L6 - Linear Kinetics

Force

the interaction of two objects that can produce a change in the state of motion of an object by pushing/pulling

1 Newton

the amount of force it takes to accelerate a 1kg body at 1m/s squared

Body mass is a vector

False, scalar

Body weight is a vector

True

Point of application

the specific point any which the force is applied to an object

Line of action

a straight line in the direction in which the force is acting

Collinear forces

multiple forces in the same line

Coplanar forces

multiple forces in one plane

Law of inertia (Newton’s 1st Law)

a body will remain at rest or continue to move with a constant velocity unless acted upon by an outside force

Inertia

used to describe an object’s resistance to motion and is directly related to the mass of the object

An increase in mass = an increase in inertia

True

To get something at rest to move, the external force needs to be greater than the inertia/mass of the object

True

Law of acceleration (Newton’s 2nd Law)

a force applied to a body causes acceleration of that body:

• of a magnitude proportional to the force

• in the direction of the force

• an inversely proportional to the body’s mass

Momentum

the quantity of motion of an object

The faster an object moves with more mass, the smaller its momentum

False, greater momentum

The total momentum of a given system remains constant in the absence of external forces

True

Linear impulse

the product of a force and the time interval over which a force acts

If an impulse acts on a system it will change the total momentum of the system

True

How can you use linear impulse to increase momentum

exerting a large force against the object for as long as possible

Action/Reaction (Newton’s 3rd Law)

for every action there is an equal and opposite reaction

Contact force

push/pull of an object in direct contact with another object

Non-contact force

no direct contact with the object

Gravity is an example of a non-contact force

True

two bodies colliding is an example of a non-contact force

False, contact force

What is the attractive force of the earth on an object called

the weight of an object

Ground reaction force

The weight of a person standing in the anatomical neutral position generates a reaction force by the ground that is equal in magnitude and opposite in direction to the weight

Factors influencing ground reaction force

• running speed (increased velocity = increased force)

• footwear

• ground surface (concrete vs grass)

• grade of surface (elevation)

Propulsive force

a force that is causing a body to speed up

Breaking force

a force that is causing a body to slow down

Why are segments often analyzed separately in joint reaction forces

because human motions are so complex

Sheer force

acts parallel to the joint

Friction

a force that acts parallel to the interface of two surfaces that are in contact during motion (opposite direction)

Static friction

friction that is large enough to disallow movement

Kinetic friction

friction that allows movement

Kinetic friction is greater than static friction

False, less

A greater coefficient of friction indicates an object is easier to slide

False, harder

A smaller coefficient of friction indicates an object is easier to slide

True

Ice has a smaller coefficient of friction than carpet

True

What is fluid resistance affected by

• density (more dense = more resistance)

• viscosity (more viscous = more resistance)

Drag force

always acts to oppose the motion

Lift force

• always acts perpendicular to the drag force

• produced by a break in the symmetry of airflow

Magnus effect

lateral deflection of an object to seek low pressure area

One body segment can exert force on another segment, causing movement at that second segment without muscle exertion

True, typically a more proximal segment exerts an inertial force on a more distal segment

Muscle can generate both a pulling and pushing force

False, only pulling

Elastic force

force applied to a material that results in a change in length of that material (ex. diving board)

Work

the product of the magnitude of the force applied to an object and the distance the object moves in the direction of applied force

Positive work

the object is moved in the same direction of the force

Negative work

the object is moved in the opposite direction of the force

Power

the amount of work done per unit of time (rate of mechanical work)

Simultaneous kinetic linking

activation of multiple muscle groups and segments at the same time

Sequential kinetic linking

activation of muscle groups and segments at different times throughout the movement

Energy

the capacity to do work

Two main types of mechanical energy

• kinetic

• potential

Kinetic energy

the energy resulting from motion

Potential energy

capacity to do work because of position or form

Strain energy

• type of potential energy that comes from object deformation

• resistance to the deformation results in stored elastic energy (strain energy)

Internal work

examining the sum of individual segment’s work by examining the mass and velocity of body segments, KE can be examined

External work

the work done by a body on an object (need to know mass and velocity of these segments/bodies)

A body behaves as if all it’s mass is concentrated at the COM

True

The human body is not a rigid object so the location of the body’s COM depends on the position of the extremities

True

What is COM important for

• stability: resistance to linear/angular acceleration

• balance: ability to control equilibrium

How to improve stability

• increase body mass

• increase friction between body and contact surface

• increase size of support base in direction of the line of action of the external force

• horizontally position COM near edge of base of support on side of oncoming force

• vertically position COM as low as possible

Pressure

the force per unit area