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HN Physics Chapter 6
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Gravity
Terms in this set (58)
What is the meaning of work in physics
what is accomplished when a force acts on an object, and the object moves through a distance
work
the product of the magnitude of the displacement times the component of the force parallel to the displacement
What is the formula for work
W=Fdcos(theta)
F= magnitude
d=distance
theta=angle between the directions of force and displacement
What type of quantity is work
scalar (no direction only magnitude)
What is the SI unit for work
joule (N x m)
What do you need for work
force and displacement
Can a force be exerted on an object but do no work
yes, if the object remains stationary
What is necessary to specify when talking about work
-whether you are talking about work done by a specific object or done on a specific object
-whether the work done is due to one particular force or the total work done by the net force on the object
What are the steps to solving work problems
-draw a free body diagram
-choose an xy coordinate system
-apply newton's laws
-find the work done by a specific force
-find the net work done on the object
What is one of the most important concepts in science
energy
What is the most crucial aspect of energy
that the sum of all types of energy, the total energy, is the same after any process as it was before
What type of quantity is energy
conserved
What is energy
the ability to do work
What are the two types of energy
potential and kinetic
What is kinetic energy
energy of motion
What is the equation for kinetic energy
KE=1/2mv^2
What is the equation for the Work net equation
Wnet=1/2mv2^2 - 1/2mv1^2
What is the work-energy principle
the net work done on an object is equal to the change in the object's kinetic energy
What does the work energy principle tell us
if (positive) net work W is done on an object, the object's kinetic energy increases by an amount W and vice versa with negative
What does a net force exerted on an object opposite to the object's direction of motion do
decreases its speed and its kinetic energy
What is kinetic energy directly proportional to
the mass of the object and the square of its speed
What type of quantity is kinetic energy
scalar
What is potential energy
the energy associated with forces that depend on the position or configuration of an object relative to its surroundings
What is the most common example of potential energy
gravitational potential energy
What is gravitational potential energy
the product of the object's weight mg and its heigh y above some reference level
What is the equation for gravitational potential energy
PEg= mgh
What is the change in gravitational potential energy equal to
the negative of the work done by gravity itself
What does gravitational potential energy depend on
the vertical height of the object above some reference level
What is the change of potential energy associated with
a particular force equal to the negative of the work done by that force when the object is moved from one point to a second point
What is an alternate definition of potential energy
the work required of a net external force to move an object without acceleration between the two points
What is the spring equation
Fs=-kx
what is the equation for potential energy
PE=1/2kx^2
What are conservative forces
forces which the work done does not depend on the path taken but only on the initial and final positions
What are nonconservative forces
any work that depends on a path
What can potential energy only be defined for
a conservative force
What is the equation for Wnc
Wnc=deltaKE + delta PE
What is Wnc
the work Wnc done by the nonconservative forces acting on an object is equal to the total change in kinetic and potential energies
what it total mechanical energy
the sum of the kinetic and potential energies at any moment
What is the equation for total mechanical energy
PE1 + KE1 = PE2 + KE2
What is the principle of conservation of mechanical energy
if only conservative forces do work, the total mechanical energy of a system neither increases nor decreases in any process. It stays constant -- it is conserved
Where does a rollar coaster car of the most kinetic energy
at the bottom of a hill
When does the rollar coaster car return to potential energy
when it comes to a stop
What are some other forms of energy
-electric energy
-nuclear energy
-thermal energy
-chemical energy
What is the atomic theory
thermal energy is the kinetic energy of rapidly moving molecules when an object is heated, the molecules that make up the object move faster
Can energy be transformed to one form to another
yes
What are some examples of the transformation of energy
-potential into kinetic
-kinetic into potential
What is a relation between work and energy
work is done when energy is transferred from one object to another
What is the law of conservation of energy
the total energy is neither increased nor decreased in any process. Energy can be transformed from one form to another, and transferred from one object to another, but the total amount remains constant
What are dissipative forces
frictional forces reduce the mechanical energy
What is Wnc in the case of friction
Wnc=-Ffrd
Final: 1/2mv1^2 + mgh1 - Ffrd = 1/2mv2^2 + mgh2
Initial: 1/2mv1^2 + mgh1 = 1/2mv2^2 + mgh2 + Ffrd
How to solve problems with conservation of energy
-draw a picture
-determine the system (objects or objects and the forces)
-ask yourself the quantity youre looking for, initial and final
-choose a reference frame
-ask if the mechanical energy is conserved (if no friction KE1+PE1 = KE2+PE2)
-apply conservation of energy (If friction Wnc=deltaKE + deltaPE)
-use the equations you develop to solve
What is power
the rate at which work is done
What is the equation for power
Paverage=work/time
P=Fd/t or Fv
What is the SI unit for power
watt (J/s)
What is the SI unit for the British system
horse power
what is the conversion factor for watts to horsepower
746 W
What is efficienty
the ratio of the useful power output of the engine Pout to the power input Pin
What is efficiency numerically
also less than 1.0 because no engine can create energy and no engine can even transform energy from one form to another without some energy going to friction, thermal energy, and other nonuseful forms of energy
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