work work Flashcards
what is work?
W = Fd (d = delta x)
W = force * displacement
the result of a force MOVING an object
does work also work independnetly on axes?
yaen
what is the joule the unit of?
work AND energy, units of N*m
what happens when the angle is 90 degrees or opposite to the displacement?
W = fdcos(theta)
at 90 degrees cos = 0, independent entirely
at 0 degrees cos = 1, simple Force*d
at 180 degrees energy is being taken away, work is negative, cos(180) = -1 thus -F*d
what do you do when force is changing?
throwback to area method:
area of work*displacement if force is constant
if force is changing, area is still work done, displacement * force/2
what is power
the rate at which work is done, or work/time (J/s)
P = W/t = Fdcos(theta)/t
simplify d/t = velocity
Depending on what you have,
P = f*v(cos(theta))
unit for power (J/s)
watts
ENERGY TYPES
- Kinetic (energy of movement)
- potential (energy of position)
- mechanical (sum of potential and kinetic
- nonmechanical (thermal, lgiht, sound, chemical, nuclear, etc. (generally microscopic forms of kinetic and/or potential))
kinetic energy equation
K = 1/2mv^2
stored/potential energy equation
U = mg(delta h)
usually given letter U for some reason
units of KE
kg * m^2/s^2 (joules)
units of PE
kg * m/s^2 * m (joules)
conservation of MECHANICAL energy and conservative vs. nonconservative forces
conservative forces conserve mechanical energy (i.e. gravity, magnetism, nuclear forces, force of a sprint) energy only changes form
nonconservative - mechanical energy —>nonmechanical, ex. rubbing hands causes heat
if i throw the tennis ball straight upwards at 5.0 m/s how high will it go?
positionf = peak of throw
Ki + Ui = Kf + Uf
Ui = 0, Kf = 0
therefore Ki = Uf
1/2mv^2 = mgh
mass cancels out
1/2(25) = 10h
h = 1.25
can potential energy be directly measured?
no
can kinetic energy be directly measured?
yes
true/false: object CAN be treated as having no internal structure despite multiple objects inside and transfer of matter/force/energy w/i system
true, though not when trying to calculate internal forces obvi
true/false: object’s internal structure cannot transfer force/matter/energy w/i a system due to conservation of mechanical energy
no bro what
potential energy only exists in what kind of system?
multiple object systems like a boy jumping and the earth pulling it down, relationship between earth and boy produces potential energy, but by by himself does not have PE
kinetic energy only exists in what kind of system?
ones in motion, but a single object can have KE by itself.
AP FRQ: A ball rolls from rest down a ramp w/some friction
A. describe what happens to the total mechanical energy of the system consisting of the ball:
B. Describe what happens to the total mechanical energy of the system conssiting of teh ball and earth
A. from the ball’s perspective (PE does not exist) kinetic energy increases as it speeds up
B. Total mechanical energy decreases due to friction transferring to thermal energy
what are the most common ways an object loses mechanical energy?
friction (thermal), sound,
true/false: energy conservation methods are generally easier/preferable when they can be applied
true
energy conservation vs. kinematics (what things can they/can they not calculate?)
describe mechanical energy when a hand is accelerating a ball upwards
both kinetic AND potential are increasing,
when is ME not conserved?
when there is a non-conservative force like friction/air resistance
when there is ANY external force doing work on the system (force of friction or force applied)
Work-energy theorem
change in energy = work
change in KINETIC energy NET WORK done on system
Ki + Ui + W = Kf + Uf
(W = F*x)
ME relationship equation (all the added stuff = the other added stuff)
Ki + Ui + W = Kf + Uf
what is the work-energy theorem most commonly used for?
finding stopping distance
take the equation
W = F * d * cos(theta)
which values are vectors? which are absolute values (not necessarily scalar)?
Force and distance are only used in magnitude, therefore are positive
degrees or theta is also always positive, as the theta here is only the DISTANCE OF ANGLE BETWEEN the force vector and distance axis
Depending on whether cos(theta) turns out to be positive or negative, W will be the same sign
