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lecture five -- linear kinetics
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Gravity
Terms in this set (24)
kinetics
branch of dynamic mechanics that describe forces that cause motion
motion
changing place/position of a body with respect to some reference point
1st law of motion
an object at rest stays at rest and an object in motion stays in motion (with same constant velocity) unless acted on by an unbalanced force
*objects want to keep doing what they're doing
*no external force acts upon an object (basis for static equilibrium) or if the net external force = 0
key to 1st law of motion
horizontal velocity of a projectile is constant and it's horizontal motion is in a straight line
seat belt vs air bag
seat belt -- absorb energy by stretching & widen area over which force is exerted
airbag -- cushion the driver increases the stopping contact time
inertia
resistance to motion, mass of an object in kg
for an object to move, inertia must be overcome...additional force is required to accelerate or to move
more mass = greater force to overcome inertia
net external force > inertia of object
linear momentum (p)
mass x linear velocity
p = mv (kg/ms/s-1)
you can think of it as the quantity of motion
very important in collisions because the result depends largely on momentum of each body involved
key points for momentum
the faster an object moves, the more momentum it has
the larger a moving object's mass, the more momentum it has
momentum depends on the mass of the object and it's velocity
momentum does not equal mass
if an object is at rest, it has no momentum -- no matter how large it's mass. momentum is not the same as inertia
conservation of momentum
when bodies exert force on one another, the total momentum in any direction remains constant unless some external force (like friction) acts on the system
when two objects collide, the momentum lost by one of the objects is gained by the other object
(think of pool when the cue ball's energy gets transferred to the other ball)
elastic collisions
when two objects collide in a head-on collision, their combined momentum is conserved
think of objects bouncing off of each other
velocity after elastic collision
involving one stationary object: the stationary will be moving, the moving will become stationary
involving two moving objects: the objects transfer their velocity following the rules of vectors
inelastic collision
momentum is conserved but instead of bouncing off each other, they stay together after the collision and move together with the same velocity
(m1v1 + m2v2)pre = (m1 +m2)v...the motion of two objects involved in a linear perfectly inelastic collision
coefficient of restitution
absolute ratio of the velocity of separation to the velocity of approach
*quantifying how elastic the collisions of an objects are -- describes how fast two objects are moving away from each other
affected by the nature of both objects in the collision
perfectly elastic collision
coefficient = 1.0 (max value)
think of the penny example
perfectly inelastic collision
coefficient = 0 (min value)
think of football where objects stay together after collision
2nd law of motion
acceleration is produced when a force acts on a mass the greater the mass (of the object being accelerated), the greater the amount of force needed to accelerate the object
net external force = mass x instantaneous acceleration of the object
key points for newton's 2nd law
force causes acceleration
acceleration is the effect of the forces
the heavier the object, it will accelerate less, or more force will be needed to cause a given acceleration
momentum and impulse
tells us what happens only at an instant in time, the acceleration experienced by a body or object at the instant the net force acts
the instantaneous acc. will change if the net force changes
stationary object
NO momentum
force application
greater the force, the greater the change in momentum
impulse-momentum relationship
the greater the impulse (J), the greater will be the change in momentum (p) or change in ft = change in mv
impulse-momentum relationship in sport
to increase the momentum of an object, either the mass or velocity of an object must change
in sport most objects we deal with constant mass (mass of a ball), therefore we need to change the velocity
3rd law of motion
For every action there is an equal and opposite reaction...or the mutual actions of two bodies upon each other are always equal and directed to contrary parts
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