Linear Kinetics

Newton’s 1st Law of Motion

Law of inertia

Newton’s 2nd Law of Motion

Law of Acceleration

Newton’s 3rd Law of Motion

Law of Reaction

Law of inertia:

A body remains at rest or in motion unless an external force acts on it

Inertia:

The tendency of an object to keep the current state of motion

Inertia is proportional to…

the mass of the object

Mass:

Measure of inertia in linear motion

Law of acceleration

An external force applied to a body causes acceleration

Force (F)=

mass(m) * acceleration(a)

Acceleration (a)=

F/m

Acceleration is proportional to…

force

Acceleration is inversely proportional to…

mass

A force of 0 means a _ velocity

constant

0 force means _ in state of motion

no change

Reaction:

Same magnitude in the opposite direction of an action

Law of reaction:

For every action there is an equal and opposite reaction

Examples of the law of reaction

* Bullet vs. gun
* Fist fighting
* Propulsion during swimming

Ground reaction force is what kind of force?

Passive

Importance of ground reaction force

It’s the major external force that accelerates the body. The only external force one can voluntarily control

Law of gravitation

All bodies are attracted to one another by their masses

Gravitation is proportional to…

the masses

Gravitational Force (F_g)=

Gravitational constant (G)*(m1m2/d²)

Gravitation is inversely proportional to…

(separation distance)^2

Weight=

(mass)*(gravitational acceleration)

Weight:

Gravity exerted to a body by the earth

What is gravitational acceleration?

The attraction Earth has on bodies, 9.81m/s^2

Friction:

Force acting at the area of contact between two surfaces

Source of horizontal ground reaction force

Friction

Magnitude is proportional to

Normal reaction force (N) and friction coefficient (μ)

Magnitude cannot exceed…

your force

Direction is opposite that of…

motion or motion tendency

Sliding friction:

Friction due to relative motion of surfaces

Rolling friction:

Friction due to temporary deformations of the surfaces

Which friction is more efficient?

Sliding

Static friction:

Friction experienced with no motion

Kinetic (dynamic) friction:

Friction experienced during motion; Force required to maintain motion once initiated

Maximum static friction is the…

force required to initiate motion

Net friction=

Force minus friction or (mass)*(acceleration)

Accleration=

Force minus friction/mass

Source of horizontal GRF

Friction as a source of propulsion

Methods to minimize friction

• Lubrication (ex. synovial fluid)

• Skating during gliding

Methods to maximize friction

• Cleats (prevent slipping)

• Skating during pushoff

Momentum:

Amount of motion

Importance of momentum

Giving & receiving impact, collision, etc.

SI unit of momentum

kg\*m/s

Momentum=

inertia (m)*velocity(v)

Principle of momentum conservation

If no external force is in play, total momentum remains constant. (Law of inertia, objects want to continue their current state of motion)

Impulse:

The total amount of force exerted over a period of time

Impulse=

Average Force (F bar)* elapsed time

SI unit for impulse

Ns

Impulse causes a change in…

momentum

To maximize giving impulse what needs to be maximized?

The momentum of the apparatus you’re putting force behind (batting)

Giving impulse can be improved by…

lengthening how much time elapses (throwing)

Impulse can be reduced by increasing…

time/area

Impulse is a _ quantity

vector

Mechanical work is a _ quantity

scalar

Mechanical work:

Force applied against a resistance; Total effect of force exertion/displacement

Work=

Average force x displacement x cosθ

SI unit of mechanical work

Joule (J)

How is positive work differentiated from negative work?

The angle between F and d, if θ > π/2 rad, the work is negative

Concentric contraction is _ work

positive

Eccentric contraction is _ work

negative

Isometric contraction has _ work

no mechanical

When working against gravity, only _ displacement is considered

vertical

Mechanical power

Rate and intensity of work; rate of energy generation/consumption

Power=

work/elapsed time or force/velocity

SI unit for power

Watt (W) or Joules/sec

Mechanical power is a _ quantity

scalar

Mechanical power=

Work (F*d*cosθ)/elapsed time

OR

Force*average velocity* cos θ

Mechanical energy:

Capacity to do mechanical work

Mechanical energy=

potential energy + kinetic energy

Potential energy:

energy due to vertical position (height)

Kinetic energy:

energy due to motion (velocity)

Mechanical energy is a _ quantity

scalar

SI unit for mechanical energy

Joule (J)

Potential energy=

mass x gravity x height

Kinetic energy=

(1/2mass) x (velocity^2)

Conservation of Mechanical energy

If gravity is the only external force acting on a system, energy remains constant.

Mechanical work=

Change in mechanical energy

Positive work _ energy of the system

increases

Negative work _ energy of the system

decreases

Friction force to initiate motion=

Static friction coefficient (μ_s) x Total Newtons

Friction force to maintain motion=

Kinetic friction coefficient (μ_k) x Total Newtons