Why Do High Jumpers' Centers of Mass Pass Underneath the Bar?

In summary, the conversation discusses the concept of centers of mass and their movement in relation to high jumpers going over bars and a hockey stick being thrown on ice. It is explained that in order to clear the bar, not all parts of the body need to pass over it simultaneously. Additionally, the conversation delves into the role of external forces, such as friction, in the movement of centers of mass. The shape and non-uniformity of objects also play a role in the rotation and stopping of rotation.
  • #1
physicsstudent12
6
0
1. How is it that when high jumpers go over bars, their centers of mass pass underneath?

2. why must a hockey stick that is thrown rotating on ice stop rotating at exactly the same time as when the center of mass comes to rest?
 
Physics news on Phys.org
  • #2
In order to get over the bar the person requires that all their mass should pass over the bar. This does not have to happen simultaneously.

They arch their bodies... think about this.
 
Last edited:
  • #3
Is #2 even true? Can't you just spin the hockey stick so it has angular velocity but no translational velocity?
 
  • #4
I just tried it with my ruler on the desktop. It is easy to slide it such that it keeps spinning after CM comes to rest. Is this really a homework question?
 
  • #5
1) In the first case , when the athlete starts running , the c.m lies within the body and while the athlete is running , he experiences external forces of friction , thus the sentre of mass with accelerate/deccelerate along with the athlete , but when he loses his contact with the ground , there is an instantaneous absence of external forces and thus c.m keeps on traveling in same horizontal path even though athlete gains height , just like "in absence of external forces the c.m of a particle in projectile motion follows the same path even when the particle explodes and the pieces trace different paths.

2) "In absence of external forces , the c.m doesnot accellerate or changes its direction, the c.m in constant motion will remain in constant motion"

BJ
 
  • #6
Dr.Brain said:
1) In the first case , when the athlete starts running , the c.m lies within the body and while the athlete is running , he experiences external forces of friction , thus the sentre of mass with accelerate/deccelerate along with the athlete , but when he loses his contact with the ground , there is an instantaneous absence of external forces and thus c.m keeps on traveling in same horizontal path even though athlete gains height , just like "in absence of external forces the c.m of a particle in projectile motion follows the same path even when the particle explodes and the pieces trace different paths.

By the time the athlete loses contact with the ground, he had better have already established an upward component of velocity to his c.m. There certainly is not an "absence of external forces"- gravity is an external force. Ignoring air resistance, the jumper's c.m. follows a parabolic path, not a horizontal line. As others have said, the entire body does not have to pass over the the bar at the same time. Doing the "Fosbury Flop", the head and shoulders pass over the bar with the c.m not yet up to the bar. Then a swing of the arms and hips shifts the c.m. forward (with the c.m. going under the bar- the body is arched so that the c.m. lies inside the arch) so that the rest if the body can go over the bar. The "Western Roll" does something similar with one side of the body passing over the bar first. A hundred years ago, people were doing the "straddle jump"- where the center of mass does go over the bar and even olympic athletes could barely clear 5 feet.

2) "In absence of external forces , the c.m doesnot accellerate or changes its direction, the c.m in constant motion will remain in constant motion"
I don't see what this has to do with rotation. In any case the question necessarily assumes friction, an external force.
 
Last edited by a moderator:
  • #7
physicsstudent12 said:
1. How is it that when high jumpers go over bars, their centers of mass pass underneath?

2. why must a hockey stick that is thrown rotating on ice stop rotating at exactly the same time as when the center of mass comes to rest?

To answer your second question, I think its because of the sticks shape. Due to the non uniformity of the stick, the centre of mass is not at the exact middle of the shaft, also, becuase of this, when the stick rotates on the ice about its centre of mass, it will have different frictional forces action on it on either side of the cm. These forces would cause the cm to move, threfore needing the rotation to be 0 when the cm is at rest. The ruler analogy does not work as a comparison to this becuase a ruler is uniform.

Regards,

Nenad
 

FAQ: Why Do High Jumpers' Centers of Mass Pass Underneath the Bar?

What is the center of mass for a high jumper?

The center of mass for a high jumper is the point at which the body's mass is evenly distributed in all directions. This point is typically located around the hips and abdomen area.

How does the center of mass affect a high jumper's performance?

The center of mass is a crucial factor in a high jumper's performance. It determines the athlete's balance and stability while jumping, and can affect the height and distance they are able to achieve.

Can the center of mass be changed in a high jumper?

Yes, the center of mass can be altered through training and technique. High jumpers can adjust their center of mass by changing their body position and positioning their limbs in specific ways during the jump.

How does the center of mass differ between male and female high jumpers?

The center of mass for male and female high jumpers is generally located at different points due to differences in body composition and proportions. Male high jumpers typically have a lower center of mass compared to female high jumpers.

What are some ways to improve the center of mass for high jumpers?

Improving strength and flexibility in the core and lower body can help high jumpers have better control over their center of mass. Additionally, practicing proper technique and body positioning during the jump can also help improve the center of mass for high jumpers.

Similar threads

Replies
17
Views
1K
Replies
7
Views
3K
Replies
2
Views
1K
Replies
13
Views
2K
Replies
4
Views
7K
Replies
5
Views
2K
Replies
2
Views
2K
Back
Top