# Velocity problem

1. Jan 31, 2007

### whiteruskii

1. The problem statement, all variables and given/known data

A pole vaulter goes 6m in the air. What is the velocity when he hits the ground?

No air resistance, just g=9.8m/s^2

2. Relevant equations
mgh = 1/2mv^2

Does an answer of 10.95 seem right? (I used 10m/s/s for gravity)

Last edited: Jan 31, 2007
2. Jan 31, 2007

### denverdoc

Well, then you need another approach, maybe attending class more regularly?

I'm not being snide but for the life of me cannot think of any other method.

The one option which might not be considered strictly as kinematics is by using energy: the potential and kinetic energies under these conditions will be constant. So develop an eqn that uses both. But given the data, this is the only approach.

Last edited: Jan 31, 2007
3. Jan 31, 2007

### whiteruskii

Attending class would be wonderful...last week was beyond my control

anyways, not to go off on a tangent, using mgh=1/2mv^2, I canceled out mass, since it's just a multiplier to both, and got an answer of approximately 10.95. Does that seem about right?

4. Jan 31, 2007

### PhanthomJay

If you use an alternate approach (and yes, there is one) using the basic kinematic equation of motion
$$v^2 = 2gh$$, you arrive at the same result (same equation), even without a knowledge of energy methods. Don't forget the units for the velocity: m/s

5. Jan 31, 2007

### Staff: Mentor

Yes that sounds about right. BTW, the other method would be to use the kinematic equation for distance, with the initial velocity = 0, and figure out how long it takes to travel the 6m distance in free-fall, then use that time in the velocity kinematic equation to figure out the person's velocity at that time. But your energy approach is better, because it's all contained in one easy equation.

BTW, are you guys classmates? Sounds like you have a conversation going on behind the scenes....

6. Jan 31, 2007

### Staff: Mentor

Doh! PhantomJay beat me to the punch.