Biomechanics - Linear Kinetics

What do you do to change velocity (momentum)?

- change amount of force - change duration of time force is applied

What is acceleration?

- effect of forces - when net force is zero object is not accelerating - net force is not zero if object is accelerating

What requires a net force to occur?

- start a movement - stop a movement - change in direction

What creates greater momentum?

- the faster an object moves - the heavier an object

120kg linebacker collides with 80kg fullback at the goal line. - fullback: 6m/s toward end zone (+) - linebacker: 5m/s opposite direction (-) Does he score?

120*5 = 600 , 80*6 = 480 480-600 = -120 -120/(120+80=200) = -.06 m/s Answer: no - Remember (-) direction is away from goal

Student = 68kg Dr. V = 120kg They are both moving at 3 m/s. Who has more momentum?

68*3 = 204 , 120*3 = 360 Thus, Dr. V has more momentum 360 - 204 = 156 = net linear momentum

Student = 68kg Dr. V = 120kg The Student moves at 5.29 m/s and Dr. V moves 3 m/s. Who has more momentum?

68*5.29 = 360 , 120*3 = 360 Thus, they have the same momentum 360-360 = 0 = net linear momentum

Mass of fist/glove = 4kg Time = .10s Velocity = 25 m/s What is the force exerted?

Equation: F*t = m (vf - vi) F*.10 = 4 (0-25) F*.10 = -100 F= -100/.10 F = -1000 N

Mass of fist/glove = 4kg Time = .10s Velocity = 50 m/s What is the force exerted?

Equation: F*t = m (vf - vi) F*.10 = 4 (0-50) F*.10 = -200 F = -200/.10 F = -2000 N

Mass of fist/glove = 4kg Time = .20s Velocity = 25m/s What is the force exerted?

Equation: F*t = m (vf - vi) F*.20 = 4 (0-25) F*.20 = -100 F = -100/.20 F = -500 N

Can internal forces change the bodies center of mass?

No; must interact with outside world to change position of center of mass

What is friction?

a force acting over the area of contact between two surfaces - direction is opposite of motion - magnitude is the product of friction

What is the 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 same direction of the force - inversely proportional to the body's mass - F = m*a

What is needed to have a net force?

acceleration

What is an external force?

act on an object as a result of its interaction with the environment surrounding it. EX: most are contact forces. gravity is non-contact

What is required to have momentum?

an external net force is required

If a net force acting on an object is zero the velocity of the object is?

constant L= constant or 0 if F=0

For bodies in motion friction is?

constant and less than maximum static friction

If a net external force action on an object is zero the object will?

continue along its original path - no chage in movement if there are forces but they sum to zero

Net external forces applied to a non-rigid body will cause?

deformation because they are resisted by internal forces

What are the two types of momentum?

elastic collision and inelastic collision

For static bodies friction is?

equal to the applied force

What type of net force is always required?

external

What is the law of reaction (Newton's 3rd law)?

for every action, there is an equal and opposite reaction - when one body exerts a force on a second, the second body exerts a reaction force that is equal in magnitude and opposite in direction

What is required to start, stop, and alter motion?

force

What are internal forces?

forces that act within the object or system whose motion is being investigated. Resist external forces EX: created by muscles, tendons, bone, ligaments, cartilage

What are centric forces?

forces that are directed through an object's center of mass. EX: - can only result in change in linear movement/position - through center mass

What are eccentric forces?

forces that are not directed through an object's center of mass EX: - result in a change in linear position AND rotation of the object - torque - not through center mass

What is the horizontal force?

friction

What happens to friction when motion occurs?

friction drops and then stays constant

What is the vertical force on a projectile?

gravity

What is the equation for impulse?

i = f*t

What causes momentum?

impulse (how much force applied and time)

What is the effect of applying force over a longer period of time?

increase

If you want to increase velocity what can you change?

increase force and time

What is directly proportional to mass of an object?

inertia - the greater the mass the greater the force necessary to alter motion

What are Newtons 3 laws?

intertia, acceleration, reaction

When objects collide what happens to linear momentum?

linear momentum is conserved - before it collides and after it collides it is equal.

The effect of a force depends on?

mass and other forces; larger the mass the smaller the effect upon it

Net external forces applied to a rigid body will cause?

motion

What causes movement?

net force

What velocity does dynamic equilibrium have?

non-zero

What is elastic collision?

objects bounce off of each other and each object transfers all of its momentum to the other EX: - pool, soccer, tennis

What is inelastic collision?

objects do not bounce off each other. They stay together and continue with the same velocity. "stick together" EX: football

What is a force?

push or pull between objects that may or may not result in a movement

what force is perpendicular?

reaction force

If the net force is zero the object will?

remain in its current state

What is inertia?

resistance to change in state of motion

What are the two types of friction?

static and dynamic

When objects collide according to elastic collision what happens to their momentum?

the objects switch linear momentum when they collide

What is impulse?

the product of a force and the time interval over which the force acts, "change in momentum"

What is linear momentum?

the product of an object's mass and its linear velocity - quantifies an objects intertia (ability to resist motion) - L = m*v

The direction of momentum and veolcity is?

the same

What is the direction of force and acceleration?

the same

What is torque?

turning effect caused by eccentric force

What is equilibrium?

unchanging/constant

Is acceleration a scalar or vector?

vector

Is momentum a scalar or vector?

vector ; it has magnitude AND direction

To change momentum you must change?

velocity or mass (often mass is constant so normally just velocity)

What velocity does static equilibrium have?

zero

When an object moves at a constant speed what is the net force?

zero

What is the equation for momentum?

L = m*v (linear momentum = mass * velocity)

What is the law of inertia (Newton's 1st law)?

a body will maintain a state of rest or constant velocity unless acted on by an external force that changes the state