

Calculate the spring force constant k.

Answer in units of N/m.

3.)A 4 kg block moving along the x-axis is acted

upon by a single horizontal force that varies

with the block’s position according to the equation

Fx = a x2 + b ,

where a = 6 N/m2, and b = −4 N. At 1.6 m,

the block is moving to the right with a speed

of 4.6 m/s.

Determine the speed of the block at 2.2 m.

Answer in units of m/s.

4.)The potential energy between two atoms in a

particular molecule has the form

U(x) =(2.6)/x^8 −(5.1)/x^4

where the units of x are length and the num-

bers 2.6 and 5.1 have appropriate units so

that U(x) has units of energy.

What is the equilibrium separation of the

atoms (that is the distance at which the force

between the atoms is zero)?

5.)A particle of mass 8.95 kg is attached to two

identical springs on a horizontal frictionless

tabletop as shown.

The springs have spring constant 37.7 N/m

and equilibrium length L = 0.74 m.

If the mass is pulled 0.428 m to the right

and then released, what is its speed when it

reaches the equilibrium point x = 0?

Answer in units of m/s.

## Homework Equations

k=.5mv^2

u=mgh

W=FD

u(spring)=1/2kx^2

F=-du/dx

## The Attempt at a Solution

1.) I guess I don't really understand what to look for in this graph, I know that for a negative slope the force is positive and S and Z are neutral equilibriums and V is an unstable equilibrium. I was not sure how to determine the maxiumu KE though..

2.) I got k=609.56 N/m for this problem but it was wrong, and I am not sure why. This is what I did:

Ui(ball)+Ui(spring)=Uf(ball)+Uf(spring)=mgh+.5kx^2=mgh+.5kx^2=(.126*9.8*.581)=.5k(.0485169)^2

And then I got k=609.56N/m But this was wrong, and I'm not sure what I was doing wrong.

3.) I got v=5.393978124 but I am unsure if I worked this out correctly

First I took the first d=1.6m and v=4.6m/s and plugged it into W=FD=KE=1/2mv^2 and got x as 2.252776065 then I took the second value of d=2.2 and used it in the same equation as be fore to get v. Now I am really not sure if this works at all, I just didn't know how to go about it.

4.) for this problem I took F=0 because there would not be force if the atoms were at equilibrium, I got the equation for F from f=-du/dx and then from there I solved for x and go 1.004866324. Now I'm not sure if that is at all right, and I don't know if that is the distance I am looking for.

5.) I honestly just don't know what to do for this one >.<Help on any of these problems would be greatly appreciated I got to all the lectures and read the book, but unfortunately I seem to be failing when it comes to applying things. I know these are a lot of questions but help would be appreciated!

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