Biomechanics Unit 2

What is the definition of Kinetics?

• The branch of Dynamics concerned with the Forces that cause Motion

What is the definition of Linear Kinetics?

• Concerned with the forces that cause Linear Motion

  • Useful in analyzing and explaining the techniques used in many sport skills.

What is Newton’s First Law?

• Law of Inertia

• Everybody continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.

• The Horizontal component of a Projectile

If No External Force acts on an object or if the Net External Force is Zero the object will?

• Remain in its state of rest or will continue moving at a constant speed in a straight line.

• Newton’s First Law

What is Linear Momentum?

• The product of an Objects Mass and its Linear Velocity

• kg x m/s

• Vector Quantity

The Greater the Mass and or the Greater the Linear Velocity the?

• Greater the Linear Momentum of the Object.

The Direction of an Object’s Momentum is determined by?

• The Direction of its Velocity

In the Absence of External Forces?

• Momentum is Conserved

If a system has a number of objects the momentum of the system is?

• Conserved

• If no External Forces act on the System

  • Therefore, if one object speeds up another on will slow down to keep momentum unchanged

  • Batting, Golf, and Racquet Sports

Friction and Air Resistance are Common External Forces that?

• Reduce Momentum

The Conservation of Momentum Principle is useful in?

• Analyzing Collisions

What is Elastic Collisions?

• Momentum is Conserved in Elastic Collisions

What are Inelastic (plastic) Collisions?

• Momentum is Conserved but objects stay together after the collision and move together with the same velocity

• Common in Contact Sports such as Football

What is the Coefficient of Restitution?

• Used to quantify the Elasticity of a Collison

• Absolute value of the ratio of the velocity of separation to the velocity of approach.

• Affected by the nature of the objects, temperature, and the velocity of the objects.

In Coefficient of Restitution. When one object is ___ and _____ only the velocity of the object before and after the collision needs to be known

• fixed and immovable

______ and ______ can be used to calculate a Coefficient of Restitution?

• Drop Height and Rebound Height.

What is the Coefficient of Restitution in Perfect Elastic?

• 1.0

What is the Coefficient of Restitution in Perfect Inelastic?

• 0

True or False:

Many Collisions are between Perfectly Elastic and Perfectly Inelastic?

• True0

What is Newton’s Second Law?

• The change of motion of an object is proportional to the forced impressed.

• Made in the direction of the straight line in which the force is impressed.

What is the summarized definition of Newton’s Second Law?

• Law of Acceleration

If a net external force is exerted on an object?

• The object will accelerate in the direction of the net external force and its acceleration will be proportional to the net external force and inversely proportionally to its mass.

Newton’s Second Law is a ___ and ___ relationship.

Cause and Effect

What is a Cause and Effect Relationship?

• A net external force causes and object to accelerate

• If an object accelerates a net external force must have acted on it.

What happens when a Elevator speeds up for Newton’s Second Law?

• The reaction force is greater than your weight and you feel heavier

What happens when a Elevator slows down for Newton’s Second Law?

• The reaction force is less than your weight, so you feel lighter.

True or False:

It is easier to accelerate an object Horizontally than Vertically?

• True

A net force is needed to ___ ___ or ___ ___ an object?

• Slow Down

• Speed Up

A net force is also needed to ___ ___?

• Change Direction

• Running a curve on ice

True or False:

A Net Friction force is needed to Change Directions?

• True

åF = ma only tells us?

• What happens at the instant the force acts.

In sport and human movement, we are more interested in the?

• Final outcome of forces acting over time.

When is Impulse Produced?

• By a Net Force acting overtime will cause a Change in Momentum of the Object

The greater the force that can be generated or longer the amount of time a force can beapploed?

• The greater the impulse that is imparted to the object

• Best situation is to do Both.

• Results in increased velocity of the object and therefore, Increased Momentum

Throwing Light Objects?

• Time of Application is the most important.

  • Ping-pong

  • Baseball

Throwing Heavy Objects?

• The Force Applied is the most important

  • Shot-Put

m (v_f – v_i) is the?

• Object’s Momentum

The Faster an Object moves and the Larger it’s Mass?

• The more Momentum it Possess.

Most Objects have a Constant Mass, so the Velocity?

• Must Change to Change Momentum.

Impulse can be also used to ?

• Decrease Momentum

What are some situations where you want to Decrease Momentum?

• Landing from a Jumo

• Catching

• Pole Vaulting

• High Jump

• Gymnastics

• Mid-sole in Running Shoes

• Falls

What are used to increase the time of the impact and proportionally decrease the average landing force.

• Landing Pads

What is the Landing with Straight Legs Example?

• Velocity is the same in Both Conditions

• The product of force and time is the same but individually they can be different.

• Increase the time to decrease impact force with the ground.

What is Newton’s Third Law?

• To every action there is always opposed an equal reaction

• Mutual actions of two bodies upon each other are always equal and directed to contrary parts.

What is the summarized definition of Newton’s Third Law?

• Law of Action-Reaction

This is an example of Newton’s Third Law:

• If object (A)exerts a force on another object (B), the other object (B) exerts?

• The same force on (A) but in the opposite direction.

What are examples of Newton’s Third Law?

• Pushing in a wall, football, boat oars and the water.

What is the football example of Newton’s Third Law?

• Action-reaction forces were the same, by because the larger player has more mass and more friction the effect of the larger players pushing force on the smaller player will be greater.

What is Newton’s Law of Universal Gravitation?

• All objects attract each other with a gravitational force that is inversely proportional to the square of the distance between the objects.

• Helped to predict the motion of the stars and planets.

The force of gravity is proportional to the mass of each of the?

• 2 bodies being attracted to each other

What are Force Platforms?

• Majority use Piezoelectric crystals

• Crystals must be connected to a charge to amplifier and an electronic circuit

• Presentation of Data

What are Piezoelectric Crystals?

• A natural occurring mineral that produces electric charge in response to deformation from an applied force

What are Force Transducers?

• Stress can be applied to a material and strain, or failure can be determined.

What do Hand Dynamometers measure?

• Grip Force

What are Pressure Sensors?

• Usually, thin mats with force sensors imbedded in them.

• Can quantify the pressure exerted on each specific area

Beginning of Chapter 4

True or False:

Causes of motion that do not rely on Newton’s Laws of Motion but rather on the relationships between work, energy, and power.

• True

What is Work in mechanics?

• Product of Force and the amount of Displacement in the Direction of that force.

• It is the mean by which energy is transferred how one object or system to another.

Because work is the product of force and displacement?

• The units for work are units of force times units of length

• Newton meters (Nm)

• Foot-pounds (ft x lb)

1 Joule is equal to how many Newton meters (Nm)

• 1 Nm

Work can be described as the work done by a ?

• Specific force or as a net work.

What is does Net Work mean?

• Net work done on an object is the work done by all the external forces acting on that object.

True or False:

Work can be Positive or Negative?

• True

Positive work is done by?

• Force acting on an object if the object is displaced in the same direction as the force

  • Example:

    • Pitcher does positive work against a baseball when throwing it

    • Weightlifter does positive work when lifting or raising the weight.

Negative work is done by?

• A force acting on an object when the object is displaced in the direction opposite the force acting on it.

• Example:

  • A first baseman does negative work against the ball when catching it.

  • A weightlifter does negative work against a weight when lowering it.

Muscles can also do ____ ____?

• Mechanical Work

When a muscle contracts?

• It pulls on its points of attachment.

True or False:

Positive work is done by a muscle when it contracts and its points of attachment move in the direction of the muscle force pulling on them.

• True

The force (muscle force) and the displacement (displacement at the point of muscle attachement) are in the same direction?

• The muscle shortens and the muscle contraction is a concentric contraction

True or False:

Negative work is done by a muscle when it contracts and its points of attachment move in the opposite direction of the muscle force pulling on them.

• True

The force (muscle force) and the displacement (displacement at the point of muscle attachment) are in opposite directions

• The muscle lengthens, and the muscle contraction is an eccentric contraction.

True or False:

Not all muscle contractions produce mechanical work.

• True

A muscle can contract and do zero mechanical work, which occurs when?

• A muscle contracts and its points of attachment do not move relative to each other.

The displacement at the point of muscle attachment is zero?

• The muscle length remains unchanged, and the muscle contraction is an Isometric contraction.

In Mechanics, energy is defined as?

• The capacity to do Work

Kinetic Energy is energy due to ?

• Motion

Potential Energy is stored energy due to?

• Position or Deformation

What are 2 types of Potential Energy?

1. Gravitational Potential Energy

2. Strain Potential Energy

What is Gravitational Potential Energy?

• Energy due to an object’s position relative to the earth

What is Strain Potential Energy?

• Result of the deformation of an object

What is Gravitational Potential Energy referred to as?

• Potential Energy

The gravitational potential energy of an object is related to?

• object’s weight and its elevation or height above the ground or some reference.

Strain potential energy is also referred to as?

• Elastic Potential Energy

• Is usually shortened to just strain energy

What is Strain Potential Energy used for/in?

• Sports

The definitions of Work and Energy indicates?

• That a relationship exists between them.

The unit of measure for work and energy is ?

• Joules (J)

The relationship is expressed by the Work Energy Principle, which states that the Net Work done by all the external forces acting on an object causes a change in?

• Kinetic Energy of the object or system.

An alternative way of expressing the Work Energy Principle is that the Work done by all the Nonconservative forces acting on an object causes?

• A change in total Mechanical Energy of the object or system.

More work = ?

• Greater change in Energy

• Increase the force and or increase the displacement in line with the force.

• A shot’s Kinetic Energy is Increased

Use Negative Work to Decrease?

• An Objects Kinetic Energy

• The displacement is increased = reducing kinetic energy

What is an example of using negative work to decrease an objects kinetic energy?

• Catching a ball

• Landing from a height

• Falling on landing pads

• Jumping in water

• Midsoles in running shoes.

Shock Absorbing Materials are?

• Energy-Absorbing Materials

Conservation of Mechanical Energy is useful when examining situations where?

• There are no External Forces other than Gravity acting on the object.

No work can be done so the Total Mechanical Energy of the object is?

• Conserved

What is the Conservation of Mechanical Energy Throwing a Ball Example?

• At release, the ball has little Potential Energy but a lot of Kinetic Energy.

• As the ball Reaches its Apex, it has a lot of Potential Energy but little Kinetic Energy.

• Opposite effect on the way back to Earth.

• Mechanical Energy is Conserved.

What is the definition of Power in Sports?

• It is related to the ability of an athlete to increase the displacement of an object while exerting a force affects performance in many skills.

What is the definition of Power in Mechanics?

• Power is the rate of doing work, or how much work is done in a specific amount of time.

Power can also be defined as the ?

• Average force times the average velocity along the line of action of that force.

Power can be measured in ?

• Horsepower or Watts (W)

1 Watt = ?

• 1 J/s