15. Equilibrium, Balance & Stability

Question 1

When are living humans motionless?

Answer 1

Never, we can’t be truly motionless as long as our hearts are pumping an we are breathing

Question 2

What is Equilibrium?

Answer 2

Equilibrium is a physical state, in which opposing forces equal out

Question 3

What is Balance?

Answer 3

• Balance is an even distribution of weight enabling someone or something to remain upright and steady
• Balance is a static concept

Question 4

What is Static equilibrium?

Answer 4

Static equilibrium occurs when an object is at rest with no net force or torque acting on it, resulting in zero acceleration.

Question 5

What is Stability?

Answer 5

• Stability is the capacity of an object to return to equilibrium or to its original position after it has been displaced
• Stability is a dynamic concept (ish)
• Reflexes

Question 6

The center of gravity of an object is its balance point or the point about which the moments of force created by the weights of all the parts of the object sum to zero.

Knowing this, how does an object stay balanced?

Answer 6

For an object to be in balance (in a state of equilibrium), the external forces acting on it or the moments of those forces must sum to zero

Question 7

How do we calculate Static Equilibrium?

Answer 7

Question 8

Calculating Static Equilibrium

Two individuals apply force to opposite sides of a frictionless swinging door. If A applies a 30 N force at a 40° angle from 45 cm from the door’s hinge and B applies force at a 90° angle 38 cm from the door’s hinge, what amount of force is applied by B if the door remains in a static position?

Answer 8

Question 9

The forearm is oriented at 90° relative to the humerus. The center of mass of the forearm and hand, 24 N, is located 16 cm from the elbow and the elbow flexor muscles attach 3.2 cm from the joint center. A 60 N weight is being held stationary in the hand 28 cm from the joint center.

Assuming the muscle attaches at an angle of 90° relative to the bone, how much force must be produced in the muscle to maintain this position in static equilibrium?

Answer 9

Question 10

What is Dynamic Equilibrium?
How do we calculate it?

Answer 10

Question 11

A 580 N skydiver in free fall is accelerating at -8.8 m/s2 rather than -9.81 m/s2 because of the force of a vertical air resistance. How much force is acting on the skydiver?

Answer 11

Slide 17

Question 12

The center of gravity of an object is its balance point or the point about which the moments of force created by the weights of all the parts of the object sum to zero.

Does this effect Stability?

Answer 12

So, stability is affected by, among other things, the position of the center of gravity

Question 13

The stability of an object is affected by the base of support, the area bound by the outermost regions of contact between a body and support surface.

How can we increase our stability while standing?

Answer 13

As the base of support increases (area of contact), stability increases

Question 14

How is Stability affected as the center of gravity moves toward the anterior-posterior edge?

Answer 14

Stability decreases as the center of gravity moves towards
the anterior-posterior edge of the base of support

red

Question 15

How is Stability affected as the center of gravity moves toward the lateral edge?

Answer 15

Stability decreases as the center of gravity moves towards the lateral edge of the base of support

blue

Question 16

How does Height affect Stability?

Answer 16

Stability decreases as the center of mass moves up the system

It would be easier to knock over a system with a higher c.o.g
(Assuming the system was rigid & falling around a fixed point)

The higher the center of mass, the greater the amount of torque its motion creates about the support surface

Question 17

How does Mass Affect Stability?

Answer 17

• According to Newton’s First Law, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force
• This resistance to change, or inertia, is proportional to mass
• As mass increases, stability increases

Question 18

How does Friction affect stability?

Answer 18

• As mass increases, the contact (reaction) force also increases, which increases friction
• So, the amount of friction present also influences the level of stability

Question 19

Humans can control stability by changing their stance and body position. Give 2 Examples of this?

Answer 19

1. A staggered stance decreases mobility but provides increased stability
2. Lowering the center of gravity and helps to maximize stability

Question 20

Who Gives a Darn About this Stuff?

Answer 20