Biomechanics Linear Kinetics

What is kinetics concerned with?

the causes of motion

Are forces a vector or scalar measure?

vector

What is the Law of Inertia?

that a body in motion will stay in motion unless acted upon by an external force

What is the Law of Acceleration?

that a force applied to a body will cause acceleration

Magnitude is proportional to what and moves in what direction?

the force applied and the same direction of the force

Force is ___ proportional to the body’s mass.

inversely

What is the Law of Action-Reaction?

when one body exerts a force on the second, the second body exerts a reaction force equal to the magnitude and in the opposite direction of the first body

What unit is force measured in?

Newtons (N)

What is the line of application?

A straight line that passes through the point where the force is applied and extends in the same direction as the force vector

What is the point of application?

the area where force is acting on an object

What does a free body diagram represent?

it depicts forces acting on a system and helps define the system

What are the two types of forces?

contact force and noncontact force

What is a contact force?

the forces exerted by one object in direct contact with another object

What is a noncontact force?

forces exerted by objects that are not in direct contact with one another

What is the gravitational constant?

6.67 × 10^-11

Can altitude and elevation affect the gravitational constant?

yes

What type of force is a ground reaction force?

contact force

What is a ground reaction force?

when the ground surface produces a reaction force against the object touching it

What path does the kinetic chain follow?

starts distally and migrates proximally

How are ground reaction forces typically measured?

with force plates

What are the three components of a ground reaction force?

vertical force, antero-posterior force, and medio-lateral force

The actual ground reaction force is what?

the resultant vector of the horizontal and vertical components

During walking, vertical GRF typically looks like what?

a positive, two peaked line

On a antero-posterior GRF graph, a negative value indicates what?

slowing down, or a braking force

On a antero-posterior GRF graph, a positive value indicates what?

speeding up, or a propulsive force

Which of the three GRF components has the most variability and typically the lowest force during forward movement?

medio-lateral force

What is the typical magnitude of vertical force during walking?

1.1-1.2 BW

What is the first peak of a vertical GRF graph called?

impact peak

What is the second peak of a vertical GRF graph called?

active peak

What is the typical magnitude of the vertical force during running?

2-3 BW

What can increase running injuries?

high vertical load rates

What can cause high vertical load rates?

strike pattern (rear-foot)

Impact peak during running is usually dependent on what?

what region of the foot impacts the ground

What vertical GRF peak has no link with injuries?

active peak

What is the typical antero-posterior GRF during walking?

.15 BW

What is the typical antero-posterior GRF during running?

.5 BW

Which typically has a larger acceleration and deceleration peak? Walking or running?

running

What is the typical range of medio-lateral GRF?

.01-.02 BW

Right and left medio-lateral GRF should be symmetrical? T/F

true

What are joint reaction forces?

forces acting between two segments

What are the other components of joint reaction force?

compressive force and shear force

What impact does an unstable knee have on joint reaction force?

it may increase compressive forces

What is momentum?

a mechanical quantity of motion that an object has

When is momentum important?

during collisions

A static object has momentum? T/F

false

What causes a change in momentum?

either a change in mass or a change in velocity

What is the momentum rule for collisions?

both objects tend to move in the direction of motion possessed by the object with the greatest momentum

What is impulse?

a change in momentum

What is the area under a force-time curve?

impulse

Is walking a mile better than running for joint problems? Why?

the total impulse is equal between walking and running due to more footfalls while walking than running

If negative and positive impulse are equal, then velocity is what?

constant

What does a larger negative than positive impulse mean?

the object is slowing down

How can you manipulate impulse during a jump landing?

by having a soft or hard landing

Why does a soft landing decrease impulse?

it increases the time it takes to land

What is a perfectly elastic collision?

a collision where no energy is lost

What do post-impact velocities depend on?

pre-impact velocities and elasticity of impact

What is deformation?

a change in an object’s shape

What is restitution?

an object returning to its original shape

The coefficient of restitution for a perfectly inelastic (plastic) collision is what?

e = 0

The coefficient of restitution for a perfectly elastic collision is what?

e = 1

What is the coefficient of restitution defined as?

The ratio of relative velocities before and after impact of two colliding objects

What is the coefficient of restitution affected by?

the object height, the landing surface, or the surrounding temperature

What does a high bounce height imply for the coef of restitution?

a high coef

What does landing on a soft surface do to the coef of restitution?

causes a low coef

Are all objects’ coef of restitution affected by temperature?

no, some are more affected than others

What is friction?

a force that interacts parallel to two surfaces in contact and opposite to the direction of motion

What is friction without movement called?

static friction

What is friction with movement called?

kinetic friction

What is another word for static friction?

traction

What does the coefficient of friction indicate?

how well something slides

What affects the coefficient of friction?

roughness of surfaces, hardness of surfaces, and molecular interaction between surfaces

How can you modify friction?

by changing either normal force or the coefficient of friction