# Find velocity.

1. May 4, 2008

### cecico

Find velocity.........

A 3.00Kg block at x=0 is moving to the right at 5.00m/s.
A horizontal force acts on the block. Its potential energy funtion is U=(-x^2)-4x+5
find the speed of the block at x=2.00m (surface is smooth)

ok the answer is already given by prof. answer is 5.74m/s but I wanna know how to solve this. I used kinetic and potential energy equation for this problem but I got 4.52m/s also I used changing kinetic and potential energy but I got 8.50m/s........I tried other things too but I got wrong....... I'm stuck with this.....

2. May 4, 2008

### Chi Meson

The answer is either 4.51 m/s or 5.45 m/s, depending on whether the force is opposite or with the direction of motion. If the answer is actually 5.74 m/s, then the question is wrong.

Clearly the function gives a potential energy of 7 J, so the block either gains or loses 7 J of KE (depending on the intended meaning of the negative quantity).

Edit: I noted rock.freak's answer, below, and then slapped my head. Only the change in PE of the system matters. Since the +5 exists at zero, and at x=2, then then change in KE is 12 J.

Awkwardly worded question.

Last edited: May 4, 2008
3. May 4, 2008

### rock.freak667

$U=-x^2-4x+5$

at x=0..what is U? Then use the law of conservation of mechanical energy.