# Simple Conservation of energy question

1. Aug 12, 2016

### Derek1997

1. The problem statement, all variables and given/known data
Pole-vaulting is a fantastic example of energy being converted from one form to another. A pole- vaulter 1.7 m tall runs at 30 km/h (8.4 m/s) with her pole before starting her jump. The kinetic energy she generates is converted into elastic potential energy of the pole bending.This is then returned to the jumper as gravitational potential energy. Assume that her centre of mass is 55% of her height:
1)-Show that the height achieved of 4.6 m is close that that predicted by simple conservation of energy equations
2)- Discuss why this calculation is not exact - including consideration of the fact that the women’s pole vault world record is over 5 meters and that it’s not possible to run faster than 30 km/h with a
Thanks

2. Relevant equations

3. The attempt at a solution
Well, I got 3.6h for my first question i did KE=P and solved for h. Although im not sure it said 4.6 and I got 3.6. Also for second part I say because since world record is 5m and not possible run more than 30, this athelte did run 30km and acheived lower than world record? thanks

2. Aug 12, 2016

### BvU

Hello Derek, ,

Culture at PF is that you show your work in detail and we'll ask questions and give hints etcetera. (It doesn't help you if we do the exercise for you )

You got 3.6 m (meter, I hope, not hours ) starting from where ?

3. Aug 12, 2016

### TomHart

I got 3.6 m too for the increase of her height.

4. Aug 12, 2016

### Derek1997

Hi, So i got 3.6 as height using the equation but I hope im in a right track, so it says height of achieved is close to 4.6m which I don't get it, but for explaining second part I said it's because even tho he/she ran for 30km/h achieved fairly low height (3.6m)

5. Aug 12, 2016

### jbriggs444

Using what equation? As BvU has suggested, you need to show your work in detail. Is the 3.6 (meters) result supposed to be the height attained?
So, how might one clear a bar at 5 meters when one only has enough kinetic energy to raise one's center of mass to [an assumed] 3.6m?

6. Aug 12, 2016

### TomHart

I think we agree that her height increased by 3.6 m, but you have to consider where the starting point is. Hint: 4.6 m is the right answer.

For part b, the problem is telling you that pole vaulters are able to clear a 5 m bar. Then they ask how that can be possible considering that she doesn't produce enough kinetic energy to be able to increase her center of mass to that height.

7. Aug 12, 2016

### Derek1997

I really appreciate your helps guys, first off, I gone 1/2mv2=mgh cancell h and solved for h which I got 3.6m. But she has a starting height of 1.7m so 1.7+3.6=5.3m?

8. Aug 13, 2016

### Tom.G

Have you ever seen a pole vaulting exihibition?
What is the body orientation of the vaulter when clearing the bar?
What sources of energy, other than legs, are available to the contestant?

9. Aug 13, 2016

### Derek1997

No, I haven't seen any pole vaulting or I did when i was really young. I guess hands?

10. Aug 13, 2016

### jbriggs444

If she has a height of 1.7 m and jumps up by 3.6 meters, that gets the top of her head over a 5.3 m bar. Would that be enough?

11. Aug 13, 2016

### TomHart

Derek, here's a YouTube clip of a pole vaulter in slow motion.

12. Aug 13, 2016

### Derek1997

Well since the record is over 5m, I guess it makes sense? so yes

13. Aug 13, 2016

### Derek1997

Thank you Tom, but what confuses me most is the 4.6m part, I don't get it.

14. Aug 13, 2016

### BvU

Make a sketchy drawing, showing clearly where the 3.6 m you calculated (correctly), is located ...

15. Aug 13, 2016

### Derek1997

O
Okay then?

16. Aug 13, 2016

### BvU

I expected a drawing, not a question. Okay what ?

17. Aug 13, 2016

### Derek1997

I can't upload it here man, But Im pretty sure I got it right(thats what I sketched) so assuming the bar is 5m he pushes himself 3.6m using spear since he is 1.7m tall he passes the 5m bar with 0.3m difference. right?

18. Aug 13, 2016

### BvU

No. Passing isn't the word. The bar will hit her throat (it was her pole). if she stays upright.

19. Aug 13, 2016

### TomHart

Hi Derek. Here is a YouTube video that describes the "Fosbury flop" high-jumping method. It's the method currently used by . . . virtually all high jumpers. It is a good visual description of the physics involved in high-jumping. Also, contrast the pole vaulter and the high jumper. Once the high jumper's feet leave the ground, there's nothing he can do to apply more energy to increase his maximum height - well, except for maybe trying to flap his arms like a bird. The pole vaulter still has his hands on the pole. Can he use that to any advantage?

20. Aug 13, 2016

### Tom.G

Yes!
Now, since you've seen the videos, when a body freely rotates, around what point in the body does it rotate?

edit: strike-out

Last edited: Aug 13, 2016