L2: Laws, Balance, Equilibrium, and Stability

Newton’s Laws explain the relationship between

force, inertia, and motion

What is Newton’s First Law?

A body in motion tends to remain in motion at the same speed in a straight line unless acted on by a force; a body at rest tends to remain at rest unless acted on by a force

What overcomes the bodys state if inertia?

Muscles produce force to start, stop, accelerate, decelerate, and change the direction of motion

• Bones (rigid bars) need to be moved by force which comes from muscles.

Define: Inertia

an objects resistance to action or change

In humans inertia is ….

body segments resistance to acceleration or deceleration

A body in motion remains constant unless…

acted on by an unbalanced force

• unbalanced forces are what allow a state of inertia to change, either accelerating or decelerating it.

I: How does human acceleration occur

We input force from muscular contraction to be greater than the forces holding us in place.

I: How does a human decelerate

Decrease in muscle force so that gravity and friction forces become the greater force

I: How does a change in trajectory occur?

To change our direction we change our muscle contractions so that more force occurs in a different plane and can then move in that direction.

Inertia in the human body is most impacted by….

Mass.

The greater the mass of an object, the greater their inertia.

Therefore more resistant to change, so, more force is required to change their state of inertia.

Sprinter in starting blocks must apply considerable force to overcome……

a state of no motion (resting) and accelerate

Runner on an indoor track must apply considerable X before hitting wall

force to overcome a state of motion with deceleration.

Changing direction…

Is costly on energy reserves because it required more force to constantly change direction.

What is Newton’s Second Law

A change in the acceleration of a body occurs in the same directions as the force that caused it. The change in acceleration is directly proportional to the force that’s applied to is and inversely proportional to mass (inertia)

Acceleration equals to…..

Force / Mass

Explain

• Acceleration (A) = Force (F) / Mass (M)

When F is applied to an object it accelerates in the direction of the force application (directly proportional)

However, how much it accelerates is proportional to the amount of F applied, divided by its M (inertia)

Define Acceleration

The rate of change in velocity ( how fast velocity is being changed)

To attain speed in moving the body….

A strong muscular force is generally necessary to overcome our mass and create acceleration.

Define: Mass

• it is different from…

• it affects….

The amount of matter in the body

• It is NOT poundage. Weight and M are different because weight is a product of gravitational pull.

  • M affects our velocity and acceleration because it is directly related to our state of inertia.

F = M x A

Greater F is required to move a greater M and accelerate it

What is Newton’s Third Law

For every action there is an opposite and equal reaction.

• For any F that is acting on an object, there is an equal and opposite F that’s going to occur from that object.

Ground-Reaction Force

As we place force on a surface by walking on it, the surface provides an equal resistance back in the opposite direction to our soles.

In Ground-Reaction Force we provide…

We provide the action force while the surface provides the reaction force

• Our feet push down and back, while the surface pushes up and forward.

Ground-reaction in different surfaces

• Track

Track resists runners propulsion force (action) and the reaction drives the runner ahead

Ground-reaction in different surfaces

• Sand

Action force given to sand is absorbed and dissipated. So, what the action force the sand receives a lesser than what we gave. As a result the sands reaction force is reduced which becomes apparent with the loss of forward force and speed.

What is friction?

Is a F that results from resistances between 2 surfaces of 2 objects that are moving upon one another.

• when 2 objects are touching there is a certain amount of friction F between them.

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The amount of friction F between objects is typically related to….

How much R is happening between the two objects

• The greater the R → harder to move → greater the friction

  • The lower the R → easier to move → lesser the friction

Do we want friction? What are some examples?

Depending on the situation we may want more or less friction

• Running: We want a certain amount of friction between our feet and ground so we have grip as we apply action force and propel forward

  • Skating: We want to decrease the amount of friction so a lower amount of energy is used to propel with less R (less muscular F required)

What are the types of friction?

Static Friction: the amount of friction between two objects that have not yet moved. (at rest)

Kinetic Friction: the amount of friction between two objects that are sliding upon one another (not at rest)

Which type of friction is greater? Why?

SF is always > KF because it is more difficult to initiate movement between 2 objects than it is to continue their movement.

SF has perpendicular forces that hold it together. When at rest the surface of the 2 objects have greater binding. When they’re moving the bonds are broken so keeping the movement continuous is easier.

How can static friction be increased?

SF can be increased by adding more weight in the perpendicular/sideways direction of the 2 objects.

• more mass between objects → the greater the SF between the 2 → the harder to initiate movement between the 2.H

How do we determine friction?

To determine the amount of friction forces, we need to consider BOTH forces that are pressing the 2 objects together and the coefficient of friction.

• depends on the harness and roughness of the two objects

What is the coefficient of friction?

The ratio between F needed to overcome the F over the F holding the surfaces together.

We need to consider…

Fn

f

u

u = f / Fn

mass, gravity, and friction force

Fn= mass of object x gravity (inherent state of inertia)

F= friction force - influenced by floor and shoes

• what is the influence of the roughness/hardness in ADDITION to Fn, which is what effects u.

u= coefficient of friction

u = f / Fn

Fn is constant, therefore what helps to determine u are our shoes and the surface we are walking on

→ which determines our likeliness to slip or not slip on the surface

If u is > 1

If u is < 1

less likely to slip

more likely to slip

Define: Balance

Ability to control equilibrium, either static or dynamic.

Define: Equilibrium

State of 0 acceleration where there is no change in the speed or direction in the body

• An objects ability to control equilibrium (modifying itself) means that the object has a certain amount of balance

What are the types of equilibrium?

Static Equilibrium: body is at rest or completely motionless

Dynamic Equilibrium: all applied and internal forces action on moving body are in balance (equal). Resulting in movement with unchanging speed or direction.

• constant state of velocity so no acceleration or change

The ability of an object to maintain a state of equilibrium (static or dynamic) means…

Means it has balance

To control equilibrium and achieve balance, X needs to be…

Stability needs to be maximized

Define Stability

An objects resistance to change (objects inherent inertia)

• … in body’s acceleration

  • … and disturbance of body’s equilibrium

How is stability maximized? By doing so we can….

By determining the body’s center of gravity and appropriately changing it.

By appropriately changing the body’s center of gravity we can maximize our balance.

Define: Center of gravity

Point of balance for an object where weight and mass are equally distributed in all directions.

Why is balance important?

Is important in resting and moving bodies and objects so they are not constantly falling

How many general factors are applicable when enhancing equilibrium, maximizing stability, and ultimately achieving balance?

Nine

1) A person has balance when…

the center of gravity falls within the base of support

2) A persons ability to achieve balance is directly ….

… directly proportional to the size of our base of support

• the larger → the more balance because there is more space to find balance within

3) A persons weight is proportional to ….

the ability to achieve balance

• inertia → you are harder to be moved if you have more mass

4) Balance is dependent upon the height …

the height of the center of gravity

• the lower the CoG, the eaiser to balance.

5) A persons ability to balance is dependent on where the CoG is…

• how can this be improved when anticipating an incoming force

Within their base of support

• if CoG is nearer to the edge of base, it is more likely they will be tipped over

• if CoG is in the middle, there is more space in all directions before they fall outside of the base.

When anticipating an oncoming force, stability may be improved by shifting our CoG in the direction of the oncoming force.

6) In anticipation of an oncoming force, stability may be increased by…

Enlarging the size of the base of support in the direction of the anticipated force.

7) Equilibrium may be enhanced by….

Increasing the friction between the body and the surfaces it contacts

→ increasing the coefficient of friction between the person and ground

8) Rotation about an…

Axis aids in balance.

→ a moving bike is easier to balance than a stationary one.

9) Kinesthetic physiological functions….

Contributes to balance.

→ our ability to feel balance is directly related to our ability to have kinesthetic awareness.

→ semi circular canals, vision, touch (pressure), and kinesthetic sense.

Walking is a series of…

Controlled falls

Explain why walking is a series of controlled falls.

The person throws the body in an out of balance for each step to occur.

If the CoG was always kept within the base of support a person cannot move through space. So, to move forces have to be thrown off balance, and allow yourself to fall outside the base of support.

(Picture of Walking)

1) Our CoG is taken outside of the base of support with right foot

2) We fall through space

3) We catch ourselves on our left foot and briefly regain balance.

4) Repeat