# Mechanics, PE to position function

1. May 14, 2009

### tjkubo

1. The problem statement, all variables and given/known data
A 3 kg object is moving along the x-axis where U(x) = 4x2. At x = -.5, v = +2. Find the object's position and KE as functions of time. Assume x = 0 at time t = 0. All forces acting on the object are conservative.

2. Relevant equations
ME = U + K
K = (1/2)mv2
F = dU/dx
F = ma

3. The attempt at a solution
Using initial conditions, ME = 4.
F = dU/dx = 8x
F = ma
8x = (3)d2x/dt2
This is where I got stuck. I was attempting to solve for x(t), find v(t), then use that to find K(t). Assuming everything else is correct, how do you solve a second order differential equation like this? Otherwise, please correct me.

2. May 14, 2009

### rock.freak667

Just solve

$$\frac{d^2x}{dt^2}- \frac{8}{3}x=0$$

Do you know how to solve a second order differential equation with constant coefficients?

EDIT: http://www.sosmath.com/diffeq/second/constantcof/constantcof.html" [Broken]

Last edited by a moderator: May 4, 2017
3. May 14, 2009

### tjkubo

I am still having trouble solving for x(t).
I got $$x=c_1e^{\sqrt{\frac{8}{3}}t}+c_2e^{-\sqrt{\frac{8}{3}}t}$$
and $$c_1+c_2=0$$
but since there is no initial value associating time and velocity, I can't find the constants.

4. May 14, 2009

### rock.freak667

How did you get ME=4 by chance?

Also F=-dU/dx not F=+dU/dx

5. May 14, 2009

### Delphi51

Can you use the "At x = -.5, v = +2" condition?
Would it work to begin with
ME = U + K
4 = 4x^2 + 1/2*mv^2 (which includes the x = -.5 condition)
4 = 4x^2 + 1.5(dx/dt)^2
The solution to this differential equation would have only one constant, which you should be able to get using the x=0 at t=0 condition.

6. May 14, 2009

### tjkubo

My bad, ME = 7. I forgot to square. (Is is correct to assume that ME is constant?)
Anyway, when I retried solving the differential equation with the initial conditions, I ended up getting 0 = 0 while solving for the constants. ????

7. May 14, 2009

### tjkubo

How would you solve this differential equation? The (dx/dt)^2 term throws me off.

8. May 15, 2009

### Delphi51

dx/dt = sqrt(2/3)*sqrt(4 - 4x^2)
sqrt(2/3) dt = dx/sqrt(4 - 4x^2)
Integrate both sides. Doesn't look bad - trig substitution if I'm not mistaken.

9. May 15, 2009

### tjkubo

Yes! I got
$$x=\frac{\sqrt{7}}{2}\sin{\sqrt{\frac{4}{3}}\,t}$$
That was much simpler than what I was doing.
Thanks Delphi!